Gram-positive bacteria evade phage predation through endolysin-mediated L-form conversion

Bacteriophages kill bacteria by osmotic lysis towards the end of the lytic cycle. In the case of Gram-positive bacteria, peptidoglycan-degrading endolysins released at the end of infection cycle cause explosive cell lysis not only of the infected host, but can also attack non-infected bystander cells. Here, we show that in osmotically stabilized environments, Listeria monocytogenes can evade phage predation by transient conversion to a cell wall-deficient L-form state. This L-form escape is triggered by endolysins disintegrating the cell wall from without, leading to turgor-driven extrusion of wall-deficient, yet viable L-form cells. Remarkably, in absence of phage predation, we show that L-forms can quickly revert to the walled state. These findings suggest that L-form conversion represents a population-level persistence mechanism to evade complete eradication by phage attack. Importantly, we also demonstrate phage-mediated L-form switching of the urinary tract pathogen Enterococcus faecalis in human urine, which underscores that this escape route may be widespread and has important implications for phage- and endolysin-based therapeutic interventions.