Listeria monocytogenes ActA: a new function for a 'classic' virulence factor.

Listeria monocytogenes (Lm) is ubiquitous and widespread in the environment. It is responsible for one of the most severe human foodborne infection. Lm is a facultative intracellular bacterium that can cross the intestinal barrier, disseminate via the bloodstream and reach the liver, spleen, central nervous system and fetus. The bacterial surface protein ActA is one of the most critical and best characterized virulence factors of Lm. It fulfills many essential functions within host cells, allowing Lm escape from autophagy and recruiting an actin polymerization complex that promotes Lm actin-based motility, cell-to-cell spread and dissemination within host tissues. We have recently shown that ActA also acts extracellularly. It mediates Lm aggregation and biofilm formation in vitro and in vivo, and long-term colonization of the gut lumen. This new property of ActA favors Lm transmission and may participate in the selective pressure on Lm to maintain ActA.