Plasmid-mediated aminoglycoside phosphotransferases in Haemophilus ducreyi

Three clinical isolates of Haemophilus ducreyi, representing at least two subtypes, were shown to be resistant to streptomycin and kanamycin. They also produced a beta-lactamase and chloramphenicol acetyltransferase and were resistant to tetracycline. In the three strains the resistance to both aminoglycoside antibiotics was encoded by a plasmid of ca. 4.7 kilobases which apparently did not carry ampicillin, chloramphenicol, or tetracycline resistance genes, as determined after transfer to Escherichia coli by transformation. Resistance to streptomycin and kanamycin was due to the presence of two aminoglycoside phosphotransferases (APH). The enzyme modifying kanamycin was a 3',5"-APH of type I [APH(3',5")-I], as inferred from its substrate profile and immunological cross-reactivity with the APH(3',5")-I encoded by the transposable element Tn903. However, the APH(3',5")-I gene in H. ducreyi did not appear to be carried by Tn903.