Correspondence

The stationary-phase acid-resistance pathways of Shigella flexneri 2457T have not previously been studied. The two acid-resistance systems, the glutamate-dependent acid-resistance (GDAR) and the oxidative pathways, reported elsewhere for Escherichia coli and S. flexneri 3136, were both detected in S. flexneri 2457T. However, S. flexneri 2457T cells grown overnight under fermentative conditions and acid-shocked in minimal media in the absence of glutamate, an acid test often described as a negative control for both pathways, were capable of surviving acid challenge. It is possible that this resistance is due to the oxidative pathway operating in a non-glucose-repressible manner, or to a novel pathway present in S. flexneri 2457T. The construction of gadB and gadC mutants ruled out any contribution by the GDAR pathway, whilst further characterizing the GDAR properties of S. flexneri 2457T. Interestingly, study of the role of rpoS in the oxidative pathway and the unusual acid-resistance phenotype revealed that the frameshift present in the 2457T rpoS gene results in expression of a truncated RpoS protein, which may be reduced in activity and is not essential for the acid-resistance phenotype of S. flexneri 2457T