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International Journal of Medical Microbiology 300 (2010) 148–154

Contents lists available at ScienceDirect

International Journal of Medical Microbiology

1438-42

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.de/ijmm

Mini Review

The wall teichoic acid and lipoteichoic acid polymers of Staphylococcus aureus

Guoqing Xia, Thomas Kohler, Andreas Peschel n

Division of Cellular and Molecular Microbiology, Institute of Medical Microbiology and Hygiene, University of Tubingen, Elfriede-Aulhorn-Straße 6, D-72076 Tubingen, Germany

a r t i c l e i n f o

Keywords:

Wall teichoic acid

Lipoteichoic acid

Glycopolymers

Cell wall

Microbe–host interaction

Gram-positive bacteria

Staphylococci

21/$ - see front matter & 2009 Elsevier GmbH

016/j.ijmm.2009.10.001

esponding author. Tel.: +49 7071 298 1515; f

ail address: andreas.peschel@uni-tuebingen.d

a b s t r a c t

Staphylococci and most other Gram-positive bacteria incorporate complex teichoic acid (TA) polymers

into their cell envelopes. Several crucial roles in Staphylococcus aureus fitness and cell wall maintenance

have been assigned to these polymers, which are either covalently linked to peptidoglycan (wall teichoic

acid, WTA) or to the cytoplasmic membrane (lipoteichoic acid, LTA). However, the exact TA structures,

functions, and biosynthetic pathways are only superficially understood. Recently, most of the enzymes

mediating TA biosynthesis have been identified and mutants lacking or with defined changes in WTA or

LTA have become available. Their characterization has revealed crucial roles of TAs in protection against

harmful molecules and environmental stresses; in control of enzymes directing cell division or

morphogenesis and of cation homeostasis; and in interaction with host or bacteriophage receptors and

biomaterials. Accordingly, several in vivo studies have demonstrated the importance of WTA and LTA in

S. aureus colonization, infection, and immune evasion. TAs and enzymes required for TA biosynthesis

represent attractive candidates for novel vaccines and antibiotics and are targeted by recently

developed antibacterial therapeutics.

& 2009 Elsevier GmbH. All rights reserved.

Introduction

Staphylococcus aureus is extremely successful in colonizing andinfecting human and animal hosts. This capacity depends onadaptation to several, completely different habitats such as (i)human skin involving exposure to dryness, high salt concentra-tion, and antimicrobial fatty acids; (ii) human nares with moistsurfaces and high amounts of antimicrobial peptides; and, (iii)during infection, otherwise sterile host tissues containingphagocytes and the entire arsenal of antimicrobial host defenses(Lowy, 1998; Gotz et al., 2007). Accordingly, S. aureus cells have toprotect themselves effectively against many different harmfulmolecules and environmental stress. This task is particularlychallenging for the cell envelope, which is directly accessible tosmall molecules. Gram-positive bacteria have been shown tomodify their cell envelopes in multiple ways to prevent the accessand damage by antimicrobial molecules. In addition to covalentmodification of peptidoglycan (PG) and phospholipids, theproduction of protective capsule and slime polymers, and thesecretion of proteinaceous evasins (Foster, 2005; Kraus andPeschel, 2008), most Gram-positive bacteria have teichoic acids(TAs) or related glycopolymers that play crucial roles in bacterialsurvival under disadvantageous conditions and in other basiccellular processes (Weidenmaier and Peschel, 2008; Kohler et al.,

. All rights reserved.

ax: +49 7071 293 435.

e (A. Peschel).

2009b). TAs are usually constitutively produced and eitherconnected to PG (wall teichoic acids, WTA) or to the cytoplasmicmembrane (lipoteichoic acids, LTA). S. aureus and most otherGram-positive bacteria produce both TA types (Fig. 1A). Severalrecent studies have shed new light on the functions andbiosynthesis of WTA and LTA, which are summarized anddiscussed in this overview article.

