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The glutamate-dependent acid-resistance system is the most effective acid tolerance pathway in  Shigella , allowing survival in extremely acidic environments. However, the regulation of this system in  Shigella  remains elusive. In the current study, we identified significant differences in the levels of glutamate decarboxylase between three  Shigella flexneri  strains with different levels of acid resistance using blue native-polyacrylamide gel electrophoresis (PAGE) and isoelectric focusing (IEF)/sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis. The results showed that the degree of acid resistance and the levels of GadA/B were significantly lower in strain 2457T compared with two other  S. flexneri  strains. It has been reported that plasmid pSf-R27 is expressed in strain 2457T but not in the other 142 sequenced  S. flexneri  isolates. pSf-R27 encodes protein Sfh, which belongs to a family of histone-like nucleoid-structuring (H-NS) proteins that participate in the transcriptional control of glutamate-dependent acid resistance, implicating pSf-R27 in the lower acid resistance of strain 2457T. Transformation of pSf-R27 or  sfh  alone into strain 301 resulted in decreased expression of GadA/B in the recombinant strains. Thus, we confirmed that H-NS family protein Sfh, bound to the  gadA/B  regulatory region and regulates the expression of glutamate decarboxylase at the transcriptional level. We also examined the acid tolerance of the wild-type and recombinant strains using flow cytometry and determined that the acid tolerance of  S. flexneri  is closely related to the expression of GadA/B. These findings further our understanding of the acid tolerance of  S. flexneri , especially via the glutamate-dependent pathway.

An H-NS Family Protein, Sfh, Regulates Acid Resistance by Inhibition of Glutamate Decarboxylase Expression in Shigella flexneri 2457T