Animal models of Salmonella infections:enteritis versus typhoid fever

Renato L. Santosa, Shuping Zhanga, Renée M. Tsolisa, Robert A. Kingsleyb,L. Garry Adamsa, Andreas J. Bäumlerb*

aDepartment of Veterinary Pathobiology, College of Veterinary Medicine, Texas A&M University, College Station, TX 77843-4467, USAbDepartment of Medical Microbiology and Immunology, College of Medicine, Texas A&M University System Health Science Center,

407 Reynolds Medical Building, College Station, TX 77843-1114, USA

ABSTRACT – The most common disease syndromes caused by Salmonella serotypes in humans,typhoid fever and enteritis, can be modeled using Salmonella enterica serotype Typhimurium infections inmice and calves, respectively. This article reviews murine typhoid and bovine enteritis and discussesstrengths, limitations and distinctive features of these animal models. © 2001 Éditions scientifiques etmédicales Elsevier SAS

murine typhoid / bovine enteritis / Salmonella pathogenesis

1. Introduction

From the standpoint of human disease, Salmonellaserotypes can be divided into three groups that causedistinctive clinical syndromes, typhoid fever, bacteremiaand enteritis. Animal models are frequently used to studythe virulence mechanisms of Salmonella serotypes that areimportant for two of these human disease syndromes,typhoid fever and enteritis. This article will review thestrengths and limitations of using different animal speciesto model infections caused by Salmonella serotypes inman. The symptoms in human patients or volunteersclosely resemble signs of disease observed in primatemodels. However, the high cost and scarcity of theseanimals has prevented the widespread use of this model.Emphasis will therefore be placed on two animal modelsthat are used most successfully for elucidating virulencemechanisms, the mouse and the calf. To give the readerbackground information on how closely the animal modelapproximates the human disease the characteristic fea-tures of typhoid fever, bacteremia and enteritis are sum-marized briefly in the following paragraphs.

Typhoid (enteric) fever is a systemic infection in manthat is caused by Salmonella serotypes which are strictlyadapted to humans or higher primates, including Salmo-nella enterica serotypes Typhi, Paratyphi A, Paratyphi B,and Paratyphi C. The disease is currently rare in the UnitedStates [1] and Europe but endemic in Asia, Africa andSouth America from where it can be imported by foreign

travel. In human volunteers, fever is the first symptom andmanifests after a median incubation period of 5–9 days,depending on the challenge dose [2]. Typhoid feverpatients are often constipated during the early stages ofinfection but about one third develop diarrhea subsequentto the onset of fever [3]. Biopsies taken from the uppersmall intestine as early as three days post experimentalinfection of volunteers with serotype Typhi reveal diffuseenteritis caused predominantly by a mononuclear leuko-cyte infiltrate [4]. Similarly, mucosal thickening of theileum due to a polymorphonuclear-poor, mononuclear-rich infiltrate is characteristic in typhoid fever patients [5].Tissue colonization of serotype Typhi may produce capil-lary thrombosis in Peyer’s patches of the terminal ileumwhich can result in hemorrhage, necrosis (usually observedin the second week of infection), ulceration and intestinalperforation (usually observed in the third week) [6].Enlargement of mesenteric lymph nodes, liver and spleenis accompanied by granulomatous lesions. The low-levelbacteremia detected in most patients is important for thesystemic spread of bacteria but endotoxin appears not toplay a major role in the pathogenesis of typhoid fever [7,8]. While rhesus monkeys are resistant to infection, typhoidfever can be induced by oral infection of chimpanzeeswith serotype Typhi [9]. In this animal model, mucosalthickening of the ileum and enlargement of Peyer’s patchesis due to a diffuse enteritis characterized by infiltration ofpredominantly mononuclear leukocytes [10]. Further-more, a reactive follicular hyperplasia of lymphoid fol-licles of Peyer’s patches is noted [11].

Enteritis is caused by any of more than 2 500 Salmo-nella serotypes, however, S. enterica serotypes Typhimu

*Correspondence and reprints.E-mail address: abaumler@tamu.edu (A.J. Bäumler).

