Synergistic enhancement of in vitro antimicrobial activity of cefmetazole and cefotiam, cefamandole or cefoperazone in combination against methicillin-sensitive and -resistant Staphylococcus aureus. II. Effect of inoculum size.

The inoculum effect was studied on the activity of cefmetazole and cefotiam, cefamandole or cefoperazone alone and in combination against 9 strains of methicillin-sensitive Staphylococcus aureus (MSSA) and 20 strains of methicillin-resistant Staphylococcus aureus (MRSA) by means of the checkerboard titration method with Mueller-Hinton agar plate using 10(6) and 10(8) CFU/ml. The antimicrobial activity against MSSA and MRSA was potentiated synergistically in combination of cefmetazole and these cephalosporins either with inoculum size 10(6) or 10(8) CFU/ml. At a concentration of cephalosporins < or = 6.25 micrograms/ml, the combination effect of cefmetazole and cefotiam or cefamandole against MRSA was more potent than that of cefmetazole and cefoperazone. With a higher inoculum size the effect was reduced. Under these conditions with a low dose level of drugs, the enhancement of the activity of cefmetazole by cefotiam was least influenced by inoculum size among cephalosporins studied.