Methicillin-resistant Staphylococcus aureus in Nosocomial Infections in the Surgical ward and Operating Room

In this study 214 strains of Staphylococcus aureus were isolated from clinical specimens on the surgical ward from 1983 to 1988 and in addition, 62 airborne strains were collected in the operating room. Highly methicillin-resistent strains of S.aureus (H-MRSA, MIC > 100 μg/ml) not detected in 1983 showed a significant increase in frequency by 1987 accounting for about 60% of MRSA (MIC≧12.5 μg/ml). Countermeasures instituted in 1987 such as the use of disinfectant chlorhexidine alcohol significantly decreased the frequency of MRSA and H-MRSA isolates in 1988. In our study of coagulase type, MRSA type IV strains were predominent until 1984, whereas after 1986 type II was prevalent. All airborne strains collected in the operating room were methicillin-sensitive S.aureus, with type VII currently epidemic. We therefore concluded that cross infection with MRSA took place on the surgical ward rather than in the operating room

β-lactamase in Gram-negative Rods : the relationship between penicillinase and R plasmids in Gram-negative rods

Concomitant with the extensive use of antibiotics, the number of multiple antibiotic-resistant strains has been increasing. Since resistance is mainly mediated by R plasmids, we undertook to investigate the characteristics of R plasmid-determined ,β-lactamase in 6 Gram-negative rods.　　　 The β-lactamase produced by each organism was classified by its substrate: type P which attacks penicillins, type C which attacks cephalosporins, and type C/P which attacks both penicillins and cephalosporins. Though the chromosomally mediated β-lactamase of almost all Gram-negative rods is classified as type C, R plasmid-mediated β-lactamase is almost equally active against both penicillins and cephalosporins. Therefore, we suggest that type C/P ,β-lactamase was mediated by R plasmids in Gram-negative rods which already produced chromosomally mediated type C β-lactamase.　　　 The strains which produced type C/P β-lactamse tended to be more resistant to antibiotics than the other β-lactamase producing strains. Among type C/P strains, the sensitivity to cephalosporins varied with the bacterial species, whereas all these strains were highly resistant to penicillins. Even for piperacillin, which is stable to cephalosporinase, the MIC at which the cumulative percentage of strains inhibited was 50% (MIC50) was over 50 μg/ml in all strains tested