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rium and Enteritidis are encountered most frequently [12].Salmonella enteritis represents the second most commoncause of bacterial food-borne disease of known etiology inthe United States [1]. The most common vehicles of trans-mission are meat, meat products, dairy products, eggs oregg products containing Salmonella serotypes eitherbecause animals are infected or because fecal contamina-tion occurs during processing [13]. The infection is local-ized to the ileum, colon and mesenteric lymph nodes andcommonly manifests within 12–72 h after ingestion ofcontaminated food with diarrhea, vomiting and abdomi-nal pain. Rectal biopsies reveal an acute enteritis charac-terized by mucosal edema and acute inflammation withpolymorphonuclear leukocytes [14, 15]. Bacteremia isuncommon and transient in this syndrome [16]. Enteritiscan be modeled by oral infection of rhesus monkeys withserotype Typhimurium [17, 18]. In this model, lesions dueto infiltration of the mucosa with polymorphonuclear leu-kocytes are first observed in the colon and subsequentlyinvolve the ileum. Blood cultures of rhesus monkeys arenegative and no significant lesions are observed in liver,spleen or bone marrow.

Bacteremia (septicemia) is the least common clinicalsyndrome in man. It is caused by the porcine-adaptedS. enterica serotype Choleraesuis and the bovine-adaptedS. enterica serotype Dublin which may enter the foodchain through undercooked pork products or unpasteur-ized milk, respectively [19–21]. The syndrome clinicallydiffers from enteritis in that bacteria are frequently isolatedfrom blood while diarrhea is observed in only about onethird of the patients infected with serotypes Dublin orCholeraesuis [19, 20, 22]. Bacteremia is often accompa-nied by a high spiking fever that distinguishes the syn-drome from typhoid fever in which a more continuousfever is observed [19]. Furthermore, bacteremia caused byserotypes Dublin or Choleraesuis often occurs withoutlocal manifestations (enteric pathology is encounteredrarely), a clinical feature that distinguishes this syndromefrom both, typhoid fever and enteritis [19]. Finally, amongpatients recovering from infection with serotype Choler-aesuis, healthy carriers are encountered less frequentlythan typically described for serotypes causing enteritis ortyphoid fever. Thus a rapid clearance of the organism fromintestinal sites distinguishes the bacteremic syndrome fromboth enteritis and typhoid fever [19].

2. The mouse model of typhoid fever

In 1892 Loeffler described the causative agent of murinetyphoid, an epidemic typhoid fever-like disease in mice.The lesions in internal organs of mice closely resembledthose observed in typhoid fever victims. Since the mouseisolate displayed similar growth characteristics as bacteriaisolated from typhoid fever patients (then known as Bacil-lus typhi) it was termed B. typhimurium (now S. entericaserotype Typhimurium) [23]. Initial studies revealed thatthe distribution of bacteria in tissue of mice infected withserotype Typhimurium [24, 25] is similar to that in typhoidfever patients [26]. These early reports therefore estab-lished murine typhoid as an animal model for the study of

typhoid fever. Serotype Typhimurium is a natural pathogenfor rodents as shown by its frequent association withdisease in this animal reservoir [27]. The interaction ofserotype Typhimurium with one of its natural hosts isconsidered by many to be a better model for studyingtyphoid fever than more recently developed artificial sys-tems (e.g., serotype Typhi infection of mice treated withiron) [28].

Today, susceptible mouse lineages (e.g., Balb/c) arewidely used to study the pathogenesis of serotype Typh-imurium infections. These animals show signs of disease(i.e. elevated temperature as indicated by ruffled fur)between 4–8 days post oral infection, however, diarrheadoes not develop. Gross pathology of the intestine com-monly reveals enlarged Peyer’s patches and a thickeningof the ileal mucosa. A diffuse enteritis is present in thesmall intestine characterized by a predominantly mono-nuclear leukocyte infiltrate as well as edematous villiwhich become shortened in height (figure 1) [29]. Follicu-lar hyperplasia of the lymphoid tissue, capillary thrombo-sis, hemorrhage, and ulcerations may be present in theterminal ileum at areas of Peyer’s patches. The intestinalepithelium in other areas of the intestine remains largelyintact (figure 1). The intestinal pathology and inflamma-tory reaction in mice is hence more similar to that oftyphoid fever patients [4–6] than it is to that caused byserotype Typhimurium in human intestines [14, 15].

