Bacterial Cell Wall Homeostasis PDF Download

The bacterial cell wall comprises a strong, covalently closed network of peptidoglycan (PG) strands. While PG synthesis is generally essential for bacterial survival, the cell wall is also by necessity a dynamic structure and undergoes constant degradation and remodeling by "autolysins," enzymes that break bonds within PG. One class of autolysin, the lytic transglycosylases (LTGs), cleaves the glycosidic linkages within PG strands. Despite LTGs having well-described biochemical properties, LTG redundancy and diversity have stymied understanding of their fundamental physiological roles. LTGs have been mostly assigned various non-essential, or poorly defined, pleiotropic functions and so there has been no clear evidence to explain why this extreme redundancy, usually indicating an essential function, is so widely conserved amongst diverse bacteria. The diarrheal pathogen Vibrio cholerae encodes eight known LTGs and inactivating single LTGs rarely generates a significant mutant phenotype from which to infer physiological importance. Therefore, rather than directly pursuing individual LTGs, we sought to explore the collective function of the entire enzymatic class by interrogating a mutant lacking all known LTGs. In doing so, we found that V. cholerae must retain at least one active LTG for survival and subsequently characterized the first truly essential role fulfilled by LTGs : clearance of PG debris from the periplasm which accumulates during normal cell wall expansion and remodeling, or during cell wall damage. Coincidentally, this addresses a fundamental question about how bacteria maintain the integrity of a dynamic cell wall through temporal separation of this LTG-mediated autolysis from synthesis, likely independent of previously hypothesized protein-protein interactions. By systematically re-introducing LTGs back into LTG-deficient mutants, we have also created a platform for empirically organizing diverse LTGs into functional families where previously they could only be categorized by their biochemistry. For example, one functional group includes LTGs that are specifically required for clearance of PG debris during septation and daughter cell separation. Another group likely contributes to the elusive, and now confirmed essential, function of releasing newly synthesized PG from the inner membrane. This platform is far from exhaustion and will continue to yield critical information about lytic transglycosylases and their relationship with cell wall homeostasis.