Enterobacter sakazakii invasion in human intestinal Caco-2 cells requires the host cell cytoskeleton and is enhanced by disruption of tight junction

Enterobacter sakazakii is an opportunistic pathogen that causes systemic bacteremia and meningitis with high mortality, and powdered infant formula is a frequent source of this bacterium. However, the mechanisms that this organism uses to invade and translocate through the intestinal barrier are unknown. Using Caco-2 epithelial cells, we were able to demonstrate penetration of E. sakazakii and to determine invasion-associated properties. We found that E. sakazakii entry and invasion were dependent on the exposure time and multiplicity of infection and required bacterial de novo protein synthesis but was independent of cell polarity in the presence of tight junctions. Moreover, the presence of actin filaments and microtubule structures was required, and disruption of the tight junction significantly enhanced the initial association with Caco-2 cells and the efficiency of invasion, which provides a possible explanation for the preferential occurrence of this infection in babies and neonates. This is the first description of E. sakazakii invasion of host intestinal cells, and our findings suggest that this emerging pathogen employs a novel invasion mechanism for development of systemic infection.