365 S. aureus and 165 Coagulase Negative Staphylococci were tested fur susceptibility to methicillin, cefazolin, cefamandole and ceftazidime by standard broth microdilution and disc method. In staphylococci resistance to methicillin normally parallels resistance to ß-lactams, and it has been suggested that cephalosporins are not he used clinically if susceptbility tests show resistance to methicillin. Populations of methicillin resistant staphylococci are made up of a mixture of methicillin-susceptible and methicillin-resisistant cells (heteroresistant isolates). This phenomenon is the cause of differences in resistance in vitro to methicillin and tested cephalosporins. Cefamandole was the cephalosporin which retained most antibacterial activity against some methicillin-resistant isolates (MlCm MRSA = 63,2 mg/L, MICm MRSA = 67.4 mg/L). This antibiotic has extremely small resistant subpopulations, only detectable by hisih -in- oculum screening or prolonged incubation of isolates in the presence of the drug. Cefazolin was the cephalosporin which had most antibacterial activity against all methicillin-susceptible isolates (MlCm MSSA = 1,3 mg/L, MICm MSCNS = 1,6 mg/L. Among the 3 studied cephalosporins ceftazidime was found to be the least active against methicillin-resistant and methicillin-susceptible Staphylococci (MICm MSSA = 25 mg/L, MICm MRSA = 334,5 mg/L, MICm MRCNS = 414,6 mg/L, MICm MSCNS = 26,7 mg/L)
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