Life in the inflamed intestine,Salmonella styleRenato L. Santos1, Manuela Raffatellu2, Charles L. Bevins3, L. Garry Adams4,Cagla Tukel3, Renee M. Tsolis3 and Andreas J. Baumler3

1 Departamento de Clınica e Cirurgia Veterinarias, Escola de Veterinaria, Universidade Federal de Minas Gerais, Belo Horizonte,

MG, Brasil2 Department of Microbiology and Molecular Genetics, University of California at Irvine, Irvine, CA, USA3 Department of Medical Microbiology and Immunology, School of Medicine, University of California at Davis, One Shields Ave.,

Davis, CA, USA4 Department of Veterinary Pathobiology, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College

Station, TX, USA

Review

Glossary

Enterobactin: also known as enterochelin, a cyclic trimer of 2,3-dihydrox-

ybenzoylserine. It binds Fe3+ with high affinity, and is internalized by E. coli and

S. Typhimurium through the energy-dependent substrate specific outer

membrane receptor FepA.

Exudative inflammation: microscopic pathological changes in tissue charac-

terized by acutely increased vascular permeability, neutrophil recruitment and

the formation of tissue exudates above surfaces or within spaces (e.g. in the

intestinal lumen).

Granulopoiesis: development of the granulocytic white blood cells, neutro-

phils, eosinophils and basophils, in the bone marrow.

Lipocalin-2: also known as siderocalin or neutrophil gelatinase-associated

lipocalin (NGAL), a 25-kDa N-glycosylated protein that exhibits antimicrobial

activity by binding enterobactin, thereby preventing bacterial iron acquisition.

Paracrine signal: a signal, such as a cytokine, that is released by one cell and

acts on a target cell located in close proximity.

Pattern recognition receptors (PRRs): receptors of the innate immune

surveillance system that recognize and respond to conserved pathogen

associated molecular patterns (PAMPs).

Pyroptosis: a caspase-1 dependent form of programmed cell death associated

with proteolytic activation of the pro-inflammatory cytokines IL-1b, and IL-18.

Type III secretion system (T3SS): protein secretion system of Gram-negative

bacteria that functions in translocating proteins, termed effectors, into the

cytosol of a host cell. These effectors, therefore, cross three biological

membranes: the bacterial cytoplasmic membrane, the bacterial outer mem-

The lower gastrointestinal tract is densely populatedwith resident microbial communities (microbiota),which do not elicit overt host responses but ratherprovide benefit to the host, including niche protectionfrom pathogens. However, introduction of bacteria intothe underlying tissue evokes acute inflammation. Non-typhoidal Salmonella serotypes (NTS) elicit this stereo-typic host response by actively penetrating the intestinalepithelium and surviving in tissue macrophages. Initialresponses generated by bacterial host cell interactionare amplified in tissue through the interleukin (IL)-18/interferon-g and IL-23/IL-17 axes, resulting in the acti-vation of mucosal barrier functions against NTS disse-mination. However, the pathogen is adapted to surviveantimicrobial defenses encountered in the lumen of theinflamed intestine. This strategy enables NTS to exploitinflammation to outcompete the intestinal microbiota,and promotes the Salmonella transmission by the fecal/oral route.

Non-typhoidal salmonellosisNon-typhoidal Salmonella serotypes (NTS) are a leadingcause of acute food borne disease worldwide (Box 1). Themost common human clinical isolates are Salmonellaenterica serotypes Typhimurium (S. Typhimurium) andEnteritidis (S. Enteritidis) (reviewed in [1]). In immuno-competent individuals, NTS are associated with gastro-enteritis, a localized infection of the terminal ileum andcolon that manifests as fever, diarrhea and intestinalcramping. However, a breach of mucosal barrier functionsin immunocompromised individuals can result in the de-velopment of a life-threatening bacteremia.

Current research on S. Typhimurium pathogenesis isbeginning to paint a novel picture of the unique challengesand opportunities encountered during life in the inflamedintestine. Recent studies have identified host factors thatare crucial for activating mucosal barrier functions in theinflamed intestine of immunocompetent hosts. In turn,these findings provide clues about the identity of mucosalbarrier defects that put immunocompromised hosts at riskof developing bacteremia. New insights into the con-

Corresponding author: Baumler, A.J. (ajbaumler@ucdavis.edu)

498 0966-842X/$ – see front matter � 2009 Elsevie

sequences of inflammation on the growth conditionsencountered by microbes residing in the intestinal lumenreveal how the pathogen might benefit from inducingantimicrobial host responses. In this report, we reviewthese novel hypotheses, which help us to understand boththe patient presentation and the selective forces shapingthe inflammation-adapted pathogenic lifestyle of S. Typhi-murium.

Virulence factors, pathogen associated molecularpatterns and the initiation of inflammatory responsesThe key virulence traits that enable S. Typhimurium toelicit inflammation are its ability to penetrate the intes-tinal epithelium and to survive within macrophages.These properties are conferred by two type III secretionsystems (T3SS) that function in injecting bacterialproteins, termed effectors, into the eukaryotic cytoplasmwhere they alter host cell physiology. The invasion-associ-ated type III secretion system (T3SS-1) allows thepathogen to induce actin rearrangements in epithelial

brane and the eukaryotic cytoplasmic membrane. T3SS is phylogenetically

related to the flagella assembly apparatus.

r Ltd. All rights reserved. doi:10.1016/j.tim.2009.08.008 Available online 12 October 2009

Box 1. Epidemiology of NTS infections

NTS are the single most common cause of death from diarrheal

disease associated with viruses, parasites or bacteria [83], and are

the leading cause of foodborne disease outbreaks in the USA [84],

costing $0.5–2.3 billion annually in medical care and lost productiv-

ity [85]. The press coverage of high profile outbreaks contributes to

a good visibility of this public health problem in the USA and

Europe. What is less known publicly is the enormous effect that NTS

infections have in developing countries, particularly in sub-Saharan

Africa. Diarrheal diseases result in an estimated 2–3 million annual

deaths among children in developing countries, a significant portion

of which is caused by NTS. In addition, NTS are a leading cause of

bacteremia in immunocompromised individuals in sub-Saharan

Africa, and symptoms of gastroenteritis are frequently absent in

these cases. The main risk factors in African children for developing

NTS bacteremia are malnutrition, AIDS and severe malarial anemia

(reviewed in [86]). The magnitude of this problem is little publicized

but it contributes considerably to morbidity and mortality in sub-

Saharan Africa. For example, NTS are currently the most common

blood isolates from children [87] and the second most common

cause of neonatal meningitis in Malawi [88], resulting in mortality

rates exceeding 20%. In contrast to that in children, NTS bacteremia

in African adults is associated with AIDS as the sole major risk

factor. Owing to the human immunodeficiency virus (HIV) epidemic,

NTS have become one of the most common blood isolates from

adults admitted to hospital, which represents an important but

under-recognized emerging infectious disease problem in Sub-

Saharan Africa [86]. Annually, about 10% of HIV positive African

adults develop invasive NTS infections, resulting in mortality rates

of greater than 20% despite antibiotic therapy.

