Pneumococcal S protein coordinates cell wall modification and repair to resist host antimicrobials.

S protein is conserved among streptococci and contributes to group A Streptococcus virulence, but the mechanisms involved are unclear. Here we used genetic, biochemical, single-molecule, in vitro and in vivo analyses to show that S protein is crucial for resistance against host-derived antimicrobials by coordinating cell wall modification and repair. We observed that S protein was localized to the streptococcal septum dependent on its transmembrane domain, while S protein function was dependent on its peptidoglycan (PG)-binding LysM domain. Direct interactions between the pneumococcal S protein and the PG synthase PBP1a as well as the PG deacetylase PgdA were detected. Loss of S protein reduced the proportion of circumferentially moving PBP1a molecules, altered streptococcal morphology and increased susceptibility to cell-wall-targeting antibiotics, suggesting that S protein activates PBP1a. Streptococcus pneumoniae ess mutants lacking the gene encoding S protein were more susceptible to human antimicrobial peptide LL-37 and lysozyme, while their virulence was decreased compared with wild-type bacteria in zebrafish and mice. These data suggest that S protein activates the PG repair and modification complex, providing defence against host-derived and environmental antimicrobials.