A total of 4 Shigella flexneri isolates showed positive sereny test indicating the capability of invasiveness. This was also confirmed by testing the adhesion and invasion to HeLa cells in-vitro. At the molecular level, inv gene was amplified and detected in the four test isolates at 320 bp. Susceptibility testing revealed multiple drug resistance in all of the isolates with two resistance patterns. Electrophoregram showed a common plasmid with 140 MDa in the plasmid profile of all isolates. Following conjugation and curing experiments, the transconjugant tSh173 gained all plasmids and became invasive to HeLa cells. Elimination of that plasmid by curing was accompanied by the loss of this virulence factor. Moreover, loss of 140 MDa plasmid was associated with the loss of ampicillin, cephradine, norfloxacin, enoxacin and chloramphenicol resistance markers. A positive direct and significant correlation was found between invasiveness transfer and the co-transfer of the above resistance markers. Thus, it appears that inv gene which mediates the invasiveness of the bacteria into the host cells is plasmid encoded and can be transferred by conjugation and expressed by the transconjugants.
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