Phage pattern-specific oxacillin-resistant and borderline oxacillin-resistant Staphylococcus aureus in U.S. hospitals: epidemiological significance.

For a 13-year period (1978 through 1990), oxacillin-resistant (MIC, greater than 4 micrograms/ml) Staphylococcus aureus (ORSA) strains were collected from Clinical Center (National Institutes of Health) patients and patients from five other U.S. hospitals. From Clinical Center patients, 251 of 253 isolates (99%) were bacteriophage typed as phage group III. Five other hospitals contributed 203 ORSA strains, of which 188 (93%) were group III. The group III ORSA strains predominantly included a characteristic core pattern of phages, 7/47/53/54/75/77. For the low-level (borderline) oxacillin-resistant strains (MIC, 2 to 4 micrograms/ml), amoxicillin-clavulanic acid combination (Augmentin) testing disclosed 62 hyper-beta-lactamase producers, of which 59 (95%) were of a separate, distinct S. aureus strain, with the phage pattern 92/94/96/292/D-11 (group V). Thus, ORSA and hyper-beta-lactamase producing S. aureus are distinct epidemic strains.