Box 2. Animal models for human gastroenteritis

Mice are naturally susceptible to infection with S. Typhimurium, and

this animal model offers some clear advantages, such as the

availability of knockout animals and immunological reagents. A

limitation of using the mouse model to study gastroenteritis is the

fact that these animals do not develop exudative inflammation in

the intestinal mucosa during S. Typhimurium infection. This

limitation can be overcome by pretreatment of mice with strepto-

mycin, which results in the consistent development of neutrophil

influx in the cecum during S. Typhimurium infection and can be

used to model acute intestinal inflammation [89]. A remaining

caveat of the mouse model is that bacteremia and systemic

dissemination are observed in mouse lineages that are either

genetically resistant or genetically susceptible to S. Typhimurium,

thus making it difficult to study mucosal barrier functions in this

model. Furthermore, development of exudative inflammation

requires a disruption of the intestinal microbiota through antibiotic

treatment. In contrast to mice, the natural or experimental infection

of calves with S. Typhimurium results in localized enteric disease

with clinical and pathological features that parallel the disease in

humans [43]. However, this model is more challenging experimen-

tally, it requires specialized animal facilities and the availability of

immunological reagents and genetic tools is limiting. A notable

difference between the mouse and the calf appears to be related to

mechanisms involved in early initiation of intestinal inflammation.

Signaling through bacterial specific TLRs does not contribute to

cecal inflammation during the first 1 or 2 days after infection of

streptomycin pretreated mice [90], whereas this mechanism does

contribute to cytokine production and neutrophil recruitment

observed within hours after S. Typhimurium infection of the bovine

ileal mucosa [9,20]. Neither mice nor calves are well suited to model

how an underlying HIV infection predisposes individuals to develop

NTS bacteremia. A model using rhesus macaques, which are

naturally susceptible to both S. Typhimurium and SIV, has recently

been developed to study this question [24].

Review Trends in Microbiology Vol.17 No.11

cells, resulting inmembrane ruffling and bacterial intern-alization (reviewed in [2]). The second type III secretionsystem (T3SS-2) is employed to alter trafficking of theSalmonella containing vacuole, resulting in macrophagesurvival (reviewed in [3]). Both type III secretion systemscontribute to intestinal inflammation after oral infectionof calves [4] or streptomycin-pretreated mice [5,6] (Box 2).

The action of T3SS-1 and T3SS-2 enables S. Typhimur-ium to reside and survive in intestinal tissues where thepathogen is located intracellularly within epithelial cellsand professional phagocytes (macrophages, dendritic cellsand neutrophils) [7]. Several mechanisms by which thesedirect interactions between bacteria and host cells result inthe production of pro-inflammatory cytokines have beenidentified by the use of tissue culture models (reviewed in[8]). These include the detection of bacterial products,termed pathogen associated molecular patterns (PAMPs),by pattern recognition receptors (PRRs) of the innateimmune system. Examples of relevant PRRs are Toll-likereceptors (TLRs) and nucleotide-binding and oligomeriza-tion domain (NOD)-like receptors (NLRs). For instance,the biofilm matrix of S. Typhimurium contains amyloidfibrils, termed curli, whose protein subunits (CsgA) aredetected by the innate immune system through TLR2 [9], aPRR that also responds to host amyloids [10]. Flagella areproteinaceous surface appendages composed of a majorsubunit (flagellin), which is recognized by TLR5, a PRRexpressed on the basolateral surface of intestinal epithelialcells [11]. Flagellin can also serve as an agonist for acytosolic NLR termed IPAF, an acronym for IL-1b convert-ing enzyme (ICE)-protease activating factor. T3SS-1 trans-locates flagellin into the cytosol of macrophages [12], aprocess that directly or indirectly results in the activationof IPAF [13]. In turn, IPAF activates ICE, also known as

caspase-1, thereby rendering this protein proteolyticallyactive. The resulting protein complex, which is known asthe inflammasome, proteolytically activates IL-1b and IL-18 [14]. A second pathway activating the inflammasomeduring S. Typhimurium infection involves injection ofSopE, a T3SS-1 effector protein [15]. Lipopolysaccharide(LPS), a component of theGram-negative outermembrane,is a potent TLR4 agonist triggering production of inter-leukin (IL)-23 by dendritic cells in response to S. Enter-itidis infection [16]. The relative contribution to intestinalinflammation of these and othermechanisms remains to beidentified in animal models (Box 2).

Direct contact between bacteria and host cells can resultin the production of neutrophil chemoattractants known asCXC chemokines. However, in vivo these mediators aresubstantially produced by epithelial cells located in thecrypts and at the base of villi, areas of the epithelium thatare not invaded by S. Typhimurium [17]. These obser-vations suggest that CXC chemokine production in theseareas of the epithelium is induced in vivo primarily byparacrine signaling mechanisms (Figure 1).

T cells and the amplification of inflammatory responsesin tissueBecause only a very small fraction of cells in infected tissuecontain bacteria, the total capacity for cytokine productionby these cells is probably limited in scope. However, hostcells infected with S. Typhimurium are a source of cyto-kines, such as IL-18 and IL-23, which help to amplify

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Figure 1. The inflammation-adapted pathogenic lifestyle of S. Typhimurium. The schematic shows host factors (red) and bacterial factors (blue) contributing to the

development of acute intestinal inflammation within hours after S. Typhimurium infection of the intestinal mucosa. The bacterium initiates intestinal inflammation through

direct interaction with host cells, resulting in a release of cytokines, such as IL-18 and IL-23. The cytokines IL-18 and IL-23 help to amplify responses in tissue by stimulating T

cells to produce IFN-g (which activates macrophage antimicrobial activities), IL-17 (which orchestrates neutrophil barrier function) and IL-22 (which stimulates epithelial

antimicrobial responses). The ability to grow on mucus carbohydrates and its resistance against antimicrobials (e.g. lipocalin-2 resistance mediated by the iroBCDEN genes)

enable S. Typhimurium to benefit from the changes encountered in the inflamed intestine, resulting in its luminal outgrowth and enhanced transmission. Although

intracellular salmonellae might lack flagellae in certain host tissues or cell types, in vivo analysis of flagellin expression suggests that intracellular bacteria isolated from

intestinal tissue contain these surface structures [97]. CD8, CD8+ ab T cell; DC, dendritic cell; MF, macrophage; NK, natural killer cell; NKT, natural killer T cell; PAMPs,

pathogen associated molecular patterns; PMN, neutrophil; dg TC, dg T cell; Th1, CD4+ ab memory type-1 T-helper cell; Th17, CD4+ ab memory type-17 T-helper cell.

