Multicopy icsA is able to suppress the virulence defect caused by the wzz(SF) mutation in Shigella flexneri

The lipopolysaccharides (LPS) of Shigella flexneri are important for virulence and their O antigen (Oag) polysaccharide chains affect IcsA (VirG)-mediated actin-based motility (ABM) within mammalian cells. S. flexneri 2a 2457T has smooth LPS whose Oag chains have two modal lengths (short (S)-type and very long (VL)-type), and has IcsA predominantly located at one pole on its cell surface. A S. flexneri 2457T wzz(SF) mutant (RMA696) has VL-type Oag but not S-type Oag chains, less IcsA detectable by immunofluorescence on its cell surface, reduced virulence and defective ABM. Introduction of a plasmid encoding IcsA into S. flexneri wzz(SF) showed that multicopy icsA could suppress the virulence defects (Sereny reaction, HeLa cell monolayer plaquing, and F-actin comet tail formation) caused by the wzz(SF) mutation suggesting that the VL-type Oag chains were masking IcsA and limiting the amount available to initiate ABM.