Most TAs exhibit zwitterionic properties because of thepresence of negatively charged phosphate groups and additionalD-alanine residues on the repeating units, which have freepositively charged amino groups (Neuhaus and Baddiley, 2003;Weidenmaier and Peschel, 2008; Kohler et al., 2009b). The knownWTA structures vary widely between bacterial species and ofteneven between clonal groups. S. aureus TAs are composed ofrepetitive polyol phosphate subunits such as ribitol phosphate(Rbo-P) or glycerol phosphate (Gro-P). As shown in Fig. 1B,S. aureus WTA is covalently linked to the 6-OH of N-acetylmuramic acid (MurNAc) via a disaccharide composed of N-acetylglucosamine (GlcNAc)-1-P and N-acetylmannosamine(ManNAc), which is followed by two units of Gro-P (Araki andIto, 1989; Brown et al., 2008). The actual WTA polymer iscomposed of 11–40 Rbo-P repeating units in most S. aureus

strains, e.g. S. aureus Copenhagen (Sanderson et al., 1962) or ofGro-P repeating units, e.g. in S. aureus strain 187 (Endl et al., 1983).Strain NM8m, a potent biofilm producer, appears to produce evenboth, poly-Rbo-P and poly-Gro-P WTA (Vinogradov et al., 2006).Staphylococcus epidermidis and some other staphylococcal speciesproduce simpler WTA structures with Gro-P units forming the

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entire polymer (Sadovskaya et al., 2004; Endl et al., 1983). Verycomplex, hexose phosphate-containing WTAs are found, e.g. inStaphylococcus hyicus or Staphylococcus auricularis (Endl et al.,1983).

Fig. 1. Schematic localization in the cell envelope (A) and structure (B) of S. aureus

wall teichoic acid (WTA) and lipoteichoic acid (LTA). P, phosphate; D-Ala,

D-alanine; GlcNAc, N-acetylglucosamine; ManNAc, N-acetylmannosamine; Mur-

NAc, N-acetylmuramic acid; Glc, Glucose.

Table 1Known and proposed functions of S. aureus WTA and LTA.

Function TA

Protection against cell damage

Resistance to antimicrobial peptides (CAMPs) D-ala of TA

Resistance to cationic antibiotics (e.g. vancomycin) D-ala of TA

Resistance to lysozyme WTA

Resistance to antimicrobial fatty acids WTA

Resistance to heat stress WTA, LTA

Resistance to low osmolarity LTA

Controlling protein machineries in the cell envelope

Cell division site placement LTA

Autolysin activity WTA, LTA

Mediating interaction with receptors and biomaterials

Adherence to epithelial and endothelial cells WTA

Binding to scavenger receptors (e.g. SrA) LTA

Activation of the complement system via MBL and ficolin LTA

Induction of inflammation via TLR2, CD36, CD14, LBP LTA

Serving as phage receptor WTA

Mediating attachment to biomaterials and biofilm formation LTA, WTA

CAMP, cationic antimicrobial peptide; SrA, scavenger receptor A; MBL, mannose-bindin

LTA polymers are attached to the cytoplasmic membrane via aglycolipid anchor, which is a diglucosyl diacylglycerol in S. aureus

and most other staphylococcal species (Wicken and Knox, 1975;Fischer, 1988). The most often found LTA backbone is formed byGro-P repeating units. As a likely consequence of the uniquebiosynthetic pathway, the structures of LTA are usually lessdiverse than those of WTA (Fischer et al., 1994 ). At the 2-hydroxylgroup of the glycerol, S. aureus LTA is substituted with D-alanylester or a-GlcNAc (Fischer, 1988).

Why does S. aureus produce TAs?

