Streptomycin and chloramphenicol resistance genes in Escherichia coli isolates from cattle, pigs, and chicken in Kenya.

The aims of this study were to determine the genetic basis of streptomycin and chloramphenicol resistance in 30 Escherichia coli isolates from food animals in Kenya and the role of plasmids in the spread of the resistance. Seven of the 29 streptomycin-resistant isolates harbored both the strA and strB genes. Twenty-one of isolates had the strA, strB, and aadA1 genes. The strA gene was disrupted by a functional trimethoprim gene, dfrA14 in 10 of the 21 isolates harboring the three streptomycin resistance genes. Physical linkage of intact strA and sul2 genes was found in two different plasmids from four isolates. Linkage of cassette-borne aadA1 and dfrA1 genes in class 1 integrons was found in two of the isolates. Chloramphenicol resistance was due to the gene catA1 in all the chloramphenicol resistant isolates. The strB, strA, and catA1 genes were transferable by conjugation and this points to the significance of conjugative resistance plasmids in the spread and persistence of streptomycin and chloramphenicol resistance in food animals in Kenya.