R Factors in Strains of Shigella dysenteriae Type 1 Isolated in the Western Hemisphere during 1969-1970

Abstract

Eighteen strains of Shigella dysenteriae type 1 isolated at various times and localities during the epidemic of dysentery in Central America and Mexico during 1969–1970 were examined for characterization of the multiple antibiotic resistance of these strains and determination of whether transferable resistance factors were present. Seventeen strains were resistant to sulfonamides, streptomycin, tetracycline, and chloramphenicol, and one was resistant to sulfonamides, streptomycin, and chloramphenicol. All strains transferred the complete pattern of resistance to Escherichia coli. Nearly all R⁺ recipients examined, regardless of the antibiotic with which they were selected, exhibited the complete resistance pattern of the donor. Frequency of R-factor transfer was similar in the four strains in which it was determined. The levels of resistance conferred by these R factors were similar in both S. dysenteriae and E. coli for all drugs except streptomycin (minimal inhibitory concentration, 125–250 μg/ml in S. dysenteriae and 8–16 μg/ml in E. coli). The findings of this study suggest that the multi-resistant strains isolated in this epidemic harbor the same R factor rather than a heterogenous group of drug-resistance factors of diverse origins.