The actual functions of TAs and the reasons why most Gram-positive bacteria produce both LTA and WTA at the same timeremain incompletely understood. WTA has been shown to bedispensable for viability of S. aureus and Bacillus subtilis underlaboratory conditions (Weidenmaier et al., 2004; D’Elia et al.,2006) but to play important roles during colonization andinfection in vivo (Weidenmaier et al., 2004, 2005; Dubail et al.,2006; Bizzini et al., 2007; D’Elia et al., 2009). In contrast, LTA isdispensable only at temperatures below 30 1C, and it turned out tobe impossible to delete both WTA and LTA at the same time sincethe two types of TA appear to compensate for one another to someextent (Oku et al., 2009; Schirner et al., 2009). The known TAfunctions can be classified in three major themes: (i) protectionagainst harmful molecules and environmental stresses, (ii) controlof enzyme activities and cation concentrations in the cellenvelope, and (iii) binding to receptors and surfaces (Table 1).

While WTA, LTA or related polymers are known to bind cellwall proteins, S-layers, or mycolic acids in other Gram-positivebacteria, staphylococci do not seem to employ LTA and WTA totether additional protection layers. Nevertheless, WTA and LTA arecrucial for preventing the passage of harmful molecules throughthe PG layers in S. aureus. WTA contributes to lysozyme resistancein concert with MurNAc O-acetylation probably by preventingbinding of the enzyme to the glycan strands of PG (Bera et al.,2007). The highly hydrophilic WTA provides resistance againsthighly hydrophobic antimicrobial fatty acids from human skin byreducing the bacterial affinity for fatty acids and may thus enableS. aureus and other skin-colonizing Gram-positive bacteria tosurvive on human skin. Of note, the level of resistance mediated

References

Peschel et al., 1999; Collins et al., 2002; Koprivnjak et al., 2002;

Weidenmaier et al., 2005

Peschel et al., 2000

Bera et al., 2007

Kohler et al., 2009a

Hoover and Gray, 1977; Vergara-Irigaray et al., 2008; Oku et al., 2009

Oku et al., 2009

Grundling and Schneewind, 2007b; Oku et al., 2009

Bierbaum and Sahl, 1985; Fedtke et al., 2007; Vergara-Irigaray et al., 2008

Weidenmaier et al., 2004, 2005, 2008

Greenberg et al., 1996; Dunne et al., 1994

Polotsky et al., 1996; Lynch et al., 2004; Nahid and Sugii, 2006

Morath et al., 2001, 2002; Hermann et al., 2002; Hoebe et al., 2005;

Draing et al., 2008

Chatterjee, 1969; Park et al., 1974

Gross et al., 2001; Fedtke et al., 2007; Vergara-Irigaray et al., 2008

g lectin; TLR2, toll-like receptor 2; LBP, lipopolysaccharide-binding protein.

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by WTA correlated with the length and hydrophobicity of theantimicrobial fatty acids (Kohler et al., 2009a). The net charge ofWTA and LTA governs the susceptibility of S. aureus and manyother Gram-positive bacteria to cationic antimicrobial peptides(CAMPs) such as defensins, cathelicidins, and kinocidins (Peschelet al., 1999; Collins et al., 2002; Koprivnjak et al., 2002;Weidenmaier et al., 2005) and to cationic antibiotics such asvancomycin (Peschel et al., 2000). In order to reduce the highlynegative net charge in the cell envelope, WTA and LTA aremodified with D-alanine, and mutants lacking this modificationare highly susceptible to CAMPs (Peschel et al., 1999). LTA- orWTA-deficient S. aureus mutants are sensitive to high tempera-tures (Hoover and Gray, 1977; Vergara-Irigaray et al., 2008; Okuet al., 2009), and LTA is important for S. aureus survival under low-osmolarity conditions for currently unclear reasons (Oku et al.,2009). Interestingly, certain antimicrobial host proteins seem tomake use of WTA, since WTA-deficient S. aureus mutants are moreresistant to mammalian group IIA phospholipase A2 and betadefensin 3 than wild-type strains (Koprivnjak et al., 2008).

