beta-Lactamase production and genetic location in Staphylococcus aureus: introduction of a beta-lactamase plasmid in strains of phage group II.

Staphylococcus aureus strains of phage group II have increased in frequency in hospital-acquired infections during recent years. A total of 184 penicillin resistant group II strains from bacteraemia cases in the years 1961-1990 were analysed for the amount of beta-lactamase produced and the location of the beta-lactamase gene. Until 1977 all strains investigated had a chromosomally located beta-lactamase gene, but since then a 21 kb beta-lactamase plasmid has increased in occurrence among group II strains, especially among those strains typable only at high phage concentrations [100 x Routine Test Dilution (RTD) and 1000 x RTD]. In 1990, 84% of the group II strains contained this plasmid. Plasmid-containing strains produced more beta-lactamase than strains without the plasmid. S. aureus strains of the 94,96 complex, which since 1984 have decreased in frequency from 18 to 9% in 1993, have remained high producers of beta-lactamase.