Furthermore, unlike the localized infection serotypeTyphimurium causes in man, susceptible mice develop asystemic disease characterized by rapid bacterial multipli-cation in the liver and spleen at a net growth rate of0.5–1.5 log/day [30, 31] which results in hepatomegalyand splenomegaly. Growth of bacteria in the mesentericlymph node, the liver and the spleen triggers the formationof acute abscesses containing predominantly polymor-phonuclear leukocytes. These microscopic lesions becomeenlarged and are gradually transformed into granulomatawith central necrosis and peripheral mononuclear leuko-cytes [32]. In these lesions, serotype Typhimurium residesintracellularly inside macrophages [33]. The granulomataobserved in mice are similar to those present in tissues ofchimpanzees infected with serotype Typhi [11]. Death ofanimals likely results from the lesions in hepatic tissuestriggered by pro-inflammatory cytokine and/or induciblenitric oxide synthase responses elicited by lipid A [34].

One obvious limitation of the mouse model of typhoidfever is that serotype Typhimurium causes enteritis ratherthan typhoid fever in humans. Thus, humans and miceexhibit strikingly different host responses to serotype Typh-imurium infections. Furthermore, serotype Typhi geneswhich are required for causing typhoid fever but which areabsent from or functionally altered in serotype Typhimu-rium cannot be studied in the mouse model. In addition,not all information obtained using the mouse model canbe directly applied to improving our understanding oftyphoid fever since some virulence factors of serotypeTyphimurium are absent from the serovar Typhi genome.For instance, the Salmonella plasmid virulence (spv)operon is required for full mouse virulence of serotypeTyphimurium [35] but is not present in serotype Typhi[36]. Finally, some genes that are present in serotype Typhi

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and required for serotype Typhimurium infection in miceare not essential for the pathogenesis of typhoid fever. Forexample, a mutation in galE renders serotype Typhimu-rium avirulent for mice [37, 38] but a serotype Typhi galEmutant is not avirulent for human volunteers [39].

Although it is important to be aware of possible limita-tions when extrapolating results from murine typhoid totyphoid fever, it is clear that the strengths of the mousemodel outweigh its shortcomings. The mouse model hasbeen extremely useful in identifying virulence mecha-nisms of serotype Typhimurium. The type III secretionsystem encoded by Salmonella pathogenicity island (SPI)2 and the spv operon, two major virulence determinants ofserotype Typhimurium, were identified during in vivoscreens performed in the mouse model [40, 41]. Further-more, the mouse model was used to establish the roleduring infection of major virulence factors identified intissue culture assays, such as the type III secretion systemencoded on SPI1 [42] and many others. The use of knock-out mice has revealed new insights into host factorsrequired for innate resistance and acquired immunity toSalmonella infections [43–51]. In addition, the mousemodel has been used successfully for identifying live

attenuated typhoid fever vaccine candidates. That is muta-tions in aroA [52], cya and crp [53], phoP [54] and htrA[55] cause optimal attenuation of serotype Typhimuriumin mice while retaining immunogenicity of the vaccinestrain. The information generated in the mouse model wasused for the design of new live attenuated serotype Typhivaccine strains carrying deletions in aroC and aroD [56],aroC, aroD and htrA [57], aroA, aroC and htrA [58], cyaand crp [59] or phoPQ [60]. These data illustrate theusefulness of the mouse model for studying the pathogen-esis of typhoid fever.

3. The bovine model of human enteritis

Since mice infected with serotype Typhimurium do notdevelop diarrhea, murine typhoid is not suited to study thepathogenesis of enteritis in humans. In contrast, natural orexperimental infection of calves with serotype Typhimu-rium results in an enteric disease with clinical and patho-logical features that parallel the disease in man. Recentstudies have thus focused on the calf as a model forSalmonella-induced enteritis.

