Aminoglycoside resistance among Pseudomonas aeruginosa isolates with an unusual disk diffusion antibiogram

In recent years, a number of clinical microbiology laboratories have isolated Pseudomonas aeruginosa with the unusual aminoglycoside disk diffusion result of resistance to both amikacin and gentamicin but susceptibility to tobramycin (ArGrTs). A total of 39 isolates of P. aeruginosa reported to have this resistance pattern were retested by the standard National Committee for Clinical Laboratory Standards disk diffusion procedure; 30 strains (77%) were confirmed to be ArGrTs. These 30 isolates were further examined for susceptibility to those aminoglycosides by agar dilution and broth micro- and macrodilution methods. Only 27, 27, and 23% of the isolates appeared to be ArGrTs by agar, broth microdilution, and broth macrodilution testing, respectively. Most of the remaining isolates were resistant to all three aminoglycosides when tested by broth dilution and resistant only to gentamicin when tested by agar dilution. The percentages of strains resistant to any particular aminoglycoside by agar dilution, broth microdilution, and broth macrodilution, respectively, were 43, 80, and 70 for amikacin, 97, 93, and 100 for gentamicin, 100, 100, and 100 for netilmicin, 30, 87, and 93 for sisomicin, and 13, 57, and 50 for tobramycin. These results indicate that strains showing the unusual aminoglycoside antibiogram are less susceptible to aminoglycosides in general and should probably be considered borderline resistant to all aminoglycosides. The efficacy of aminoglycosides in the treatment of infections produced by these strains is unknown.