Standardization of a rapid microbiologic assay for aminoglycosides using Enterobacter cloacae

The standardization of a rapid serum aminoglycoside assay using Enterobacter cloacae is described. This includes the sensitivity testing of the organism and its performance on various media, with Mueller-Hinton agar being the medium of choice. The precision and reproducibility of the assay, as measured by the within-run and between-run coefficients of variation, were 5.0 and 5.9, respectively. A significant positive correlation was obtained between the microbiologic assay for gentamicin and a 125I-labeled gentamicin radioimmunoassay with the use of both normal and uremic sera. When known amounts of gentamicin, tobramycin, and amikacin were added to antibiotic-free sera from normal persons, recovery rates of 80.0% to 97.9% were found. In the case of gentamicin, recovery rates of 85.0% to 97.9% were found with the use of sera from patients undergoing either hemodialysis or peritoneal dialysis. There were no effects on the recovery rates of the aminoglycosides from normal serum if high concentrations of clindamycin, methicillin, ampicillin, penicillin G, cephalothin, cefamandole or cefoxitin were also present in the sera. The newer cephalosporins, cefamandole and cefoxitin, had no in-vitro effect on the Kirby-Bauer sensitivity patterns of gentamicin, tobramycin, or amikacin, when tested against the assay organism.