Review Trends in Microbiology Vol.17 No.11

responses in tissue through paracrine signaling mechan-isms (Figure 1). T cells play an important role in amplifyinginflammatory responses induced byS.Typhimurium in theintestinal mucosa by contributing to the production of keycytokines, including interferon (IFN)-g, IL-17 and IL-22[18]. IL-18 can stimulate antigen-experienced T cells torapidly secrete IFN-g during bacterial infection by anantigen-independent mechanism, thereby significantlyamplifying early effector responses in vivo [19]. Caspase-1, which proteolyticallly activates IL-18, is essential forinducing IFN-g expression in the cecal mucosa 12 hoursafter infecting mice with S. Typhimurium [20], and con-tributes to pathological changes observed 2 days afterinfection in the cecum [15]. However, the inflammasomeis not required for inducing expression of IL-17 [20].

The cytokine IL-6 in combination with transforminggrowth factor (TGF)-b can initiate the differentiation ofnaıve T cells into antigen-specific IL-17-producing CD4+

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memory T cells (Th17 cells) [21–23], an adaptive responsethat develops over a period of several days. However, adramatic increase in IL-17 and IL-22 expression isobserved in bovine or simian ligated ileal loops within2–5 hours after infection of naıve animals with S. Typhi-murium, pointing to an innate mechanism for inducing theproduction of these cytokines in the intestinal mucosaearly after infection [24,25]. In a naıve host, this early,innate amplification of cytokine expression probablyinvolves antigen-experienced T cells that secrete IL-17and IL-22 in response to an antigen-independent stimu-lation with IL-23. Studies in the mouse model show thatIL-23 is essential for the expression of IL-17, and contrib-utes to the production of IL-22 by T cells in the intestinalmucosa early during S. Typhimurium infection [26]. Inaddition to T cells, natural killer (NK) cells [27,28] anddendritic cells [29] are sources for IL-22 production in theintestine. The receptor for IL-23 is expressed on the surface

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of several distinct intestinal T cell populations, includingTh17 cells, gd T cells and NK T cells [26,30–32]. Thenumber of gd T cells expressing the receptor for IL-23increases within 2 days after S. Typhimurium infection,whereas expression by other T cell subsets remains con-stant [26]. IL-23 does not affect production of IFN-g, whoseexpression in the intestine is induced normally during S.Typhimurium infection of IL-23 deficient mice [26].

To summarise, an innate, T cell-dependent amplifica-tion of inflammatory responses proceeds through two inde-pendent pathways, which are initiated by IL-18 and IL-23,respectively. Consistent with the idea that their pro-duction results from an amplification of responses, IFN-g, IL-17 and IL-22 are among the cytokines whose expres-sion is most prominently induced during S. Typhimuriuminfection of the intestinal mucosa [18,24].

IL-17, neutrophils and the mucosal barrier to S.

Typhimurium disseminationThe cytokine storm ensuing from the amplification ofinflammatory responses in the intestinal mucosa resultsin the activation of two broad arms of an innate immuneresponse. One important component of this antimicrobialresponse is the recruitment of neutrophils. IL-17 is acytokine involved in orchestrating this arm of the hostresponse, in part by stimulating epithelial cells to secreteCXC chemokines and by stimulating granulopoiesis in thebone marrow through inducing production of granulocytecolony-stimulating factor (G-CSF) [33]. During S. Typhi-murium infection, IL-17 deficient mice exhibit a defect inrecruiting neutrophils into the intestinal mucosa [24]. Anadditional mechanism contributing to CXC chemokineproduction in epithelial cells might be mediated throughIL-1b [34], which becomes proteolytically activated by thecaspase-1 inflammasome. Consistent with this idea, cas-pase-1 deficient mice exhibit reduced CXC chemokineproduction in the cecal mucosa early (12 hours) after S.Typhimurium infection [20](Figure 1).

Neutrophils help to prevent S. Typhimurium dissemi-nation from the gut, a notion supported by clinical data,showing that neutropenia is a risk factor for bacteremiawith NTS [35,36]. The neutrophil barrier orchestrated bythe IL-23/IL-17 axis is thought to constitute a defenseagainst extracellular bacteria [37], which at first seemsto be at odds with the fact that this host defense is effectiveagainst the facultative intracellular S. Typhimurium. Apossible explanation is the brief extracellular phase thatmust exist when S. Typhimurium transits from epithelialcells to phagocytes or when it moves to a new host cell afterkilling a macrophage through pyroptosis [38], which mightrender the pathogen susceptible to neutrophil attack. S.Typhimurium typically is not observed extracellularly ininfected intestinal tissue [7]. However, after depletion ofneutrophils, bacteria grow extracellularly, which demon-strates the importance of this arm of the host defense incontrolling the extracellular phase of S. Typhimurium invivo [39,40]. In summary, clinical and experimental datasupport a role for neutrophil recruitment in checkingsystemic dissemination of S. Typhimurium from the gut.

Patients with AIDS have an increased risk of developingNTS bacteremia (Box 1), and experimental evidence from a

rhesus macaque model points to IL-17 deficiency as theunderlying immune defect responsible for this clinicalobservation. Simian immunodeficiency virus (SIV) inducesa depletion of Th17 cells in the ileal mucosa of rhesusmacaques, which results in increased translocation of S.Typhimurium to the mesenteric lymph nodes [24]. Inter-estingly, SIV-infected rhesus macaques exhibit a defect inproducing IL-17 and IL-22 in response to S. Typhimuriumchallenge, whereas production of IFN-g is normal. IL-17deficiency [24] or depletion of CD4+ T cells [41] in miceresults in increased bacterial translocation from the gut.These data suggest that Th17 depletion impairs the muco-sal neutrophil barrier, which is responsible for theincreased risk of individuals infected with HIV for devel-oping NTS bacteremia. HIV positive individuals also exhi-bit an increased translocation of normal gut microbiota,which does not manifest as bacteremia. Instead, transloca-tion of microbiota leads to chronic immune activation byintroducing LPS into the circulation, which presumablyoriginates from killed gut commensals [42]. In contrast tothe resident microbiota, S. Typhimurium possesses potentvirulence factors, such as T3SS-2, and the resulting abilityto resist host defenses upon crossing the mucosal barriermight help explain why NTS are leading blood isolatesfrom AIDS patients in sub-Saharan Africa (Box 1).