TAs seem to affect the function of important housekeepingenzyme machineries in Gram-positive cell envelopes. It isassumed that LTA interacts with components of the membrane-bound cell division machinery and contributes to its properpositioning or regulation because depletion of LTA leads toS. aureus cells with distorted shapes and division sites (Grundlingand Schneewind, 2007a; Oku et al., 2009). In accord with thisfinding, the LTA polymerase LtaS has been shown to be mainlylocated in the division sites in both vegetative and sporulatingB. subtilis cells (Schirner et al., 2009). WTA and LTA also play aprofound role in controlling the activity of autolysins (Rice andBayles, 2008), which show high affinities for LTA in vitro(Bierbaum and Sahl, 1985, 1987; Giudicelli and Tomasz, 1984).S. aureus mutants with reduced LTA amount or lacking LTAexhibited strongly reduced autolysis (Fedtke et al., 2007; Okuet al., 2009), and mutants lacking WTA have profoundly increasedautolysis activities compared to wild-type cells (Vergara-Irigarayet al., 2008), indicating a complex regulatory interaction of LTA,WTA, and autolysins. Accordingly, WTA inhibits the side walllocalization of the LysM domain containing autolysin LytF, the DL-endopeptidase activity of which is required for cell separation inB. subtilis (Yamamoto et al., 2008). The capacity of TAs to controlcell envelope enzymes may be related to the ion exchanger-likeproperties of TAs and the capacity to bind magnesium ions withparticularly high affinity (Heptinstall et al., 1970). Accordingly, anLTA-depleted B. subtilis mutant shows a higher requirement forMg2 + and increased susceptibility to toxic Mn2 + (Schirner et al.,2009).

TAs are exposed at the bacterial surface and can interact withhost cells, bacteriophages, or inert surfaces (Table 1). A strongimpact on adherence to biomaterials and biofilm formation hasbeen observed in S. aureus and Enterococcus faecalis mutantsdeficient in TA alanyl esters or with altered TA amounts in in vitroand in vivo models (Gross et al., 2001; Fabretti et al., 2006; Fedtkeet al., 2007; Vergara-Irigaray et al., 2008). Many staphylococcalbacteriophages use WTA as receptors probably by lectin-mediatedbinding to the GlcNAc residues on WTA (Chatterjee, 1969; Parket al., 1974). As a consequence, differences in WTA structure play acrucial role in staphylococcal phage typing (Pantucek et al., 2004).WTA and LTA have also been implicated in S. aureus interactionwith a wide range of host receptors and are thought to shape theentire colonization and infection process, ranging from initialadherence, activation of innate immunity, to the elicitation ofadaptive immune reactions (Weidenmaier and Peschel, 2008).WTA contributes to binding of S. aureus to epithelial andendothelial cells most probably via lectin-like receptors, whichremain to be identified (Weidenmaier et al., 2004, 2005, 2008).

Accordingly, S. aureus mutants lacking WTA have lost the ability tocolonize the nose in animal models or to leave the bloodstreamand infect sub-endothelial tissues in endovascular infections. LTAhas been shown to bind to soluble C-type lectins such as themannose-binding lectin (MBL) and L-ficolin, thereby activatingthe lectin-initiated complement pathway, which leads to bacterialopsonization and release of chemotactic complement splitproducts (Polotsky et al., 1996; Lynch et al., 2004; Nahid andSugii, 2006). A number of studies have indicated that LTA activatesthe innate immune system via Toll-like receptor 2 (TLR2) (Morathet al., 2001, 2002; Hermann et al., 2002; Hoebe et al., 2005;Tapping and Tobias, 2003; von Aulock et al., 2007; Draing et al.,2008). However, most of the commonly used LTA preparations arecontaminated with bacterial lipopeptides, which account for alarge percentage of the TLR2-mediated proinflammatory activity(Hashimoto et al., 2006, 2007; Zahringer et al., 2008), and theproinflammatory potency of LTA remains a matter of vigorousdebate. S. aureus TAs are well-known targets for antibodies(Kumar et al., 2005), and TAs have been developed as vaccines(Schaffer and Lee, 2009). Indeed, promising results have beenobtained through passive vaccination with a humanized mono-clonal antibody targeting staphylococcal LTA (Weisman, 2007;Weisman et al., 2009). Previous studies have also led to theassumption that the zwitterionic S. aureus WTA might beprocessed and presented by antigen-presenting cells and serveas an antigen for specific T cells (Tzianabos et al., 2001). Thus, TAsare most critical in S. aureus–host interaction and representattractive targets for new anti-colonization or anti-infectionstrategies.

