Imipenem and meropenem activity against mecA-positive homogeneously and heterogeneously oxacillin-resistant and mecA-negative oxacillin-borderline-susceptible staphylococci.
Microbroth dilution and disk-diffusion testing of imipenem and meropenem was performed at 35 and 30 degrees C against 61 phenotypic expression class 3,4 and 9 phenotypic expression class 1,2 oxacillin-resistant isolates of Staphylococcus aureus (ORSA), 51 oxacillin-borderline-susceptible isolates of S. aureus (BORSA), and 37 phenotypic expression class 3,4 and 9 phenotypic expression class 1,2 isolates of Staphylococcus epidermidis (ORSE). Imipenem MIC ranges at 35 degree C were 0.6 to > 64 micrograms/ml for class 3,4 ORSA, 0.03 to 0.25 micrograms/ml for class 1,2 ORSA, 0.015 to 0.12 micrograms/ml for BORSA, 0.03 to 64 micrograms/ml for class 3,4 ORSE, and 0.12 to 8 micrograms/ml for class 1,2 ORSE. Corresponding values for meropenem were 0.5 to 64 micrograms/ml, 0.12 to 4 micrograms/ml, 0.06 to 1 microgram/ml, 0.5 to 64 micrograms/ml, and 1 to 8 microgram/ml. MIC ranges at 30 degrees C did not differ by more than 1 log2 dilution from those at 35 degrees C. After 24 h incubation of disk-diffusion tests at 35 degrees C, 44% of class 3,4 and 100% of class 1,2 ORSA isolates were imipenem-susceptible; after an additional 24 h at 25 degrees C, 39 and 100% of these isolates, respectively, remained susceptible to imipenem. Similar values were obtained with 24 h incubation at 30 degrees C followed by 24 h at 25 degrees C. All BORSA isolates were susceptible to imipenem. Of the ORSE isolates, 22 and 78% of isolates in classes 3,4 and 1,2, respectively, were susceptible at 24 h with little change after an additional 24 h at 25 degrees C. Similar trends were observed with meropenem. In parallel disk-diffusion studies with oxacillin, false-susceptibility rates of 5% of class 3,4 and 44% class 1,2 ORSA isolates after 24 h of incubation at 35 degrees C were reduced to 3 and 0%, respectively, after an additional 24 h of incubation at 25 degrees C. Imipenem- and meropenem-resistant subpopulations of oxacillin-resistant staphylococci did not seem to be detected by altered susceptibility testing conditions.
Knapp
C C
CC
Department of Clinical Pathology, Cleveland Clinic Foundation, OH 44195-5140, USA.
Ludwig
M D
MD
Washington
J A
JA
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000032:analysis; D004352:Drug Resistance, Microbial; D015378:Imipenem / Q000494:pharmacology*; D000077731:Meropenem; D008826:Microbial Sensitivity Tests; D010068:Oxacillin / Q000494:pharmacology*; D013210:Staphylococcus / Q000187:drug effects*; D013845:Thienamycins / Q000494:pharmacology*
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D013845:Thienamycins; D015378:Imipenem; D000077731:Meropenem; D010068:Oxacillin