In addition to the benefits conferred upon the host,neutrophil influx can also cause collateral tissue damageand is associated with necrosis of the surface mucosa inlarge areas of the terminal ileum and colon [4]. The neu-trophil-associated injury to the intestinal epithelium andthe resulting loss of epithelial barrier function lead toleakage of extravascular fluids, thereby contributing todiarrhea [43]. Neutrophils might also contribute to diar-rhea by stimulating chloride secretion in intestinal epi-thelial cells [44]. Thus neutrophil recruitment might be adouble-edged sword that is potentially harmful to the hostbecause the resulting diarrhea can lead to severe dehy-dration. IL-17 deficiency is predicted to impair neutrophilrecruitment and neutrophil functionality during AIDS,which might explain why NTS bacteremia frequentlymanifests in the absence of gastroenteritis in these indi-viduals [45,46] (Box 1).

IL-22, epithelial cells and antimicrobial effectorsreleased to the intestinal lumenA second arm of the antimicrobial innate immune responseencountered during acute inflammation is epithelial-derived antimicrobial proteins and peptides (Box 3). Animportant cytokine for orchestrating this arm of theresponse is IL-22, which induces expression in epithelialcells of host defense effector molecules that seem to bedirected against luminal bacteria (Figure 1). During Citro-bacter rodentium infection of mice, the IL-23/IL-22 axis isrequired for expression in the colonic mucosa of calprotec-tin, an antimicrobial protein mediating zinc deprivation,and RegIIIg (regenerating islet-derived 3 gamma), a bac-tericidal C-type lectin [29]. RegIIIg expression in the cecalmucosa is markedly increased by an IL-23 dependentmechanism during S. Typhimurium infection of strepto-mycin-pretreated mice [26]. Secretion of RegIIIg into theintestinal lumen of wild type mice contributes to clearance

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Box 3. Antimicrobial effectors of the intestinal epithelium

A variety of molecules in the mammalian intestine might impair

growth and survival of microbes in this anaerobic environment. One

source of inhibitory molecules stems from many of the colonizing

microbes themselves. Bacteriocins, including colicins, microcins,

lantibiotics and others, are proteinaceous toxins that inhibit the

growth of related bacteria [91]. These molecules probably con-

tribute to niche protection and might inhibit colonization by

potential pathogens. The other source of inhibitory molecules are

products of the host, some of which are host defense molecules per

se, whereas others have primary function in nutrient absorption. For

example, bile salts and hydrolytic enzymes required for digestion

are toxic for microbes and might contribute to controlling microbial

proliferation and survival. Within the group of primary host defense

molecules, epithelial cells make various peptides along the intest-

inal tract with some constitutively expressed and others transcrip-

tionally induced. In the small intestine, Paneth cells are the source of

abundant quantities of antimicrobials in most mammals [92]. These

granule-rich secretory cells reside at the base of small intestinal

pouches called crypts. Among the most abundant and extensively

studied antimicrobials of Paneth cells are the defensins [93].

Constitutively expressed at high levels, the Paneth cell defensins

typically have membrane-targeted antimicrobial activity against a

wide range of bacteria, and some fungi and parasites. Transgenic

and knockout mouse models have provided evidence that Paneth

cell defensins contribute to the host defense against bacterial

pathogens [94,95]. Other secretory granule-associated constitutive

antimicrobials of these cells are lysozyme and secretory phospho-

lipase A2. Paneth cells also make some inducible antimicrobials,

including RegIIIg, a C-type lectin with selective activity against

Gram-positive bacteria [48,96]. In the colon, epithelial cells inducibly

express a collection of antimicrobial peptides and proteins, includ-

ing Resistin-like beta (RELM-b), RegIIIg, calprotectin, and b-defen-

sins.

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of luminal bacteria, including Listeria monocytogenes andvancomycin-resistant Enterococcus, but this response isabsent inmice deficient formyeloid differentiation primaryresponse protein 88 (MyD88) [47,48], an adaptor proteinfor all TLRs except TLR3.

In vitro, IL-22 induces the expression in human colonicepithelial cells of inducible nitric oxide synthase (iNOS),mucin (MUC4) and lipocalin-2, an antimicrobial proteinthat prevents bacterial iron acquisition [49]. Whereaslipocalin-2 secretion is induced upon IL-22 stimulation,S. Typhimurium infection of colonic epithelial cell linesdoes not induce expression, suggesting that activation ofthis epithelial antimicrobial response requires paracrineIL-22 signaling rather than direct interaction of bacteriawith enterocytes. In vivo, epithelial cells in the ilealmucosa of rhesus macaques produce large quantities oflipocalin-2 in response toS.Typhimurium infection, result-ing in accumulation of this antimicrobial in the intestinallumen [49]. Other antimicrobial proteins and peptideswhose transcripts are prominently induced in the intesti-nal mucosa during S. Typhimurium infection includeiNOS, calprotectin, MUC4, dual oxidase 2 (Duox2) andbovine enteric b defensin [18,24,25].

The inflammation-adapted pathogenic lifestyle of S.

TyphimuriumFrom these discussions, it can be concluded that acuteintestinal inflammation causes dramatic changes in theenvironment encountered by luminal bacteria through atleast two distinct mechanisms [50]. First, the release of

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antimicrobial products orchestrated through the IL-23/IL-22 axis converts the intestinal lumen in an increasinglyhostile environment. Second, coordination of neutrophilrecruitment through the IL-23/IL-17 axis is associatedwith diarrhea, during the course of which the intestinebecomes devoid of contents, thereby severely limiting thepresence of nutrients that normally support bacterialgrowth in the intestinal lumen. Together, these dramaticchanges in the luminal environment trigger changes in thecomposition of the intestinal microbiota [51]. The numbersof commensal microbes, mostly belonging to the Firmicutesand Bacteroides phyla, are reduced [51,52]. Although S.Typhimurium cannot resist the onslaught of neutrophils inintestinal tissue, the pathogen appears to be custom-builtto bloom in the lumen of the inflamed intestine, because itsnumbers in this niche increase dramatically during inflam-mation [52,53].

The strategy by which S. Typhimurium appears to beable to adjust to the limited nutrient availability in theinflamed intestine is to specialize in mucus carbohydratesas a major source of high-energy nutrients. This pathwaymight take advantage of the increased mucus (MUC4)production elicited by IL-22 during S. Typhimurium in-fection [49]. Flagella-mediatedmotility and chemotaxis arerequired for enhanced growth in the inflamed intestine,because D-galactose gradients emanating from the mucuslayer drive S. Typhimurium chemotaxis towards this niche[54]. Once it reaches the mucus, S. Typhimurium canattach using Std fimbriae, an adhesin that binds terminala(1,2)fucose moieties in mucus carbohydrates of the mur-ine cecum [55]. Mucus colonization (Figure 2) enables S.Typhimurium to efficiently utilize mucus carbohydrates,which are thought to be a good energy source for thepathogen [54].

