Reliability of the microdilution technic for detection of methicillin-resistant strains of staphylococcus aureus

To determine whether the microdilution technic for antimicrobic susceptibility testing could detect the hetero-resistance of "methicillin"-resistant Staphylococcus aureus, 100 stock cultures (54 methicillin-susceptible and 46 methicillin-resistant) were tested against methicillin, oxacillin, nafcillin and cephalothin. Each drug was diluted in Mueller-Hinton broth and in Mueller-Hinton broth with 5% NaCl. Minimum inhibitory concentration (MIC) determinations were made after 20-24 hours at 35 C and again after 48 hours of incubation. With one exception, methicillin resistance was detected in Mueller-Hinton broth when the trays were reincubated and read after 48 hours. Earlier detection of methicillin resistance was possible when the penicillinase-stable penicillins were diluted in broth containing 5% NaCl. Further studies suggest that the addition of NaCl to the broth medium might improve the stabilities of methicillin, oxacillin, and nafcillin during storage of the frozen microdilution trays. Either nafcillin or oxacillin would appear to be the best representative of the penicillinase-stable penicillins. Standardized disk-diffusion tests with cephalothin disks indicated that most methicillin-resistant strains are susceptible to cephalothin. However, there is reason to believe that such in-vitro test results might be inappropriate. With microdilution tests, most methicillin-resistant isolates were more resistant to cephalothin than were the methicillin-susceptible strains, but the MIC's for both types of staphylococci were generally within the range of concentrations that can be obtained during therapy. The addition of NaCl to the microdilution tests with cephalothin did not alter the test results significantly.