The Listeria monocytogenes virulence factor InlJ is specifically expressed in vivo and behaves as an adhesin.

The food-borne pathogen Listeria monocytogenes is adapted to a diversity of environments, such as soil, food, body fluids, and the cytosol of eukaryotic cells. The transition between saprophytic and pathogenic life is mediated through complex regulatory pathways that modulate the expression of virulence factors. Here we examined the expression of inlJ, a recently identified gene encoding a protein of the LPXTG-internalin family and involved in pathogenesis. We show that inlJ expression is controlled neither by the major listerial regulator of virulence genes, PrfA, nor by AxyR, a putative AraC regulator encoded by a gene adjacent to inlJ and divergently transcribed. The InlJ protein is not produced by bacteria grown in vitro in brain heart infusion medium or replicating in the cytosol of tissue-cultured cells. In contrast, it is efficiently produced and localized at the surface of bacteria present in the liver and blood of infected animals. Strikingly, the expression of inlJ by a heterologous promoter in L. monocytogenes or L. innocua promotes bacterial adherence to human cells in vitro. Taken together, these results strongly suggest that InlJ acts as a novel L. monocytogenes sortase-anchored adhesin specifically expressed during infection in vivo.