To exploit inflammation, S. Typhimurium must alsopossess virulence genes that confer resistance to antimi-crobial defense mechanisms encountered during growth inthe mucus layer; however, the underlying mechanisms arelargely unknown. Support for this idea comes from studieson lipocalin-2 resistance of S. Typhimurium. Lipocalin-2 isan antimicrobial secreted into the lumen of the inflamedintestine [49], which prevents bacterial iron acquisition bybinding enterobactin (also known as enterochelin), a lowmolecular weight iron chelator produced by most membersof the Enterobacteriaceae, including S. Typhimurium [56–

58]. The iroBCDEN gene cluster of S. Typhimuriumencodes proteins involved in the biosynthesis and uptakeof salmochelin, a glycosylated derivative of enterobactin[59]. Salmochelin is not bound by lipocalin-2, and itsproduction therefore confers resistance against this anti-microbial protein [60,61]. Mutational inactivation of theiroBCDEN gene cluster renders S. Typhimurium sensitiveto lipocalin-2, because it now depends on enterobactin foriron acquisition. The iroBCDEN gene cluster confers agrowth advantage in the inflamed intestine, whereas nobenefit is observed in the absence of intestinal inflam-mation or in lipocalin-2 deficient mice [49]. These datasuggest that the iroBCDEN gene cluster enables S. Typhi-murium to cope with the increased lipocalin-2 concen-tration encountered in the lumen of the inflamedintestine. In addition,S.Typhimuriumhas virulence genes

Figure 2. Two distinct S. Typhimurium populations are present in the intestine.

Detection of the S. Typhimurium O-antigen by immunohistochemistry

(streptavidin–biotin peroxidase; brown precipitate) in a section of the ileal

mucosa from a rhesus macaque ligated ileal loop, 5 hours after infection. The

image shows the coexistence of two distinct S. Typhimurium populations, one

invading intestinal tissue and triggering inflammation (arrow), the other benefiting

from the resulting environmental changes in the luminal mucus layer (arrowhead).

The section was counterstained with hematoxylin (blue). Scale bar = 100 mm.

Review Trends in Microbiology Vol.17 No.11

that confer resistance to antimicrobial peptides, includingdefensins [62–64], which might serve a similar purpose.Collectively, these data are beginning to support the con-cept that resistance to epithelial-derived antimicrobialproteins and peptides represents a specific adaptation ofS. Typhimurium to life in the inflamed intestine.

The picture emerging from these studies is that efficientutilization of mucus carbohydrates and an intrinsic resist-ance to inducible antimicrobials produced by epithelialcells might allow S. Typhimurium to exploit inflammationby gaining an advantage during its competition with othermicrobes for intestinal colonization. This virulencestrategy enables S. Typhimurium to thrive in the lumenof the acutely inflamed intestine. The benefit of this tacticis that suppression of the competing microbiota by intes-tinal inflammatory responses increases the numbers of S.Typhimurium in the intestinal contents [52,53], therebypromoting its transmission by the fecal/oral route [65](Figure 1).

IFN-g, macrophages and self-destructive cooperationProduction of IFN-g in the early phase of intestinal inflam-mation contributes to antimicrobial responses in the intes-tinalmucosa [66] and is caspase-1 dependent [20].With theonset of adaptive immunity, IL-12 dependent mechanismsfurther increase IFN-g levels, thereby restricting bacterialgrowth in macrophages, and the pathogen is cleared afterthe development of antigen-specific CD4+ T-cell responses[67]. Thus, a location in tissue is a dead end, and thebacterial cells triggering intestinal inflammation by invad-

ing the intestinal mucosa cannot benefit from the resultingoutgrowth of their comrades in the intestinal lumen. It hasbeen proposed that such a self-destructive cooperation canevolve through population heterogeneity, where one frac-tion is capable of invading intestinal tissue and the otherfraction can benefit from the resulting environmentalchanges in the intestinal lumen (Figure 2) [68]. Forexample, Std fimbriae are only expressed by a fraction ofbacteria in a population, which is probably due to phasevariation [69]. As a result, bacterial cells expressing thisadhesin might be able to colonize the mucus layer [55],whereas the remaining bacterial cells might be able to passthrough it and invade the intestinal epithelium. These orother mechanisms might enable S. Typhimurium to suc-cessfully use self-destructive cooperation to thrive in thelumen of the inflamed intestine.

Concluding remarks and future directionsRecent advances in understanding the pathogenesis of S.Typhimurium-induced gastroenteritis draw a fascinatingpicture of a pathogen that triggers intestinal inflammationto create specific growth conditions in the intestinal lumen,which enable it to outcompete the residentmicrobiota. Thisinflammation-adapted lifestyle requires S. Typhimuriumto be resistant against many of the host defense mechan-isms encountered in the lumen of the inflamed intestine.Neutrophils constitiute one defense to which S. Typhimur-ium is not resistant, as they can check systemic spread ofthe pathogen in immunocompetent individuals. However,in immunocompromised individuals, defects in neutrophilrecruitment and neutrophil antimicrobial activity can giverise to a fulminant bacteremia, which often develops in theabsence of gastroenteritis. Once S. Typhimurium entersthe bloodstream of immunocompromised individuals, itsconsiderable resistance against antimicrobial proteins andpeptides leaves the host with precious few innate defenses,which might explain the high mortality rates associatedwith NTS bacteremia.

The evolution of this inflammation-adapted pathogeniclifestyle was driven by acquisition of key virulence deter-minants through horizontal gene transfer [70]. After itsplit from the closely related Escherichia coli lineage some100–140 million years ago [71,72], the genus Salmonellaacquired the Salmonella pathogenicity island (SPI) 1, agenetic region carrying genes encoding the T3SS-1secretion apparatus [73]. SPI1 encodes only two effectorproteins, SipA and SipC, which are involved in inducingactin rearrangements required for bacterial entry intoepithelial cells. Other T3SS-1 effector proteins requiredfor invasion [74], including SopB, SopE2 and SopD, wereacquired through independent horizontal gene-transferevents that occurred at a similar time during evolution,as indicated by the presence of the corresponding genes inall members of the genus Salmonella [75]. Genes on SPI2encoding the T3SS-2 secretion apparatus were acquiredsome 71–100 million years ago by the S. enterica lineageafter its divergence from S. bongori [76–78], the secondspecies within the genus Salmonella.

