Outsider to insider: resetting the natural host niche of commensal E. coli K-12.

The status of E. coli K-12 as an exclusively non-invasive, non-pathogenic bacterium has almost been incontrovertible. Our recent finding that a mutation in one of its main architectural protein, HU, converts E. coli K-12 to an actively invasive form suggests that gaining host cell entry might be an expedient survival tactic for traditional commensals during certain altered host conditions. The mutant E. coli (SK3842) exhibits properties usually associated with pathogenic bacteria: host cell invasion, phagosomal disruption and intracellular replication. However, unlike the situation with some pathogens, internalized SK3842 imparts anti-apoptotic and cyto-protective effects rather than lethality on the host cell, both in vitro and in vivo. Here, we show that SK3842 also provides colonization resistance against other invasive pathogens--a trait not shared by the parental commensal strain. Thus, the altered lifestyle of SK3842 encompasses characteristics both from traditional pathogens as well as beneficial probiotic strains.