Studies on multiple-resistant Staphylococcus aureus. (III). Difference in the incidence of multiple-resistant strains of Staphylococcus aureus among institutions and specimens, and relationship between their resistance and β-lactamase activity

The sensitivity to chemotherapeutic agents of 200 isolates of Staphylococcus aureus, isolated in seven institutions in the northeast of Japan, was examined by the microbroth dilution method using the Dynatech M1C-2000. Also, β-lactamase activity was semi-quantitatively determined by disk acido-metry. In this study, we investigated a total of 24 chemotherapeutic agents : five penicillins (ampicil-3in, amoxicillin,. piperacillin, methicillin and cloxacillin), eight cephems (cefazolin, cefotiam, cefa-mandole, cefmetazole, ceftizoxime, cefoperazone, ceftazidime and latamoxef [moxalactam]), three aminoglycosides (gentamicin, dibekacin and amikacin), a tetracycline (minocycline), two new-quino-Iones (norfloxacin and ofloxacin), and others including clindamycin, rifampicin, vancomycin, fusidic acid and fosfomycin. The incidence of methicillin-resistant (MIC≧12.5 μg/ml) strains of S. aureus was 25-5% (51/200), and that of cefazolin-resistant (MIC≧l2.5 μg/ml) strains was 21.0% (42/200) respectively. The in¬cidence of MRSA was found to vary from 0%-40%, and the difference was statistically significant. Also, the incidence of MRSA in strains recovered from pus was significantly higher than from other specimens such as sputum, otorrhea, urine and blood. Out of a total of 200 strains, 171 were found to produce penicillinase. The incidence of the penicillinase-positive strains, especially those which show moderate or strong activity, was statistically higher in MRSA than in MSSA (methicilliu-sensitive S. aureus) and statistically significant (P<0.01). The recovery of sensitivity to methicillin after 20 hr incubation at 44°C was demonstrated in 30% of MRSA. Based on our findings, we sug¬gest that the mechanism of resistance in MRSA is in greater part related to β-lactamase activity. Almost all of the MRSA were sensitive to cloxacillin, amikacin, cefamandole, cefmetazole, minocy¬cline, rifampicin, ofloxacin, norfloxacin, vancomycin and fusidic acid.