Whereas commensal E. coli serotypes are adapted tocolonize the normal intestine, acquisition of SPI1 and SPI2enabled Salmonella serotypes to trigger acute intestinal

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inflammation in their vertebrate hosts. To survive theconsequences of acute intestinal inflammation, acquisitionof SPI1 and SPI2 by the Salmonella lineage was probablyaccompanied by horizontal transfer of genes that enablethe pathogen to survive the dramatic environmentalchanges encountered in the lumen of the inflamed gut.Consistent with this idea, the iroBCDEN gene cluster ispresent in all members of S. enterica, suggesting that thesegenes were acquired at a similar time during evolution asSPI2 [79,80]. The iroBCDEN gene cluster is absent fromthe genomes of commensal E. coli isolates but can be foundin uropathogenic E. coli (UPEC) isolates [81,82], which areable to grow and survive at the mucosal surface of theinflamed bladder. These observations support the conceptthat lipocalin-2 resistance evolved as an adaptation tocolonize inflamed mucosal surfaces.

It seems likely that acquisition of SPI1 and SPI2 wasaccompanied by horizontal transfer of additional genesmediating efficient utilization of mucus carbohydratesand an intrinsic resistance against inducible antimicro-bials produced by epithelial cells. Identification of theseSalmonella-specific genes and their respective roles duringthe inflammation-adapted pathogenic lifestyle of S. Typhi-murium represents an exciting area for future study.

AcknowledgementsWe would like to thank Sebastian Winter for critical reading of themanuscript. This work was supported by Public Health Service grantsAI050553 (R.M.T.), AI076246 (A.J.B., C.L.B. and L.G.A.), AI040124,AI044170 and AI079173 (A.J.B.). CT is supported by ScientistDevelopment Grant 0835248N from the American Heart Association.

References1 Rabsch, W. et al. (2001) Non-typhoidal salmonellosis: emerging

problems. Microbes Infect. 3, 237–2472 Zhou, D. and Galan, J. (2001) Salmonella entry into host cells: the work

in concert of type III secreted effector proteins.Microbes Infect. 3, 1293–

12983 Abrahams, G.L. and Hensel, M. (2006) Manipulating cellular transport

and immune responses: dynamic interactions between intracellularSalmonella enterica and its host cells. Cell. Microbiol. 8, 728–737

4 Tsolis, R.M. et al. (1999) Contribution of Salmonella typhimuriumvirulence factors to diarrheal disease in calves. Infect. Immun. 67,4879–4885

5 Barthel, M. et al. (2003) Pretreatment of mice with streptomycinprovides a Salmonella enterica serovar Typhimurium colitis modelthat allows analysis of both pathogen and host. Infect. Immun. 71,2839–2858

6 Coburn, B. et al. (2005) Salmonella enterica serovar Typhimuriumpathogenicity island 2 is necessary for complete virulence in amouse model of infectious enterocolitis. Infect. Immun. 73, 3219–

32277 Santos, R.L. et al. (2002)Morphologic andmolecular characterization of

Salmonella typhimurium infection in neonatal calves. Vet. Pathol. 39,200–215

8 Tukel, C. et al. (2006) Neutrophil influx during non-typhoidalsalmonellosis: who is in the driver’s seat? FEMS Immunol. Med.Microbiol 46, 320–329

9 Tukel, C. et al. (2005) CsgA is a pathogen-associated molecular patternof Salmonella enterica serotype Typhimurium that is recognized byToll-like receptor 2. Mol. Microbiol. 58, 289–304

10 Tukel, C¸. et al. (2009) Responses to amyloids of microbial and host

origin are mediated through Toll-like receptor 2. Cell Host Microbe 6,45–53

11 Gewirtz, A.T. et al. (2001) Cutting edge: bacterial flagellin activatesbasolaterally expressed TLR5 to induce epithelial proinflammatorygene expression. J. Immunol. 167, 1882–1885

504

12 Sun, Y.H. et al. (2007) Injection of flagellin into the host cell cytosol bySalmonella enterica serotype Typhimurium. J. Biol. Chem. 282, 33897–

3390113 Miao, E.A. et al. (2006) Cytoplasmic flagellin activates caspase-1 and

secretion of interleukin 1beta via Ipaf. Nat. Immunol. 7, 569–57514 Franchi, L. et al. (2006) Cytosolic flagellin requires Ipaf for activation of

caspase-1 and interleukin 1beta in salmonella-infected macrophages.Nat. Immunol. 7, 576–582

15 Muller, A.J. et al. (2009) The S. Typhimurium effector SopE inducescaspase-1 activation in stromal cells to initiate gut inflammation. CellHost Microbe 6, 125–136

16 Siegemund, S. et al. (2007) Production of IL-12, IL-23 and IL-27p28 bybone marrow-derived conventional dendritic cells rather thanmacrophages after LPS/TLR4-dependent induction by Salmonellaenteritidis. Immunobiology 212, 739–750

17 Zhang, S. et al. (2003) Secreted effector proteins of Salmonella entericaserotype Typhimurium elicit host-specific chemokine profiles in animalmodels of typhoid fever and enterocolitis. Infect. Immun. 71, 4795–4803

18 Godinez, I. et al. (2008) T cells help to amplify inflammatory responsesinduced by Salmonella enterica serotype Typhimurium in theintestinal mucosa. Infect. Immun. 76, 2008–2017

19 Srinivasan, A. et al. (2007) Innate immune activation of CD4 T cells insalmonella-infected mice is dependent on IL-18. J. Immunol. 178,6342–6349

20 Winter, S.E. et al. (2009) Contribution of flagellin pattern recognition tointestinal inflammation during Salmonella enterica serotypeTyphimurium infection. Infect. Immun. 77, 1904–1916

21 Kimura, A. et al. (2007) IL-6-dependent and -independent pathways inthe development of interleukin 17-producing T helper cells. Proc. Natl.Acad. Sci. U. S. A. 104, 12099–12104

22 Veldhoen, M. et al. (2006) TGFbeta in the context of an inflammatorycytokine milieu supports de novo differentiation of IL-17-producing Tcells. Immunity 24, 179–189

23 Acosta-Rodriguez, E.V. et al. (2007) Interleukins 1beta and 6 but nottransforming growth factor-beta are essential for the differentiation ofinterleukin 17-producing human T helper cells. Nat. Immunol. 8, 942–

94924 Raffatellu, M. et al. (2008) Simian immunodeficiency virus-induced

mucosal interleukin-17 deficiency promotes Salmonella disseminationfrom the gut. Nat. Med. 14, 421–428

25 Raffatellu, M. et al. (2007) The capsule encoding the viaB locus reducesinterleukin-17 expression and mucosal innate responses in the bovineintestinal mucosa during infection with Salmonella enterica serotypeTyphi. Infect. Immun. 75, 4342–4350