Although cattle may be infected with many Salmonellaserotypes [61, 62], the two serotypes most frequentlyassociated with disease are Dublin and Typhimurium[63–65]. In humans these serotypes are associated withdifferent disease syndromes, namely bacteremia andenteritis, respectively [20, 21]. Similarly, the disease causedby serotype Dublin in cattle differs in several aspects fromthat caused by serotype Typhimurium. Unlike serotypeTyphimurium, serotype Dublin may cause abortion inpregnant cows and heifers with no other clinical signs ofinfection [66, 67]. Infections with serotype Dublin mayresult in chronic carriage as opposed to only transientcarriage observed in cattle infected with serotype Typh-imurium [68, 69]. Eventually these carriers may haveactivation of the infection associated with clinical signs[63]. Like serotype Typhimurium infections, the mostimportant clinical manifestation of serotype Dublin infec-tion in young calves is diarrhea [70, 71]. However, sero-type Typhimurium induces greater secretory and inflam-matory responses than serotype Dublin in bovine ligatedileal loops [72]. Furthermore, serotype Dublin causes amore invasive infection in calves which can manifest asmeningoencephalitis, polyarthritis, osteomyelitis, andpneumonia that eventually occurs in the absence of adiarrheal disease [67]. The spv operon, which is requiredduring the systemic phase of infection in mice, is essentialfor full virulence of serotype Dublin in calves [73, 74] butplays little role during the localized infection caused byserotype Typhimurium calves [75]. Considering the inva-sive character of serotype Dublin infections in both cattleand humans, the enteric disease caused by serotype Typh-imurium in calves appears to be better suited to for mod-eling Salmonella-induced enteritis in man.

Upon oral infection with serotype Typhimurium calvesdevelop clinical signs within 12–48 h [75], which is simi-lar to the short incubation period observed in rhesusmonkeys [17] or human volunteer studies [76]. Clinicalsigns of disease during experimental infection include

Figure 1. Hematoxylin and eosin stained histological sections ofthe murine ileum (bar = 100 µm). Upper half: uninfected con-trol. Lower half: ileum of Balb/c mouse 5 days post infection witha lethal dose of serotype Typhimurium strain ATCC14028. Notea diffuse enteritis associated with edema which is caused byinfiltration of predominantly mononuclear leukocytes.

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diarrhea, anorexia, fever, dehydration and prostration [75,77, 78]. Usually oral inoculations with 104–107 colony-forming units (CFU) causes transient diarrhea which per-sists for 2–8 days, whereas lethality may be caused atdoses between 108 and 1011 CFU [75, 77–79]. Morbidityand mortality are inversely proportional to age [78] andapproximately 75% of natural serotype Typhimurium infec-tions occur in calves less than 2 months of age, before theanimals are weaned [65]. Similarly, the incidence ofSalmonella-induced enteritis in the US is highest amongchildren aged less than 1 year with 111 cases per 100 000population [80].

Calves infected with serotype Typhimurium tend tohave a more severe dehydration than in other neonataldiarrheal diseases, resulting in an average weight loss of18.7% [81]. The most marked hematological change ininfected calves is a transient leukopenia with neutropeniaat the early stages, reflecting the initial influx of polymor-phonuclear leukocytes into the intestinal mucosa, whileneutrophilia develops at later stages of the infection [78,82]. The levels of plasma sodium and calcium aredecreased possibly as a result of the electrolyte loss duringserotype Typhimurium-induced diarrhea [83]. Calves withSalmonella-induced diarrhea frequently develop meta-bolic acidosis with blood pH measurements of 7.0 or less[67]. Fluid and electrolyte imbalances associated withacidosis have also been reported for Salmonella-inducedenteritis in man [84].

The most severe pathological changes in calves infectedwith serotype Typhimurium are restricted to the intestine[75, 77]. Grossly, the mesenteric lymph nodes are oftenenlarged and the caudal portion of the small intestine(caudal jejunum and ileum) is congested and distendedwith fluid. A fibrino-purulent enteritis is commonly seenalthough the aspect of the exudate may vary from mucoidto fibrino-necrotic. A distinct fibrino-necrotic pseudomem-brane is sometimes attached to the surface of Peyer’spatches and, in severe cases, may cover the mucosa of theterminal 5 m of the ileum and the cranial 1–2 m of thecolon (figure 2) [75, 77]. Microscopically, marked lym-phoid depletion is evident in the lymphoid nodules at thePeyer’s patches [75]. However, lymphoid depletion is notrestricted to the intestinal wall and may be observed inother lymphoid organs such as mesenteric lymph nodes,thymus and spleen [85]. Furthermore, experimentallyinfected animals develop a fibrino-purulent necrotizingenteritis characterized by a severe diffuse infiltration ofpolymorphonuclear leukocytes associated with necrosisof the upper mucosa [75, 77]. Similar lesions occur innaturally infected calves [86]. The intestinal pathologyand the pattern of inflammatory reaction (influx of pre-dominantly polymorphonuclear leukocytes) observed incalves is thus strikingly similar to that of serotypeTyphimurium-induced enteritis in non-human primates[17, 18] and in humans [14, 15]. In sharp contrast, miceinfected with serotype Typhimurium develop a diffuseenteritis in the small intestine characterized by localizederosion of intestinal epithelium and a predominantlymononuclear leukocyte infiltrate that is not associatedwith diarrhea (figure 1) [29]. Therefore, polymorpho-

nuclear leukocytes are proposed to play a decisive role inthe pathogenesis of serotype Typhimurium-induced diar-rhea.

