Beta-lactam stability in frozen microdilution PASCO MIC panels using strains with known resistance mechanisms as biosensors.

The stability of amoxicillin/clavulanate, piperacillin/tazobactam, cefepime, imipenem, and meropenem in PASCO (PASCO System, DIFCO Laboratories, Detroit, MI, USA) frozen microdilution susceptibility panels stored for 16 weeks at -20 degrees C and -70 degrees C was evaluated. The increase in MIC values for the five American-Type Culture Collection (ATCC) quality control strains for susceptibility testing recommended by the National Committee for Clinical Laboratory Standards (NCCLS) and for 13 strains with different well-characterized resistance mechanisms was indicative of bioactivity deterioration. The overall agreement (+/- 1 twofold dilution) at purchase between the MIC values of PASCO frozen microdilution susceptibility panels and the standard agar dilution method was 97.7%. Minimum inhibitory concentration values for the associations of amoxicillin/clavulanate and piperacillin/tazobactam remained unchanged for the study period at -70 degrees C. In contrast, a carbapenem bioactivity decrease was detected only with strains having well-characterized resistance mechanisms from the 12th week onwards. At -20 degrees C, antibiotic deterioration with these latter strains was observed earlier than with ATCC strains: the activity of meropenem and imipenem remained unchanged only for the first 2 weeks, while a loss of activity was detected for amoxicillin/clavulanate and piperacillin/tazobactam at the 7th and 10th week, respectively. Cefepime was highly stable both at -20 degrees C and -70 degrees C. Strains with well-characterized resistance mechanisms should be used in routine quality control studies of antibiotic stability for susceptibility testing panels during the storage period.