26 Godinez, I. et al. (2009) Interleukin-23 orchestrates mucosal responsesto Salmonella enterica serotype Typhimurium in the intestine. Infect.Immun. 77, 387–398

27 Satoh-Takayama, N. et al. (2008) Microbial flora drives interleukin 22production in intestinal NKp46+ cells that provide innate mucosalimmune defense. Immunity 29, 958–970

28 Cella, M. et al. (2009) A human natural killer cell subset providesan innate source of IL-22 for mucosal immunity. Nature 457,722–725

29 Zheng, Y. et al. (2008) Interleukin-22 mediates early host defenseagainst attaching and effacing bacterial pathogens. Nat. Med. 14,282–289

30 Ivanov, I.I. et al. (2008) Specific microbiota direct the differentiation ofIL-17-producing T-helper cells in the mucosa of the small intestine.Cell Host Microbe 4, 337–349

31 Rachitskaya, A.V. et al. (2008) Cutting edge: NKT cells constitutivelyexpress IL-23 receptor and RORgammat and rapidly produce IL-17upon receptor ligation in an IL-6-independent fashion. J. Immunol.180, 5167–5171

32 Goto, M. et al. (2009) Murine NKT cells produce Th17 cytokineinterleukin-22. Cell. Immunol. 254, 81–84

33 Ye, P. et al. (2001) Requirement of interleukin 17 receptor signaling forlung CXC chemokine and granulocyte colony-stimulating factorexpression, neutrophil recruitment, and host defense. J. Exp. Med.194, 519–527

34 Cromwell, O. et al. (1992) Expression and generation of interleukin-8,IL-6 and granulocyte-macrophage colony-stimulating factor bybronchial epithelial cells and enhancement by IL-1 beta and tumournecrosis factor-alpha. Immunol. 77, 330–337

Review Trends in Microbiology Vol.17 No.11

35 Noriega, L.M. et al. (1994) Salmonella infections in a cancer center.Support Care Cancer 2, 116–122

36 Tumbarello, M. et al. (1995) The impact of bacteraemia on HIVinfection. Nine years experience in a large Italian universityhospital. J. Infect. 31, 123–131

37 Aujla, S.J. et al. (2007) Th17 cells and mucosal host defense. Seminarsin Immunol. 19, 377–382

38 Bergsbaken, T. et al. (2009) Pyroptosis: host cell death andinflammation. Nat. Reviews 7, 99–109

39 Vassiloyanakopoulos, A.P. et al. (1998) The crucial role ofpolymorphonuclear leukocytes in resistance to Salmonella dublininfections in genetically susceptible and resistant mice. Proc. Natl.Acad. Sci. U. S. A. 95, 7676–7681

40 Conlan, J.W. (1996) Neutrophils prevent extracellular colonization ofthe livermicrovasculature bySalmonella typhimurium. Infect. Immun.64, 1043–1047

41 Gautreaux, M.D. et al. (1994) T lymphocytes in host defense againstbacterial translocation from the gastrointestinal tract. Infect. Immun.62, 2874–2884

42 Brenchley, J.M. et al. (2006) Microbial translocation is a cause ofsystemic immune activation in chronic HIV infection. Nat. Med. 12,1365–1371

43 Zhang, S. et al. (2003) Molecular pathogenesis of Salmonellaenterica serotype Typhimurium- induced diarrhea. Infect. Immun.71, 1–12

44 Madara, J.L. et al. (1993) 5’-adenosine monophosphate is theneutrophil-derived paracrine factor that elicits chloride secretionfrom T84 intestinal epithelial cell monolayers. J. Clin. Invest. 91,2320–2325

45 De Wit, S. et al. (1988) Salmonella bacteremia in African patients withhuman immunodeficiency virus infection.Eur J. Clin.Microbiol. Infect.Dis 7, 45–47

46 Brown, M. and Eykyn, S.J. (2000) Non-typhoidal Salmonellabacteraemia without gastroenteritis: a marker of underlyingimmunosuppression. Review of cases at St. Thomas’ Hospital 1970–

1999. J. Infect. 41, 256–25947 Brandl, K. et al. (2008) Vancomycin-resistant enterococci exploit

antibiotic-induced innate immune deficits. Nature 455, 804–80748 Brandl, K. et al. (2007) MyD88-mediated signals induce the

bactericidal lectin RegIII gamma and protect mice against intestinalListeria monocytogenes infection. J. Exp. Med. 204, 1891–1900

49 Raffatellu, M. et al. (2009) Lipocalin-2 resistance confers an advantageto Salmonella enterica serotype Typhimurium for growth and survivalin the inflamed intestine. Cell Host Microbe 5, 476–486

50 Stecher, B. and Hardt, W.D. (2008) The role of microbiota in infectiousdisease. Trends Microbiol. 16, 107–114

51 Lupp, C. et al. (2007) Host-mediated inflammation disrupts theintestinal microbiota and promotes the overgrowth ofEnterobacteriaceae. Cell Host Microbe 2, 119–129

52 Stecher, B. et al. (2007) Salmonella enterica serovar Typhimuriumexploits inflammation to compete with the intestinal microbiota. PLoSBiol. 5, 2177–2189

53 Barman, M. et al. (2008) Enteric salmonellosis disrupts the microbialecology of the murine gastrointestinal tract. Infect. Immun. 76, 907–

91554 Stecher, B. et al. (2008)Motility allows S. Typhimurium to benefit from

the mucosal defence. Cell. Microbiol. 10, 1166–118055 Chessa, D. et al. (2009) Salmonella enterica serotype Typhimurium Std

fimbriae bind terminal alpha(1,2)fucose residues in the cecal mucosa.Mol. Microbiol. 71, 864–875

56 Berger, T. et al. (2006) Lipocalin 2-deficient mice exhibit increasedsensitivity to Escherichia coli infection but not to ischemia-reperfusioninjury. Proc. Natl. Acad. Sci. U. S. A. 103, 1834–1839

57 Flo, T.H. et al. (2004) Lipocalin 2 mediates an innate immuneresponse to bacterial infection by sequestrating iron. Nature 432,917–921

58 Goetz, D.H. et al. (2002) The neutrophil lipocalin NGAL is abacteriostatic agent that interferes with siderophore-mediated ironacquisition. Mol. Cell 10, 1033–1043

59 Hantke, K. et al. (2003) Salmochelins, siderophores of Salmonellaenterica and uropathogenic Escherichia coli strains, are recognizedby the outer membrane receptor IroN. Proc. Natl. Acad. Sci. U. S. A.100, 3677–3682

60 Crouch, M.L. et al. (2008) Biosynthesis and IroC-dependent export ofthe siderophore salmochelin are essential for virulence of Salmonellaenterica serovar Typhimurium. Mol. Microbiol. 67, 971–983

