Resistance of Pseudomonas aeruginosa to Gentamicin and Related Aminoglycoside Antibiotics

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This study was undertaken to investigate biochemical, genetic, and epidemiological aspects of resistance to aminoglycoside antibiotics among 650 consecutive isolates of Pseudomonas aeruginosa from Parkland Memorial Hospital, Dallas, Tex. In 364 strains, minimal inhibitory concentrations were 25 μg/ml or greater for gentamicin (G), tobramycin (T) or kanamycin (K). Four patterns of resistance were noted: (A) G, T, K (four strains), (B) G, K (23 strains), (C) T, K (one strain), and (D) K (336 strains). Gentamicin acetyltransferase (GAT) activities were associated with resistance to gentamicin in strains of groups A and B, whereas kanamycin phosphotransferase activity was found in strains of group D. The GAT from group B strains acetylates both gentamicin and tobramycin. Resistance to gentamicin and susceptibility to tobramycin may reflect the fact that the Km's for tobramycin (25 to 44 μg/ml) of GAT activities in these group B strains are much greater than the Km's for gentamicin (1.9 to 2.7 μg/ml) and exceed the minimal inhibitory concentrations for tobramycin (1.25 to 7.5 μg/ml). GAT from strains of group A was associated with resistance to G, T, and K. Gentamicin acetyltransferases can be distinguished by their specificities for aminoglycoside substrates. The substrate specificity of GAT from group B strains is similar to that reported for GATI, but the specificity of GAT from group A strains differs from those described for GATI and GATII. Conjugal transfer of gentamicin or tobramycin resistance from our strains of P. aeruginosa to various potential recipient strains was not observed. Pyocin typing showed that many, but not all, of the strains resistant to gentamicin were similar, and retrospective epidemiological investigation revealed that these strains were isolated almost exclusively from patients in the adult and pediatric burn intensive care units and geographically continguous areas of the hospital.

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