Listeriolysin S: A bacteriocin from Listeria monocytogenes that induces membrane permeabilization in a contact-dependent manner

Abstract

Significance Listeria monocytogenes (Lm) is a bacterial pathogen that causes listeriosis, a foodborne disease characterized by gastroenteritis, meningitis, bacteremia, and abortions in pregnant women. The most severe human listeriosis outbreaks are associated with a subset of Lm hypervirulent clones that encode the bacteriocin Listeriolysin S (LLS), which modifies the gut microbiota and allows efficient Lm gut colonization and invasion of deeper organs. Our present work identifies the killing mechanism displayed by LLS to outcompete gut commensal bacteria, demonstrating that it induces membrane permeabilization and membrane depolarization of target bacteria. Moreover, we show that LLS is a thiazole/oxazole–modified microcin that displays a contact-dependent inhibition mechanism. Listeriolysin S (LLS) is a thiazole/oxazole–modified microcin (TOMM) produced by hypervirulent clones of Listeria monocytogenes. LLS targets specific gram-positive bacteria and modulates the host intestinal microbiota composition. To characterize the mechanism of LLS transfer to target bacteria and its bactericidal function, we first investigated its subcellular distribution in LLS-producer bacteria. Using subcellular fractionation assays, transmission electron microscopy, and single-molecule superresolution microscopy, we identified that LLS remains associated with the bacterial cell membrane and cytoplasm and is not secreted to the bacterial extracellular space. Only living LLS-producer bacteria (and not purified LLS-positive bacterial membranes) display bactericidal activity. Applying transwell coculture systems and microfluidic-coupled microscopy, we determined that LLS requires direct contact between LLS-producer and -target bacteria in order to display bactericidal activity, and thus behaves as a contact-dependent bacteriocin. Contact-dependent exposure to LLS leads to permeabilization/depolarization of the target bacterial cell membrane and adenosine triphosphate (ATP) release. Additionally, we show that lipoteichoic acids (LTAs) can interact with LLS and that LTA decorations influence bacterial susceptibility to LLS. Overall, our results suggest that LLS is a TOMM that displays a contact-dependent inhibition mechanism.