RESULTS: A PCR of the gtrIC cluster showed that serotype 1c isolates from different geographical origins were genetically conserved. An analysis of sequences flanking the gtrIC cluster revealed remnants of a prophage genome, in particular integrase and tRNA(Pro) genes. Meanwhile, Southern blot analyses on serotype 1c, 1a and 1b strains indicated that all the tested serotype 1c strains may have had a common origin that has since remained distinct from the closely related 1a and 1b serotypes. The identification of prophage genes upstream of the gtrIC cluster is consistent with the notion of bacteriophage-mediated integration of the gtrIC cluster into a pre-existing serotype.
CONCLUSIONS: This is the first study to show that serotype 1c isolates from different geographical origins share an identical pattern of genetic arrangement, suggesting that serotype 1c strains may have originated from a single parental strain. Analysis of the sequence around the gtrIC cluster revealed a new site for the integration of the serotype converting phages of S. flexneri. Understanding the origin of new pathogenic serotypes and the molecular basis of serotype conversion in S. flexneri would provide information for developing cross-reactive Shigella vaccines.
DATA DESCRIPTION: We tested the effects of SdsR and SdsRv2 on fluoroquinolone resistance in S. sonnei in vivo. SdsRv2 is a synthetic version which promotes higher binding stability to tolC mRNA. Overexpression of either SdsR or SdsRv2 lowers the expression of tolC mRNA. Interestingly, SdsR and SdsRv2 promote the growth of S. sonnei in the presence of a sub-inhibitory concentration of norfloxacin. Mutant carrying SdsRv2 showed the highest growth advantage. This phenotype is opposite to the effect of SdsR reported in E. coli. This study is an example that demonstrates the difference in the phenotypic effect of a highly conserved sRNA in two closely related bacteria.