Use of a heavy inoculum in the in vitro evaluation of the anti-staphylococcal activity of 19 cephalosporins

Abstract

The in vitro activity of 19 cephalosporins against 105 clinical isolates of Staphylococcus aureus and S. epidermidis was determined by using a heavy inoculum, i.e., 10(8) to 10(9) organisms per ml, to maximally challenge the antibiotics. The anti-staphylococcal activities of cephaloridine and 87/312 were consistently decreased by the use of a heavy inoculum when compared with the activity obtained with two less-concentrated inocula. The activity of most of the other compounds was also decreased with the use of a heavy inoculum, but this was observed only with selected isolates. Cephapirin, cephalothin, and cefazaflur were the most active drugs against the methicillin-susceptible isolates. Cephaloridine, cefamandole, cefazaflur, and 87/312 had substantial activity against methicillin-resistant staphylococci even with heavy inocula. With the exception of cefaclor against S. aureus, the orally absorbed cephalosporins were generally one-half to one-sixteenth as active as the parenterally administered cephalosporins. The median minimal inhibitory concentrations of five of the 12 parenteral cephalosporins were lower with the methicillin-susceptible S. aureus than with the methicillin-susceptible S. epidermidis strains.