Antimicrobial resistance and R factors in Salmonella isolated in California (1971-1972).

The antimicrobial resistance of 2,246 strains of Salmonella isolated from humans in California was determined. Resistance to one or more of the 12 antimicrobial agents tested was found in 32% of the isolates. Salmonella typhimurium strains represented 31% of the serotypes isolated; 49% of these strains were resistant. Fifty-one percent of S. heidelberg, 42.5% of S. newport, and 40% of S. saint paul strains were resistant. Seventy-seven percent of all resistant serotypes were resistant to two or more of the antimicrobial agents tested. R factors were demonstrated in 70% of the multiply resistant strains. Resistance of the Salmonella strains to one or more of the 12 antimicrobials tested and the frequency of resistance to tetracycline and ampicillin were significantly greater than resistance reported in other studies performed in this country. Chloramphenicol resistance was encountered in 33 (1.5%) of the Salmonella isolates; 19 of these were S. typhi demonstrating a pattern of resistance to streptomycin, sulfisoxazole, tetracycline, and chloramphenicol and carrying an R factor capable of transferring the complete pattern of resistance.