How does S. aureus synthesize WTA and LTA?

Despite of the structural similarity, the biosynthesis of WTAand LTA relies on profoundly different pathways and precursormolecules. S. aureus requires at least 12 genes for biosynthesis ofpoly-Rbo-P WTA while only three genes seem to be required forbiosynthesis of poly-Gro-P LTA backbones. Further genes arerequired for subsequent modification with D-alanine and hexoses.In contrast, synthesis of the extremely complex WTA-like polymerof Streptococcus agalactiae representing the Lancefield group Bantigen is supposed to depend on more than 120 genes (Sutcliffeet al., 2008). Again, it remains enigmatic why Gram-positivebacteria invest so much genetic information and energy in themaintenance of different pathways ultimately leading to verysimilar polymers.

The biosynthetic pathways of WTA and related PG-anchoredpolymers share some widely conserved principles such as the useof undecaprenylphosphate (C55-P) as lipid carrier during theassembly process and the genomic organization of most biosyn-thetic genes in clusters (Brown et al., 2008; Xia and Peschel, 2008;Meredith et al., 2008). Biosynthesis of WTA is initiated on C55-P,which is also used for the biosynthesis of PG or capsularpolysaccharide at the inner leaflet of the cytoplasmic membrane.Fig. 2A shows the current model of S. aureus WTA biosynthesis,which can be divided into five steps. The nomenclature of genesand proteins has been subject to some confusion recently. Wetried to omit renaming here and use names that follow thefunction and allocation of genes in gene clusters. Accordingly, thegene acronym ‘tag’ is used for genes involved in conserved stepsthat are shared by both, poly-Gro-P and poly-Rbo-P WTAbiosynthetic pathways while ‘tar’ is used only for those genesthat are additionally required for incorporation of Rbo-P units (Xiaand Peschel, 2008). WTA biosynthesis is initiated by the synthesisof a canonical disaccharide linkage unit, which requires theenzymes TagO and TagA transferring GlcNAc-1-phosphate and

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Fig. 2. Pathways of S. aureus wall teichoic acid (WTA) biosynthesis (A), lipoteichoic acid (LTA) biosynthesis (B), and D-alanine incorporation into LTA and WTA (C).

Reproduced from Kohler et al. (2009b) with permission. CDP-Gro, cytidyldiphosphate-glycerol; CDP-Rbo, cytidyldiphosphate-ribitol; Glc, glucose; GlcNAc, N-

acetylglucosamine; Gro, glycerol; Gro-P, glycerolphosphate; ManNAc, N-acetylmannosamine; MurNAc, N-acetyl muramic acid; Rbo-P, ribitol phosphate; Rib-P, ribulose-

5-phosphate; UDP-Glc, uridine-5�-diphosphate-glucose; UDP-GlcNAc, uridine-5�-diphosphate-N-acetyl-glucosamine; UDP-ManNAc, uridine-5� diphosphate-N-acetyl-

mannosamine.

G. Xia et al. / International Journal of Medical Microbiology 300 (2010) 148–154 151

ManNAc, respectively, from UDP-activated precursor molecules toC55-P (Soldo et al., 2002a; Ginsberg et al., 2006). UDP-ManNAc,which is otherwise not used in the S. aureus primary metabolism,is probably generated from UDP-GlcNAc by MnaA as in B. subtilis

