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Outer membrane protein A (OmpA) is a multifaceted predominant outer membrane protein of  Escherichia coli  and other  Enterobacteriaceae  whose role in the pathogenesis of various bacterial infections has recently been recognized. Here, the role of OmpA on the virulence of  Shigella flexneri  has been investigated. An  ompA  mutant of wild-type  S. flexneri  5a strain M90T was constructed (strain HND92) and it was shown to be severely impaired in cell-to-cell spreading since it failed to plaque on HeLa cell monolayers. The lack of OmpA significantly reduced the levels of IcsA while the levels of cell associated and released IcsP-cleaved 95 kDa amino-terminal portion of the mature protein were similar. Nevertheless, the  ompA  mutant displayed IcsA exposed across the entire bacterial surface. Surprisingly, the  ompA  mutant produced proper F-actin comet tails, indicating that the aberrant IcsA exposition at bacterial lateral surface did not affect proper activation of actin-nucleating proteins, suggesting that the absence of OmpA likely unmasks mature or cell associated IcsA at bacterial lateral surface. Moreover, the  ompA  mutant was able to invade and to multiply within HeLa cell monolayers, although internalized bacteria were found to be entrapped within the host cell cytoplasm. We found that the  ompA  mutant produced significantly less protrusions than the wild-type strain, indicating that this defect could be responsible of its inability to plaque. Although we could not definitely rule out that the  ompA  mutation might exert pleiotropic effects on other  S. flexneri  genes, complementation of the  ompA  mutation with a recombinant plasmid carrying the  S. flexneri ompA  gene clearly indicated that a functional OmpA protein is required and sufficient for proper IcsA exposition, plaque and protrusion formation. Moreover, an independent  ompA  mutant was generated. Since we found that both mutants displayed identical virulence profile, these results further supported the findings presented in this study.

Outer Membrane Protein A (OmpA): A New Player in Shigella flexneri Protrusion Formation and Inter-Cellular Spreading