Molecular analysis of virulence plasmids of invasive shigella species

A total of 48 isolates of Shigella species obtained from the Military Central Laboratories was examined for serotype, antibiotic resistance and invasiveness. The transferability and molecular structure of the virulence plasmids from each isolate were also examined. Twenty strains were evasive and 70% of these virulent strains were multidrug resistant. Ten virulent strains [tra+] transferred ampicillin resistance and virulence simultaneously, i.e. they carried conjugative plasmids; while the other strains [10 strains] were tra-. By plasmid analysis and DNA homology, almost all the virulent strains carried a large plasmid of 230 Kb, and 2-3 small cryptic plasmids [6.4, 3.3 and 2.7 Kb]. The results suggested that the cryptic plasmids can be transferred to other strains of the same species by conjugation or mobilization, predicting that the virulent multidrug resistant strains may no longer be restricted to a narrow range of serotypes in the near future. Further genetic analysis of virulence plasmids may be essential for the development of recombinant vaccines against Shigella species