Activity of ten cephalosporins on biomass of methicillin-susceptible and -resistant Staphylococcus aureus.

The growth curves automatically recorded and printed during the action of 10 cephalosporins on methicillin-susceptible and methicillin-resistant Staphylococcus aureus showed the following. (i) The biomass of methicillin-susceptible S. aureus exposed to the cephalosporins increased before lysis occurred (inoculum, 10(6) colony-forming units per ml). Lysis was more rapid with cephalothin and cephaloridine, whose minimal inhibitory concentrations were lowest. (ii) The same biomass increase followed by lysis occurred with methicillin-resistant S. aureus, and the speed of lysis was not different from those of cephalothin (without any regrowth), cefoxitin (with regrowth of a few strains), and cephaloridine (regrowth of all strains), with methicillin-susceptible strains. A 2-log increase of inoculum (10(8) colony-forming units per ml) did not modify significantly the speed of lysis with cephalothin, cephaloridine, and cefoxitin, but regrowth sometimes occurred. The early transitory lysis caused by cephaloridine, cephalothin, cefamandole, and cefoxitin was not suppressed by preincubation with 32 mug of methicillin per ml, but regrowth occurred more frequently. No lysis could be observed with cefazolin, cefotaxime, cephalexin, cephradine, cefuroxime, and cefaclor unless high concentrations were achieved. (iii) From a practical point of view, the early response of the growth curve (4 h) could not determine in every case whether a strain of S. aureus was resistant or susceptible to cephalosporin. A further study of the growth curve (18 of 24 h) was necessary for this purpose. Results obtained after a few hours with automated systems should be interpreted with great caution.