(Soldo et al., 2002b) and Listeria monocytogenes (Dubail et al.,2006). Incorporation of the preformed repeating units into WTA ismediated by a group of proteins, which comprise both, primingand polymerizing enzymes. The poly-Gro-P WTA of B. subtilis 168depends on the primase TagB adding the first Gro-P to the C55-P-carried disaccharide linkage unit (Bhavsar et al., 2005; Ginsberg etal., 2006) and the polymerase TagF, which adds approximately 35Gro-P units (Schertzer and Brown, 2003). The situation is morecomplicated in S. aureus Rbo-P WTA biosynthesis. Here the TagBreaction is followed by addition of only one additional Gro-P unitmediated by the TarF enzyme (Brown et al., 2008). The activatedprecursor molecule CDP-glycerol is generated by TagD (Park et al.,1993; Badurina et al., 2003). Subsequently, the TarL polymerasesynthesizes the poly-Rbo-P (Brown et al., 2008; Meredith et al.,

2008; Pereira et al., 2008; Ishimoto and Strominger, 1966), whileTarIJ are responsible for generating the precursor CDP-ribitol(Pereira and Brown, 2004). No Rbo-P primase seems to beinvolved in S. aureus, whereas such an enzyme (TarK) has beenimplicated in Rbo-P WTA biosynthesis in B. subtilis W23 (Meredithet al., 2008; Pereira et al., 2008). A major challenge in studyingenzyme functions results from the fact that the three genes tarIJL

involved in generation and incorporation of Rbo-P are duplicatedin S. aureus (Qian et al., 2006). Recent studies indicate that the twofunctionally redundant TarL-like enzymes mediate the same typesof reaction albeit leading to WTA of different chain length andelectrophoretic mobility (Meredith et al., 2008; Pereira et al.,2008). Apparently, S. aureus can modulate its WTA structureaccording to bacterial density and environmental changes, sinceone of the tarL genes (also been named tarK) is repressed by theagr quorum-sensing system (Meredith et al., 2008). Eventually,the WTA polymers are translocated to the outer membrane leafletby an ABC transporter formed by TagG and TagH (Lazarevic and

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Karamata, 1995) and transferred from C55-P to the 6-OH group ofMurNAc in the PG by a yet unknown enzyme (Mauck and Glaser,1972; Fiedler et al., 1974; Bhavsar and Brown, 2006).

The Gro-P repeating units of LTA are not derived from anucleotide-activated precursor but from phosphatidylglycerol, amajor constituent of bacterial membranes (Glaser and Lindsay,1974) (Fig. 2B). LTA is polymerized directly on the glycolipidserving as the membrane anchor for LTA instead of C55-P, which isthe second major difference between LTA and WTA biosynthesis(Koch et al., 1984; Fischer, 1988). The glycolipid is generated bythe YpfP enzyme, which adds two glucose residues from UDP-glucose to diacylglycerol (Jorasch et al., 1998, 2000; Kiriukhin etal., 2001). A membrane protein encoded by the ltaA gene isrequired for efficient LTA biosynthesis and is thought to be aflippase that translocates the glycolipid from the inner to theouter leaflet of the cytoplasmic membrane (Grundling andSchneewind, 2007a). Recently, the LTA polymerase LtaS has beenidentified and cloned, which utilizes Gro-P units from phospha-tidylglycerol to synthesize the LTA polymer at the outer surface ofthe cytoplasmic membrane (Grundling and Schneewind, 2007b).Amazingly, deletion of ypfP does not block biosynthesis of LTA butleads to synthesis of LTA polymer that is attached to diacylglycerol(Kiriukhin et al., 2001; Fedtke et al., 2007). For unknown reasons,ypfP mutants can produce either unaltered or strongly reducedamounts of LTA compared to the wild-type strains depending onthe S. aureus strain background (Fedtke et al., 2007).