The ileal ligated loop model, adapted from other spe-cies to calves [87] can be used for studying the earlypathologic changes occurring during infection [88] andfor characterizing virulence factors [89]. A drawback ofthe ligated intestinal loop model is that it is only suited forstudying early steps during infection. As a result, dataobtained using ileal loop experiments may not be predic-tive of the degree of attenuation observed during an oralinfection. For instance, a sopB (sigD) mutant of serotypeTyphimurium causes significantly less fluid accumulationand polymorphonuclear leukocyte influx in bovine ilealloops, suggestive of a role in diarrheal disease (Santos, R.L.et al., Infect. Immun. 69 (2001) 4610–4617). However, aserotype Typhimurium sopB mutant causes diarrhea, intes-tinal lesions and lethal morbidity in calves infected orally[75]. These data suggest that the onset of fluid accumula-tion and polymorphonuclear leukocyte influx is merelydelayed in a sopB mutant. Despite this shortcoming, thebovine ligated ileal loop model clearly is a valuable tool tostudy early steps of enteric disease in cattle.

Ultrastructural studies of infected ileal ligated loopsdemonstrate that serotype Typhimurium induces ruffling of

Figure 2. Gross pathology of the bovine ileum. Upper half:uninfected control. Lower half: 12 h post infection with a lethaldose of serotype Typhimurium strain ATCC14028. Note thesevere acute fibrinopurulent necrotizing enteritis withpseudomembrane formation.

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the plasma membrane at the apical side of intestinalepithelial cells [88], invading either M cells or absorptiveenterocytes (figure 3) [90]. In ligated ileal loops, bacteriacan be detected inside epithelial cells as early as 20 minpost infection, and in macrophages in the lamina propriacontaining bacteria were detected at 60 min after infec-tion [88]. A blunting of villi is observed soon after infectionof loops with serotype Typhimurium (figure 4). Althoughthe mechanism of Salmonella-induced diarrhea is still notclear, some previous reports indicate that it is distinct froma secretory diarrhea such as that caused by cholera toxin[18, 84, 91]. As discussed in the following section, viru-lence factors essential for serotype Typhimurium to causedisease in calves are implicated in inducing influx ofpolymorphonuclear leukocytes. Infiltration with polymor-phonuclear leukocytes is evident within 1 h after infectionin ligated ileal loops and this inflammatory response pre-

cedes fluid secretion (Santos, R.L. et al., Infect. Immun. 69(2001) 4610–4617). Within 12 h post infection the intes-tinal epithelium is largely destroyed and a fibrinopurulentexudate forms in the intestinal lumen (figure 5). Sincepolymorphonuclear leukocyte influx and damage to theintestinal epithelium precede fluid accumulation in intes-tinal loops it can be speculated that serotype Typhimuriumcauses a predominantly exudative diarrhea.

In summary, the calf is an excellent model for studyingSalmonella-induced enteritis since serotype Typhimuriumis a natural pathogen of cattle in which it causes similarsigns of disease and pathology as observed in humans.

4. The relative importance of serotypeTyphimurium virulence factors duringinfection of mice and calves

As outlined above, serotype Typhimurium is used bothin the mouse model of typhoid fever and the bovine model

Figure 3. Transmission electron micrographs of serotype Typh-imurium strain ATCC14028 invading epithelial cells in bovineligated ileal loops (bar = 2 µm). Upper half: bacterial invasion ofenterocytes associated with ruffling of the membrane at the apicalsurface. Lower half: bacterial invasion of M cells.

Figure 4. Scanning electron micrographs of bovine ligated ilealloops (bar = 50 µm). Upper half: uninfected control. Lower half:3 h post infection with serotype Typhimurium strainATCC14028. Note the marked blunting of intestinal villi.

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of human enteritis. The analysis of serotype Typhimuriummutant phenotypes in mice and calves reveals that majorvirulence determinants do not contribute equally to murinetyphoid and bovine enteritis.

