Search

A survey in 2000 to detect methicillin-resistant Staphylococcus aureus (MRSA) colonization in Vancouver downtown east side injection drug users (IDUs) revealed an MRSA nasal colonization incidence of 7.4%. This is a follow-up study to determine the current prevalence of MRSA colonization and to further characterize the isolates and risk factors for colonization. In this point prevalence study of MRSA nasal carriage among IDUs, nasal swabs were cultured to detect S. aureus. Isolates were studied for their antimicrobial susceptibility patterns and the presence of mecA and Panton-Valentine leukocidin (PVL) genes and by pulsed-field gel electrophoresis (PFGE). S. aureus was isolated from 119 of 301 (39.5%) samples; three (2.5%) participants had both methicillin-sensitive S. aureus (MSSA) and MRSA, resulting in 122 isolates. Of these, 54.1% were MSSA and 45.9% were MRSA, with an overall MRSA rate of 18.6%. USA-300 (CMRSA-10) accounted for 75% of all MRSA isolates; 25% were USA-500 (CMRSA-5). None of the USA-500 isolates were positive for PVL; 41 (97.6%) USA-300 isolates contained PVL. One MSSA isolate, from an individual also carrying USA-300, was positive for PVL. The PFGE pattern of this MSSA isolate was related to that of the MRSA strain. The antibiograms of USA-300 compared to USA-500 isolates showed 100% versus 7.1% susceptibility to trimethoprim-sulfamethoxazole (TMP-SMX) and 54.8% versus 7.1% susceptibility to clindamycin. MRSA nasal colonization in this population has increased significantly within the last 6 years, with USA-300 replacing the previous strain. Most of these strains are PVL positive, and all are susceptible to TMP-SMX.

In a study comparing the BACTEC 9240 (Becton Dickinson, Canada Inc., Mississauga, Ontario, Canada) and the BacT/Alert (Organon Teknika, Scarborough, Ontario, Canada) blood culture systems, 6,437 sets of four bottles of 10,412 cultures submitted were deemed acceptable for evaluation. There were 586 clinically significant isolates from 544 positive blood cultures. Of the 544 positive blood cultures, 329 were positive in both systems, 157 were positive in the BACTEC 9240 system alone, and 58 were positive in the BacT/Alert system alone (P < 0.001). These cultures represented 421 bacteremic episodes, of which 290 were detected by both systems, 84 were detected by the BACTEC 9240 system alone, and 47 were detected by the BacT/Alert system alone (P < 0.001). The difference between the two systems in terms of microbial recovery was attributable largely to differences in the ability to grow staphylococci. Of 82 monomicrobial isolates of Staphylococcus aureus, 46 were isolated by both systems, 30 were isolated by the BACTEC 9240 system alone, and 6 were isolated by the BacT/Alert system alone (P < 0.0001). A similar pattern was noted in the case of coagulase-negative staphylococci, of which 48 were isolated in both systems, 46 were isolated in the BACTEC 9240 system alone, and 5 were isolated in the BacT/Alert system alone (P < 0.0001). The false-positive rate in the BACTEC 9240 system was 0.5%, and that in the BacT/Alert system was 1.2%. Both systems provided satisfactory service mechanically, and in terms of computer software and costs, both systems were similar. The differences in the recovery of organisms, notably staphylococci, may be due in part to the fact that in the present study the BacT/Alert medium did not contain antimicrobial agent-removing devices, whereas the BACTEC 9240 medium did. Until this difference is resolved, we consider the BACTEC 9240 system to more adequately meet our hospital's requirements.

The E test (AB Biodisk NA Inc.) was compared with standard reference methods using the National Committee for Clinical Laboratory Standards's recommendations for determining the MICs of four selected antibiotics against 208 clinical isolates of gram-positive bacteria. These bacteria included 32 strains of Streptococcus pneumoniae, 25 strains of Enterococcus faecium, 20 strains of oxacillin-sensitive Staphylococcus aureus (OSSA), 96 strains of oxacillin-resistant S. aureus (ORSA), and 35 strains of coagulase-negative staphylococci. Evaluation included MIC accuracy within 1 dilution, reproducibility testing, and cost analysis. There was 94% agreement between the E test and the reference method in testing S. pneumoniae and penicillin G. There was 92% agreement with ampicillin and 100% agreement with vancomycin in testing E. faecium isolates. Accuracy of the oxacillin E test with staphylococci was significantly improved by the use of salt-supplemented Mueller-Hinton agar, for an agreement of 100% with coagulase-negative staphylococci and oxacillin-sensitive S. aureus and that of 85% with oxacillin-resistant S. aureus, with no major discrepancies. The E test with American Type Culture Collection isolates and clinical strains gave excellent reproducibility and was less costly than microdilution panels when used to test fewer than three antibiotics. The E test is a simple, reliable, reproducible, and cost-effective method for MIC determination for gram-positive organisms.