An increase in methicillin-resistant Staphylococcus aureus (MRSA) infections prompted a study of MRSA during a 21-month period in a 600-bed university hospital in southern Texas. MRSA cases were classified as community, nosocomial, or transfer cases. A case-control study of risk factors for community MRSA compared with community methicillin-susceptible S. aureus (MSSA) was performed. Pulsed field gel electrophoresis (PFGE) of whole cell DNA typing was used as a marker of strain identity for 31 consecutive isolates collected during the last 8 months of the study. During the 21 months there were 170 patients with MRSA infection or colonization, an incidence of 0.2 per 1,000 patient-days. Ninety-nine (58%) of 170 isolates were from community cases; the community to nosocomial case ratio was 2:1. No significant risk factors differentiated patients with community MRSA compared with community MSSA. Most community MRSA isolates studied (15 [68%] of 22) had distinct PFGE patterns, as did many nosocomial MRSA isolates (4 [44%] of 9). MRSA isolates were commonly present on admission to the hospital, and multiple MRSA strains were demonstrated among both community and hospital isolates.
During a 19-month period from April 1993 to October 1994, 41 isolates of vancomycin-resistant Enterococcus faecium (VREF) were detected in seven different hospitals in a city in southern Texas. A case-control study to determine the risk factors for acquisition was done in the hospital in which the majority of isolates were detected. Pulsed-field gel electrophoresis (PFGE) of whole-cell DNA was used to determine strain identity. Thirty-five (85%) of the 41 VREF isolates were of the vanB phenotype. Of these, 32 (91%) of 35 were the same strain by PFGE typing. The same vanB strain was documented in five different hospitals in the city. In contrast, 4 (67%) of 6 of the vanA phenotype VREF isolates were distinct strains by PFGE typing. Significant risk factors for colonization or infection with VREF were prior exposure to antibiotics (P = .04), the previous use of third-generation cephalosporins (P = .03), and the previous use of parenteral vancomycin (P = .002). Infection-control and antibiotic-utilization measures were implemented to control cross-transmission and selection of VREF isolates. During the emergence of VREF in our city, clonal dissemination of a single strain of vanB VREF among six hospitals was documented. Limited cross-transmission of vanA phenotype VREF isolates occurred, but most vanA VREF isolates were distinct strains selected in individual hospital environments.
To determine the epidemiology of bacteremias due to pneumococci not susceptible to penicillin (PNSP) at a university hospital, active microbiologic surveillance of bacteremias due to PNSP was done for 28 months. Controls were bacteremias caused by penicillin-susceptible pneumococci. Antimicrobial susceptibilities for alternative antibiotics were determined. Pulsed-field gel electrophoresis (PFGE) and serotyping were used as markers of strain identity. Of 113 pneumococcal isolates, 14 (13%) were intermediate or resistant to penicillin (MIC > or = 0.1 microgram/mL). Twelve PNSP were resistant to other drugs: chloramphenicol (5), tetracycline (6), trimethoprim-sulfamethoxazole (5), cefotaxime (1), and erythromycin (1). Independently significant risk factors associated with PNSP bacteremia were sepsis and prior treatment with beta-lactam antibiotics. PFGE revealed 10 distinguishable patterns among 12 isolates available for typing. In general, PFGE typing correlated with serotyping. It also distinguished some isolates of the same serotype. PFGE typing and serotyping suggest that the frequency of PNSP in the San Antonio, Texas, area is not due to dissemination of a single clonal strain.
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