Drug Resistance and Distribution of R Factors among Escherichia coli Strains

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Abstract
Three hundred and thirty‐eight strains of Escherichia coli from clinical sources were examined for their drug resistances and R factors in 1971. With reference to four drugs, i.e., tetracycline(TC), chloramphenicol(CM), streptomycin(SM) and sulfanilamide(SA), the SA‐resistant strains were isolated most frequently followed by SM‐, TC‐ and CM‐resistant ones in decreasing order. Among those resistant to the aforementioned four drugs, 20.3, 19.0, 7.7 and 18.3% were single‐, double‐, triple‐ and quadruple‐resistant, respectively. The R factors were demonstrated at a frequency of 29.2% among these resistant strains. From the strains possessing quadruple‐resistance, the conjugationally transferable R factors were demonstrated most frequently (60.5%) followed by triple‐ (59.1%), double‐ (21.7%) and single‐resistant strains (4.5%). The isolation frequencies of strains resistant to kanamycin (KM) and ampicillin (APC) were 1.5 and 4.7%, respectively; being fewer in numbers compared with those resistant to the four drugs. Strains resistant to cephaloridine, nalidixic acid, Colistin, gentamicin C complex and furatridine were not isolated. Strains resistant to KM and APC were demonstrated mostly from those possessing multiple resistance with reference to the aforementioned four drugs. Among the strains carrying either APC‐ or KM‐resistance, 76.9 and 100% respectively, were found to carry R factors, indicating that most of the APC‐ and KM‐resistances were conferred by the R factors. Twenty percent of the R+ strains with reference to resistance to the four drugs carried two types of R factors in a cell, being in a hetero‐R state, and 66.7% of the strains carrying R factors with APC‐ or KM‐resistance were in a hetero‐R state.