The type strains of Vi-phage type E1, M1 and A of together with drug-resistant and drug-sensitive strains of phage types E1 and M1 isolated in 1992 from patients associated with India or Pakistan, and a drug-resistant strain of phage type A isolated in South Africa in 1991, were characterised with respect to the presence of plasmids conferring resistance to antimicrobial drugs and their chromosomal insertion sequence IS profiles. The three type strains, the drug-sensitive strains of Vi-phage types E1 and M1, and a strain of phage type M1 resistant to ampicillin and trimethoprim but not to chloramphenicol, did not contain plasmids. In contrast, for strains of phage types E1 and M1 resistant to chloramphenicol, ampicillin and trimethoprim, and for the drug-resistant strain of phage type A, the complete spectrum of resistance was encoded by high molecular mass plasmids belonging to the H incompatibility group. Characterisation of IS profiles demonstrated that at least 13 IS copies were distributed on the chromosome of all strains tested. Although the IS profiles of the type strains of Vi-phage types A, E1 and M1 were identical, it was possible to distinguish between drug-sensitive and drug-resistant strains of Vi-phage types E1 and M1 isolated from patients infected in India and Pakistan by this method. It was concluded that although IS typing is not as discriminatory as phage typing for the primary subdivision of it may be useful for certain epidemiological investigations and, in particular, for investigating the origins of strains with multiple drug resistance.


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