Two Roles, One Pheromone: A Listeria Peptide that Contributes to Both Vacuole Escape and Viability

by Diandra M. Vaval Taylor

Abstract

The bacterium, Listeria monocytogenes (Lm), transitions from an environmental bacterium to a facultative intracellular pathogen following entry into mammalian host cells, where infections can lead to serious invasive disease and even death. It has been previously shown that pPplA, a small peptide pheromone secreted by Lm, contributes to bacterial virulence by enhancing the escape of Lm from the vacuoles of non-professional phagocytic cells. Peptide pheromones are generally associated with the coordination of multicellular activities such as biofilm formation, however pPplA is required within the cell vacuole where Lm exists without bacterial neighbors. Evidence suggests that the secretion and re-uptake of the pPplA pheromone within the vacuole signals a confined space to Lm and leads to specific changes in patterns of bacterial gene expression that promote vacuole membrane disruption. The receptor for pPplA binding has not yet been identified, however during the process of constructing genetic mutations to facilitate receptor identification, it became apparent that the complete loss of the pheromone negatively impacted bacterial viability. Transduction of a complete deletion of pplA into a wild type Lm strain resulted in low recovery of transductant colonies that only appeared after several days of incubation. Whole genome sequencing of independent transductants revealed mutations in genes encoding enzymes associated with RNA metabolism; subsequent RNA transcript analysis has identified several pheromone-regulated gene products that could potentially negatively impact Lm fitness. Overall these results strongly suggest that second site suppressor mutations are required to restore full viability to Lm mutants lacking the pPplA pheromone. The finding that a secreted pheromone is required for bacterial viability in broth culture is surprising and novel, and suggests that there are thus far unidentified critical roles for these peptide signals in processes central to bacterial survival.