A very constant trait of most TAs is the modification withD-alanine after biosynthesis of the polymers is completed.D-Alanine can be repeatedly incorporated into a given molecule,since the D-alanine esters are rather labile and get easily lost(Koch et al., 1985). The dltABCD genes responsible for D-alanineactivation and incorporation into WTA and LTA are highlyconserved and always seem to form an operon (Debabov et al.,1996; Neuhaus et al., 1996; Neuhaus and Baddiley, 2003). TA netcharge is strongly affected by this modification (Peschel et al.,1999). The dlt operons of S. aureus and S. epidermidis are controlledby cations (Koprivnjak et al., 2006) and in response to CAMPchallenge via the GraXRS (also named ApsXRS) regulatory system,which is in accord with the crucial role of TA alanylation inbacterial resistance to CAMPs (Li et al., 2007a, b; Bera et al., 2007;Kraus et al., 2008). Among the four dlt genes, the gene dltC

encodes a D-alanyl carrier protein (Dcp) and dltA a ligase (Dcl),which catalyze the formation of D-alanyl-Dcp in the cytoplasm(Fig. 2C). DltB, an integral membrane protein and DltD, amembrane-tethered hydrophilic protein, seem to be required forthe translocation and incorporation of D-alanine into TAs(Debabov et al., 2000; Neuhaus and Baddiley, 2003).

Conclusions and perspectives

The WTA biosynthetic enzymes seem to form a membrane-associated complex in B. subtilis, indicating that the biosynthesisof TAs is a highly organized process (Formstone et al., 2008). It canbe assumed that the TA and PG biosynthetic machineries arecoordinated in sophisticated ways, maybe in cooperation withcytoskeletal proteins and with the cell division apparatus(Schirner et al., 2009). How similar or different the rod-shapedB. subtilis and the spherical S. aureus are in this respect remains tobe analyzed. While we are only beginning to understand the rolesof WTA and LTA it is clear now that many of the biosyntheticenzymes represent attractive targets for new anti-infective agents.Recently, proteins involved in Rbo-P biosynthesis and incorpora-tion have attracted particular interest, since WTA with Rbo-Presidues are found in several human pathogens such as Staphy-

lococcus saprophyticus (Schumacher-Perdreau et al., 1978),

L. monocytogenes (Uchikawa et al., 1986), and Streptococcus

pneumoniae (Fischer et al., 1993; Baur et al., 2009) in addition toS. aureus. Recently described crystal structures of proteins such asTagD (Fong et al., 2006), TarI (Baur et al., 2009), LtaS (Schirneret al., 2009; Lu et al., 2009), DltA (Du et al., 2008; Osman et al.,2009), and the Bacillus anthracis MnaA homolog (Velloso et al.,2008) represent the basis for rational drug design. Accordingly,specific inhibitors of DltA have recently been shown to renderbacteria highly susceptible to CAMPs and cationic antibiotics (Mayet al., 2005) and to be very effective in clearing Gram-positiveinfections in vivo (Escaich et al., 2007). In addition, TAs have beenshown to hold promise as targets for active or passive vaccines(Theilacker et al., 2004; Kumar et al., 2005; Weisman, 2007;Weisman et al., 2009). A broader view on the diversity andvariability of TA structures can be achieved in the near future withimproved glycobiochemical methods. The availability of genomicand metagenomic databases represents a valuable basis forpredicting TA biosynthetic pathways by bioinformatic methods.

Future studies will allow correlating structural features of TAswith certain functions in the physiology of cell envelope orbacteria–host interaction by making use of the increasing numberof defined bacterial mutants with altered or lacking TAs. Thecontroversial proinflammatory capacity of LTA and the potential ofzwitterionic TAs to activate specific T cells upon processing andpresentation by MHC class II molecules are major open questions,which need to be approached. Furthermore, many host receptorsrecognizing and binding TAs remain to be identified. According tocell- and species-specific differences in expression of TA-bindingmolecules, it is tempting to assume that the enormous diversity ofTA structures plays a role in bacterial cell and host tropism, ineluding the host immune system, and in adapting to varyingenvironmental conditions.

Acknowledgments

The authors acknowledge support by grants from the GermanResearch Foundation (TR-SFB34, SFB766, FOR449, GRK685,SPP1130), the German Ministry of Education and Research(SkinStaph), and the IZKF program of the Medical Faculty,University of Tubingen. We apologize to those investigatorswhose work is not cited in this article. Unfortunately, spacerestrictions do not allow us to cite all of the relevant literature werefer to or all investigators who have contributed significantly tothis important and growing field.

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