61 Fischbach,M.A. et al. (2006) The pathogen-associated iroA gene clustermediates bacterial evasion of lipocalin 2. Proc. Natl. Acad. Sci. U. S. A.103, 16502–16507

62 Gunn, J.S. (2008) The Salmonella PmrAB regulon: lipopolysaccharidemodifications, antimicrobial peptide resistance and more. TrendsMicrobiol. 16, 284–290

63 Bader, M.W. et al. (2005) Recognition of antimicrobial peptides by abacterial sensor kinase. Cell 122, 461–472

64 Shi, Y. et al. (2004) PhoP-regulated Salmonella resistance to theantimicrobial peptides magainin 2 and polymyxin B. Mol. Microbiol.53, 229–241

65 Lawley, T.D. et al. (2008) Host transmission of Salmonella entericaserovar Typhimurium is controlled by virulence factors and indigenousintestinal microbiota. Infect. Immun. 76, 403–416

66 Rhee, S.J. et al. (2005) Developmentally regulated intestinalexpression of IFN-gamma and its target genes and the age-specificresponse to enteric Salmonella infection. J. Immunol. 175, 1127–

113667 Mastroeni, P. (2002) Immunity to systemic Salmonella infections.

Curr. Mol. Med. 2, 393–40668 Ackermann, M. et al. (2008) Self-destructive cooperation mediated by

phenotypic noise. Nature 454, 987–99069 Chessa, D. et al. (2008) RosE represses Std fimbrial expression in

Salmonella enterica serotype Typhimurium. Mol. Microbiol. 68, 573–

58770 Baumler, A.J. (1997) The record of horizontal gene transfer in

Salmonella. Trends Microbiol. 5, 318–32271 Ochman, H. andWilson, A.C. (1987) Evolution in bacteria: evidence for

a universal substitution rate in cellular genomes. J. Mol. Evol. 26, 74–

8672 Doolittle, R.F. et al. (1996) Determining divergence times of the major

kingdoms of living organisms with a protein clock. Science 271, 470–

47773 Mills, D.M. et al. (1995) A 40 kb chromosomal fragment encoding

Salmonella typhimurium invasion genes is absent from thecorresponding region of the Escherichia coli K-12 chromosome. Mol.Microbiol. 15, 749–759

74 Raffatellu, M. et al. (2005) SipA, SopA, SopB, SopD and SopE2contribute to Salmonella enterica serotype Typhimurium invasion ofepithelial cells. Infect. Immun. 73, 146–154

75 Mirold, S. et al. (2001) Salmonella host cell invasion emerged byacquisition of a mosaic of separate genetic elements, includingSalmonella pathogenicity island 1 (SPI1), SPI5, and sopE2. J.Bacteriol. 183, 2348–2358

76 Hensel, M. et al. (1997) Analysis of the boundaries of Salmonellapathogenicity island 2 and the corresponding chromosomal region ofEscherichia coli K-12. J. Bacteriol. 179, 1105–1111

77 Ochman, H. et al. (1996) Identification of a pathogenicity island forSalmonella survival in host cells. Proc. Natl. Acad. Sci. U. S. A. 93,7800–7804

78 Vernikos, G.S. et al. (2007) Genetic flux over time in the Salmonellalineage. Genome Biol. 8, R100

79 Baumler, A.J. et al. (1997) Rapid detection of Salmonella enterica withprimers specific for iroB. J. Clin. Microbiol. 35, 1224–1230

80 Baumler, A.J. et al. (1998) IroN, a novel outer membrane siderophorereceptor characteristic of Salmonella enterica. J. Bacteriol. 180, 1446–

145381 Sorsa, L.J. et al. (2003) Characterization of an iroBCDEN gene cluster

on a transmissible plasmid of uropathogenic Escherichia coli: evidencefor horizontal transfer of a chromosomal virulence factor. Infect.Immun. 71, 3285–3293

82 Bauer, R.J. et al. (2002) Molecular epidemiology of 3 putative virulencegenes forEscherichia coli urinary tract infection—usp, iha, and iroN(E.coli). J. Infect. Dis. 185, 1521–1524

83 Mead, P.S. et al. (1999) Food-related illness and death in the UnitedStates. Emerg. Infect. Dis. 5, 607–625

84 Olsen, S.J. et al. (2000) Surveillance for foodborne-disease outbreaks–United States, 1993-1997. M.M.W.R. C.D.C. Surveill. Summ. 49, 1–62

85 Frenzen, P. et al. (1999) Salmonella cost estimate update usingFoodNet data. Food Rev. 22, 10–15

505

Review Trends in Microbiology Vol.17 No.11

86 Gordon, M.A. (2008) Salmonella infections in immunocompromisedadults. J. Infect. 56, 413–422

87 Walsh, A.L. et al. (2000) Bacteremia in febrile Malawian children:clinical and microbiologic features. Pediatr. Infect. Dis J. 19, 312–318

88 Molyneux, E.M. et al. (2000) Salmonella meningitis in children inBlantyre, Malawi, 1996–1999. Ann. Trop. Paediatr. 20, 41–44

89 Hapfelmeier, S. and Hardt, W.D. (2005) A mouse model for S.Typhimurium-induced enterocolitis. Trends Microbiol. 13, 497–503

90 Hapfelmeier, S. et al. (2005) The Salmonella pathogenicity island(SPI)-2 and SPI-1 type III secretion systems allow Salmonellaserovar Typhimurium to trigger colitis via MyD88-dependent andMyD88-independent mechanisms. J. Immunol. 174, 1675–1685

91 Baba, T. and Schneewind, O. (1998) Instruments of microbialwarfare: bacteriocin synthesis, toxicity and immunity. TrendsMicrobiol. 6, 66–71

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92 Porter, E.M. et al. (2002) The multifaceted Paneth cell. Cell Mol. LifeSci 59, 156–170

93 Ouellette, A.J. (2006) Paneth cell alpha-defensin synthesis andfunction. Curr. Top. Microbiol. Immunol. 306, 1–25

94 Salzman, N.H. et al. (2003) Protection against enteric salmonellosis intransgenic mice expressing a human intestinal defensin. Nature 422,522–526

95 Wilson, C.L. et al. (1999) Regulation of intestinal alpha-defensinactivation by the metalloproteinase matrilysin in innate hostdefense. Science 286, 113–117

96 Cash, H.L. et al. (2006) Symbiotic bacteria direct expression of anintestinal bactericidal lectin. Science 313, 1126–1130

97 Cummings, L.A. et al. (2006) In vivo, fliC expression by Salmonellaenterica serovar Typhimurium is heterogeneous, regulated by ClpX,and anatomically restricted. Mol. Microbiol. 61, 795–809

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