Surveillance of multidrug-resistant strains of Salmonella enterica serotype Typhimurium in southern Italy in the years 1992-1997.

Spread of multidrug-resistant strains of Salmonella enterica serotype Typhimurium (S. typhimurium) is increasingly reported worldwide. The presence of a pattern of resistance to ampicillin, chloramphenicol, streptomycin, sulfonamides and tetracycline (ACSSuT), in some cases associated to trimethoprim and infrequently to quinolones, is of particular concern. This resistance pattern appears to be chromosomally encoded and, in most epidemiological studies, closely related to definitive type 104 (DT104). In southern Italy multidrug-resistant isolates of S. typhimurium had been identified since 1980, but only during 1992 S. typhimurium strains with chromosomally encoded drug resistance were first isolated from domestic animals. One hundred fifty-five isolates--52.5% of the multidrug-resistant strains identified in the years 1992-1997--were submitted to phage typing and plasmid profile analysis. Ribotyping was also performed in comparison with a random sample of 150 strains susceptible or resistant to three or less antibiotics identified in the same interval of time. Four ribotypes (RTs)--1, 5, 8, and 48--included approximately 90% of the multiresistant strains, RT8 accounting for 61.2%. Phage type (PT) 193 is the most prevalent phage type. Phage typing and ribotyping suggest that few bacterial clones are involved in spread of multidrug-resistant S. typhimurium strains in southern Italy.