Identification and characterization of a virulence-associated gene in Shigella dysenteriae
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Shigella species, including Shigella dysenteriae, are causative agents of bacillary dysentery, a disease characterized by severe diarrhea and blood in the stool. Although a number of genes required for virulence have been characterized in S. dysenteriae, others remain to be identified. These additional virulence-associated genes can be identified by screening mutants of S. dysenteriae in a plaque assay. The plaque assay is used to infer virulence by measuring the ability of the bacteria to invade, grow within, and spread between eukaryotic cells, resulting in the formation of plaques, small holes in a monolayer of eukaryotic cells. Mutants that have lost the ability to form normal plaques are necessarily avirulent. A previously described non-directed mutant, SDU380, lost the ability to form plaques. Further characterization revealed that this strain had sustained a 33 kilobase deletion. None of the genes in the deleted region are known virulence genes. The purpose of this study was to identify and characterize a novel virulence gene in this deleted region that resulted in the inability of SDU380 to form plaques. Different mutant strains missing only some of the genes deleted in SDU380 were constructed and tested for virulence in a plaque assay. This analysis revealed that the loss of the gene yciB is responsible for the failure of SDU380 to form plaques. The role of the protein YciB in S. dysenteriae virulence is unknown, though the yciB gene is a known virulence-associated gene in Shigella flexneri, a species with a similar lifestyle to that of S. dysenteriae. In S. flexneri, YciB plays a role in septation, an essential step in cell division. Further analysis showed that loss of YciB in S. dysenteriae also results in an intracellular growth defect. However, this defect does not seem to be due to loss of ability to undergo septation and no growth defect is observed in vitro.