Phenyllactic acid affects cell wall thickness by targeting the synthesis of peptidoglycan in Listeria monocytogenes

Abstract

Phenyllactic acid (PLA) exhibits antibacterial activity against Listeria monocytogenes which is a significant foodborne pathogen. In this study, effects of PLA on the cell survival, cell morphology, cell wall thickness, extraction of cell wall peptidoglycan and expression levels of genes involved in the synthesis of cell wall peptidoglycan were investigated using L. monocytogenes 10403S, which were either PLA shocked for 30 min (PLA shock) or grown in medium with PLA (PLA growth). Flow cytometry analysis demonstrated that at the same concentration, PLA shock resulted in a higher proportion of dead and injured cells than those from PLA growth. Images of field emission scanning electron microscopy showed that PLA growth had almost no effect on cell morphology, while damages on L. monocytogenes cells were observed after PLA shock. Results from transmission electron microscopy showed that the cell walls became thinner after PLA shock while PLA growth resulted in thicker cell walls. Coincidently, lower extraction yields were obtained after PLA shock while PLA growth led to higher extraction yields compared to the corresponding control group. PLA shock inhibited expression of genes associated with the production of lipid II and polymerization of peptidoglycan, while PLA growth induced expression of genes responsible for the production and translocation of lipid II. Our results suggest that L. monocytogenes responds differently to PLA shock and PLA growth. PLA affects the cell wall thickness of L. monocytogenes through regulating the synthesis of peptidoglycan.