Shigella flexneri cell-to-cell spread, and growth and inflammation in mice, is limited by the outer membrane protease IcsP

The Shigella flexneri autotransporter protein IcsA is essential for intra- and intercellular spread, and icsA mutants are attenuated in several models. However, the pathogenic significance of the outer membrane protease IcsP, which orchestrates the polar distribution of IcsA on the bacterial surface, remains unclear. To further examine this point, we constructed icsP mutants in the two most commonly studied S. flexneri strains and evaluated their in vitro and in vivo performance relative to wild type. Both icsP mutants showed aberrant surface distribution of IcsA, but the in vitro consequences depended upon the cell line being used to assess bacterial motility and plaque formation. Evaluating the behaviour of the mutants in two mouse models suggested functional expression of icsP might limit bacterial persistence and the associated inflammation in host tissues, consistent with the findings in one of the three cell lines used.