Multiplicity of S almonella entry mechanisms, a new paradigm for S almonella pathogenesis

The S almonella enterica species includes about 2600 diverse serotypes, most of which cause a wide range of food‐ and water‐borne diseases ranging from self‐limiting gastroenteritis to typhoid fever in both humans and animals. Moreover, some serotypes are restricted to a few animal species, whereas other serotypes are able to infect plants as well as cold‐ and warm‐blooded animals. An essential feature of the pathogenicity of S almonella is its capacity to cross a number of barriers requiring invasion of a large variety of phagocytic and nonphagocytic cells. The aim of this review is to describe the different entry pathways used by S almonella serotypes to enter different nonphagocytic cell types. Until recently, it was accepted that S almonella invasion of eukaryotic cells required only the type III secretion system ( T3SS ) encoded by the S almonella pathogenicity island‐1. However, recent evidence shows that S almonella can cause infection in a T3SS ‐1‐independent manner. Currently, two outer membrane proteins Rck and PagN have been clearly identified as S almonella invasins. As Rck mediates a Z ipper‐like entry mechanism, S almonella is therefore the first bacterium shown to be able to induce both Z ipper and T rigger mechanisms to invade host cells. In addition to these known entry pathways, recent data have shown that unknown entry routes could be used according to the serotype, the host and the cell type considered, inducing either Z ipper‐like or T rigger‐like entry processes. The new paradigm presented here should change our classic view of S almonella pathogenicity. It could also modify our understanding of the mechanisms leading to the different S almonella ‐induced diseases and to S almonella‐ host specificity.