title: Beta-lactam stability in frozen microdilution PASCO MIC panels using strains with known resistance mechanisms as biosensors.
abstract: 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.
authors.affiliation: Servicio de Microbiología, Hospital Ramón y Cajal, Madrid, Spain.
authors.forename: S
authors.initials: S
authors.lastname: Valdezate
authors.forename: J
authors.initials: J
authors.lastname: Martínez-Beltrán
authors.forename: L
authors.initials: L
authors.lastname: de Rafael
authors.forename: F
authors.initials: F
authors.lastname: Baquero
authors.forename: R
authors.initials: R
authors.lastname: Cantón
mesh_terms: D000900:Anti-Bacterial Agents / Q000032:analysis*; D016972:Enterobacter cloacae / Q000187:drug effects; D004755:Enterobacteriaceae / Q000187:drug effects; D013293:Enterococcus faecalis / Q000187:drug effects; D004926:Escherichia coli / Q000187:drug effects; D005615:Freezing; D007709:Klebsiella / Q000187:drug effects; D007711:Klebsiella pneumoniae / Q000187:drug effects; D008826:Microbial Sensitivity Tests / Q000379:methods*; D011514:Proteus vulgaris / Q000187:drug effects; D011550:Pseudomonas aeruginosa / Q000187:drug effects; D013211:Staphylococcus aureus / Q000187:drug effects; D018440:beta-Lactam Resistance; D047090:beta-Lactams
chemical_list: D000900:Anti-Bacterial Agents; D047090:beta-Lactams