Staphylococcus aureus peptidoglycan (PG) is densely functionalized with anionic polymers called wall teichoic acids (WTAs). These polymers contain three tailoring modifications: d -alanylation, α- O -GlcNAcylation, and β- O -GlcNAcylation. Here we describe the discovery and biochemical characterization of a unique glycosyltransferase, TarS, that attaches β- O -GlcNAc (β- O - N -acetyl- d -glucosamine) residues to S. aureus WTAs. We report that methicillin resistant S. aureus (MRSA) is sensitized to β-lactams upon tarS deletion. Unlike strains completely lacking WTAs, which are also sensitive to β-lactams, Δ tarS strains have no growth or cell division defects. Because neither α- O -GlcNAc nor β- O -Glucose modifications can confer resistance, the resistance phenotype requires a highly specific chemical modification of the WTA backbone, β- O -GlcNAc residues. These data suggest β- O -GlcNAcylated WTAs scaffold factors required for MRSA resistance. The β- O -GlcNAc transferase identified here, TarS, is a unique target for antimicrobials that sensitize MRSA to β-lactams.

Methicillin-resistantStaphylococcus aureus (MRSA) is a frequent cause of difficult-to-treat, often fatal infections in humans 1,2 . Most humans have antibodies against S. aureus, but these are highly variable and often not protective in immunocompromised patients 3 . Previous vaccine development programs have not been successful 4 . A large percentage of human antibodies against S. aureus target wall teichoic acid (WTA), a ribitol-phosphate (RboP) surface polymer modified with N-acetylglucosamine (GlcNAc) 5,6 . It is currently unknown whether the immune evasion capacities of MRSA are due to variation of dominant surface epitopes such as those associated with WTA. Here we show that a considerable proportion of the prominent healthcare-associated and livestock-associated MRSA clones CC5 and CC398, respectively, contain prophages that encode an alternative WTA glycosyltransferase. This enzyme, TarP, transfers GlcNAc to a different hydroxyl group of the WTA RboP than the standard enzyme TarS 7 , with important consequences for immune recognition. TarP-glycosylated WTA elicits 7.5-40-fold lower levels of immunoglobulin G in mice than TarS-modified WTA. Consistent with this, human sera contained only low levels of antibodies against TarP-modified WTA. Notably, mice immunized with TarS-modified WTA were not protected against infection with tarP-expressing MRSA, indicating that TarP is crucial for the capacity of S. aureus to evade host defences. High-resolution structural analyses of TarP bound to WTA components and uridine diphosphate GlcNAc (UDP-GlcNAc) explain the mechanism of altered RboP glycosylation and form a template for targeted inhibition of TarP. Our study reveals an immune evasion strategy of S. aureus based on averting the immunogenicity of its dominant glycoantigen WTA. These results will help with the identification of invariant S. aureus vaccine antigens and may enable the development of TarP inhibitors as a new strategy for rendering MRSA susceptible to human host defences.Novel prevention and treatment strategies against major antibioticresistant pathogens such as MRSA are urgently needed but are not within reach because some of the most critical virulence strategies of these pathogens are not understood 8 . The pathogenic potential of prominent healthcare-associated (HA)-MRSA and recently emerged livestock-associated (LA)-MRSA strains is thought to rely on particularly effective immune evasion strategies, whereas communityassociated (CA)-MRSA strains often produce more aggressive toxins 1,2 . Most humans have high overall levels of antibodies against S. aureus as a consequence of preceding infections, but antibody titres differ strongly for specific antigens and are often not protective in immunocompromised patients, for reasons that are not clear 3 . A large percentage of human antibodies against S. aureus is directed against WTA 5,9,10 , which is largely invariant. However, some S. aureus lineages produce altered WTA, which modulates, for instance, phage susceptibility 7,11 .To investigate whether ...

Most of the dissimilarity between Staphylococcus aureus strains is due to the presence of mobile genetic elements such as bacteriophages or pathogenicity islands. These elements provide the bacteria with additional genes that enable them to establish a new lifestyle that is often accompanied by a shift to increased pathogenicity or a jump to a new host. S. aureus phages may carry genes coding for diverse virulence factors such as Panton-Valentine leukocidin, staphylokinase, enterotoxins, chemotaxis-inhibitory proteins, or exfoliative toxins. Phages also mediate the transfer of pathogenicity islands in a highly coordinated manner and are the primary vehicle for the horizontal transfer of chromosomal and extra-chromosomal genes. Here, we summarise recent advances regarding phage classification, genome organisation and function of S. aureus phages with a particular emphasis on their role in the evolution of the bacterial host.