Streptococci and staphylococci: Laboratory considerations for in vitro susceptibility testing

Antimicrobial susceptibility of Abiotrophia adiacens and Abiotrophia defectiva

The susceptibilities of 27 Abiotrophia adiacens (proposed reclassification Granulicatella adiacens comb.nov., Collins & Lawson, 2000) and 12 Abiotrophia defectiva isolates were tested by microdilution in pyridoxal hydrochloride and lysed horse blood supplemented Mueller-Hinton broth. According to NCCLS interpretative criteria for Streptococcus spp. not Streptococcus pneumoniae, the susceptibilities of A. adiacens and A. defectiva were, respectively: penicillin, 55% and 8%; amoxicillin, 81% and 92%; ceftriaxone, 63% and 83%; meropenem, 96% and 100%; and 100% for both species with clindamycin, rifampin, levofloxacin, ofloxacin, quinupristin/dalfopristin, and vancomycin.

Rapid dissemination of Staphylococcus aureus with classic oxacillin resistance phenotype at a new university hospital

To determine the prevalence rates of oxacillin-resistant Staphylococcus aureus (ORSA) at a new university hospital since its opening, the results of disk diffusion tests on all clinical isolates, recovered between 1990 and 1998 at the National Cheng Kung University Hospital, were reviewed. In order, to investigate the mechanisms of oxacillin resistance among strains of S. aureus in Taiwan, MICs were determined by an agar dilution method, and polymerase chain reaction and colony hybridization assays were performed on 288 isolates collected during November 1998 to detect the mecA gene. The prevalence rates of ORSA increased rapidly from 14.1% in 1990 to 61.0% in 1998. The increasing rates were most rapid in the first four-year period, ranging from 11.6 to 106.7% per year, and became steady after 1994, ranging from 1.8% to 11.6%. Of 288 clinical isolates collected in November 1998, 206 (71.5%) were resistant to oxacillin (MIC >/= 16 mg/L), and four were borderline resistant (MIC 2-8 mg/L). All 210 strains possessed the mec A gene (classic resistance). The present study demonstrated that ORSA could disseminate in a new hospital with great speed, and indicated that all ORSA strains in Taiwan revealed classic resistance phenotype.

Detection of oxacillin-resistance in Staphylococcus aureus by MicroScan MIC panels in comparison to four other methods

Two hundred fifty-two isolates of Staphylococcus aureus were tested for oxacillin susceptibility by MicroScan Gram positive overnight and rapid MIC panels. Results were compared with nonautomated methods including disk diffusion, MRSA Crystal ID, and Etests using MRSA Screen Agar as reference. One hundred sixty-nine isolates (67.1%) were oxacillin-susceptible and 83 (32.9%) were resistant. All methods agreed for 234 (92.9%) isolates. Very major error rates were 1.2% for disk diffusion, 3.6% for Etest, and 0 for all other methods. Major error rates were 5.3% for MicroScan overnight panels, 3% for rapid panels, 2.4% for disk diffusion, 1.2% for Etest, and 0.6% for MRSA Crystal ID. Nine oxacillin-susceptible isolates with borderline MICs and discrepant results for 1 or more methods were tested for the mec A gene and all were negative. Each was susceptible to beta lactam/beta lactamase inhibitor combinations, suggesting that false resistance may have been due to excessive beta lactamase production. Oxacillin-resistant S. aureus with borderline MICs determined by MicroScan should be confirmed by an alternate method. The most practical and cost-effective means among those we tested is the MRSA Screen Agar.