Deception point: Peptidoglycan modification as a means of immune evasion

The ability to evade host immune surveillance is a critical virulence determinant for any pathogenic microorganism. The intracellular pathogen Listeria monocytogenes is highly adept at reaching privileged niches within the host without alerting immune responses. Upon infection, Listeria penetrates epithelial cells in the gastrointestinal tract and spreads silently from cell to cell. Once this barrier is breached, it has the ability to escape the phagosome and replicate within phagocytic cells, using them as vehicles to disseminate throughout the host (reviewed in ref. 1). The capacity to defend against the onslaught of immune defenses is another key aspect to being a successful pathogen. In this issue of PNAS, Boneca et al. (2) demonstrate that Listeria modifies peptidoglycan (PG) in its cell wall, thereby conferring resistance to the host bacteriolytic product, lysozyme. By preventing bacterial degradation and subsequent release of immunostimulants, PG modification is therefore also a means by which Listeria evades immune detection by host pattern–recognition receptors (PRRs). Importantly, although many pathogens are known to modify their PG, this report by Boneca et al. (2) is the first to demonstrate that PG N-deacetylation has an important role in virulence and evasion of host defense.