Survey of aminoglycoside resistance patterns

We studied the susceptibility of 545 gram-negative clinical isolates to six aminoglycosides: gentamicin, tobramycin, netilmicin, amikacin, 2'-N-ethylnetilmicin, and 6'-N-ethylnetilmicin. The resistant strains (359) have been classified into aminoglycoside resistance (patterns) phenotypes (AGRPs) corresponding to known resistance mechanisms. The results clearly demonstrated that most strains of Proteus spp., Klebsiella spp., Escherichia coli and Pseudomonas aeruginosa appeared to be able to adenylate the 2'-hydroxyl group (ANT(2'')) of aminoglycosides. Of the AGRPs reflecting acetylating activity, the AAC(3)-II and AAC(3)-I predominated. The AGRP AAC(2') was limited mainly to Proteus and Providencia isolates. It is noteworthy that permeability type resistance (PTR) was observed only among P. aeruginosa strains. Statistical evaluation of the results, using a discriminant regression analysis showed that PTR P. aeruginosa strains were isolated less often from urine than from other clinical sources (P less than 0.05). On the contrary the ANT(2'') Proteus strains were more common isolates from urinary infections as compared to those from other sources (P less than 0.05). Other correlations between frequency of AGRPs, bacterial groups, and various clinical sources were not statistically significant. However such differences in the frequency of occurrence of AGRPs among diverse bacterial groups were noted in relation to hospitals.