A number of virulence determinants required for growthat systemic sites of infection in mice are less importantduring the localized infection caused by serotype Typh-imurium in calves. For instance, LPS core biosynthesisgenes, the spv operon and SPI2 are required for growth ofserotype Typhimurium at systemic sites of infection inmice [92–94]. A serotype Typhimurium galE mutant isavirulent in the murine typhoid model [37, 38] but causesmortality when administered orally to calves at a dose of1010 CFU/animal [95]. The spv operon is required for fullmouse virulence [35] but serotype Typhimurium spvmutants cause severe intestinal lesions and mortality atwild-type level in calves infected orally at a dose of1010 CFU/animal [75]. A mutation in SPI2 renders sero-type Typhimurium avirulent (> 10 000-fold attenuated) formice [41] because bacteria have a reduced ability to

replicate in macrophages [96]. In calves, on the otherhand, inactivation of SPI2 results in a reduced severity ofintestinal lesions but only in a modest attenuation (< 15-fold) of serotype typhimurium during an oral infection[75].

While mutations in galE, spv and SPI2 lead to a greaterattenuation of serotype Typhimurium in the mouse than inthe calf model, the opposite is true for mutations in genesrequired during the intestinal phase of infection. Forexample, the invasion associated type III secretion systemencoded by SPI1 is required for serotype Typhimuriumcolonization of murine Peyer’s patches [42] and the bovinesmall intestine [83, 97]. Mutations that prevent type IIIsecretion (prgH) or translocation (sipB, sipC, sipD) ofeffector proteins into the host cell cytosol result in aviru-lence and the absence of diarrhea in the calf model [75,83]. A mutation in hilA, encoding a positive regulator ofSPI1 [98], renders serotype Typhimurium avirulent duringoral infection of calves [75] and results in strongly reducedfluid accumulation and inflammation in bovine ligatedileal loops [99]. However, a mutation in hilA has little orno effect on mouse virulence of this pathogen [99]. Thesedata demonstrate that attenuation in the mouse modelcaused by mutations in serotype Typhimurium virulencegenes are not always predictive of the degree of attenua-tion observed in the calf.

5. Conclusions

Virulence is a complex phenotype which fully mani-fests only during host pathogen interactions in vivo. Study-ing a natural infection in an animal which develops signsof disease similar to those observed in humans is thusessential for a complete understanding of Salmonellapathogenesis. For instance, in vitro experiments usingepithelial cell lines have shown that the SPI1 secretedeffector protein SipA (SspA) binds and stabilizes actinfilaments, modulates the actin-bundling activity of T-plastinand contributes to transepithelial migration of polymor-phonuclear leukocytes [100–102]. A mutation in sipA(sspA) of serotype Typhimurium results neither in attenua-tion in the mouse typhoid model nor in an obvious inva-sion defect in tissue culture cells, although entry is delayedat early time points [102–104]. Furthermore, this mutationcauses neither reduced invasion of the bovine intestinalmucosa nor does it reduce the severity of diarrhea orintestinal lesions during an oral infection of calves [83].However, a sipA mutant is unable to cause mortality incalves when administered orally at a dose of 1010 CFU/animal, a phenotype that is not predicted by experimentalinfection of mice [83]. Furthermore, there is currently noobvious connection between the role of SipA in modulat-ing processes in epithelial cell lines in vitro and its involve-ment in causing lethal morbidity in calves. This exampleillustrates the importance of using an animal model, whichresembles the natural course of infection and the typicalsigns of disease to elucidate mechanisms important for thepathogenesis of enteritis. Thus, an appropriate animalmodel is still an invaluable tool for the study of humandisease syndromes caused by Salmonella serotypes.

Figure 5. Hematoxylin and eosin stained histological sections ofbovine ligated ileal loops (bar = 100 µm). Upper half: uninfectedcontrol. Lower half: 12 h post infection with serotype Typhimu-rium strain ATCC14028. Note diffuse infiltration with predomi-nantly polymorphonuclear leukocytes, destruction of intestinalepithelium (with the exception of epithelial cells in the crypts),and fibrinopurulent exudate in the intestinal lumen.

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Acknowledgments

This material is based in part upon work supported bythe Texas Advanced Research (Technology) Program undergrant number 000089-0051-1999. Work in A. Bäumler’slaboratory is supported by Public Health Service grants#AI40124 and #AI44170.

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