I always thought that the "susceptibility hierarchy" or activity of the aminoglycosides against gram-negative bacilli was as follows: amikacin (most active), followed by tobramycin, followed by gentamicin. However, occasionally, we obtain amikacin "R", tobramycin "R", and gentamicin "S" results. Retesting by a second method confirms the results. When this happens, should we repeat the test? Should we be editing our final report, e.g., amikacin "R", tobramycin "R", and gentamicin "R"?

Answer

No, reports for aminoglycosides should not be edited, although it may be helpful to repeat the test.

Approximately 10% of Enterobacteriaceae in the United States are resistant to gentamicin. Isolates resistant to gentamicin may or may not be resistant to tobramycin depending on the mechanisms of resistance. Strains that produce acetyltransferease will not be resistant to tobramycin but those that produce adenylyltransferase will be resistant to both gentamicin and tobramycin. Amikacin is typically more active against Enterobacteriaceae than is gentamicin or tobramycin. Some Enterobacteriaceae produce an aminoglycoside acetyltransferase enzyme, AAC(6’)-I, that results in a gentamicin-S, tobramycin-R, and amikacin-R phenotype. AAC(6’)-I is common among Serratia spp. but less frequent in other Enterobacteriaceae. Consequently, gentamicin-S, tobramycin-R, and amikacin-R results should be checked if encountered in Enterobacteriaceae other than Serratia spp. The isolate identification should also be checked. Less than 1.5% of Enterobacteriaceae in the United States are resistant to amikacin.

Aminoglycoside resistance is more common in Pseudomonas aeruginosa than in Enterobacteriaceae and P. aeruginosa isolates that are resistant to gentamicin may or may not be resistant to tobramycin. Some isolates may appear tobramycin-S, gentamicin-R, and amikacin-R where the tobramycin-S results are just below the resistant breakpoint, but this is likely a false susceptible result and the organism should be retested. P. aeruginosa may be resistant to gentamicin, tobramycin, and amikacin such that none of these would be useful therapeutically. Approximately 5% of P. aeruginosa in the United States are resistant to amikacin but 20% or greater may be resistant to gentamicin and tobramycin.

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