[Penicillin-resistance as indicator of resistance of Staphylococcus aureus towards cephalosporines and structure-related substances (author's transl)].

Abstract

81 strains of Staphylococcus aureus (41 methicillin-resistant and 40 -sensitive ones) were tested against older and newer cephalosporines in both broth-dilution and agardiffusion-tests using Mueller-Hinton (MH)-broth and MH-agar respectively in order to establish the degree of parallel-resistance. The substances used were cephalothin, cefazolin, cephalexin, cefamandol, cefuroxim, cefoxitin, cefotaxim and cefsulodin. Furthermore, for reasons of comparison the relatively new substance "Oxabetalaktam" was included in the investigation. As shown in Fig. 1 and Table 1 all methicillin-resistant strains required at the average at least 10 times the concentrations of cephalosporine (excepting cefsulodin) which was necessary to inhibit methicillin-sensitive strains. Again excepting cefsulodin, for each cephalosporine there was a clear bimodal distribution indicating a clear separation of both populations of strains: methicillin-sensitive and -resistant ones. Cephalothin cannot be used as test substance in agardiffusion-tests with staphylococci as there is no correlation between MIC and the inhibition zone size (Fig. 2). This is not necessary, anyway, since all methicillin-resistant strains must be regarded as resistant against virtually all cephalosporines available on the market (with the possible exception of cefamandol). By contrast, all methicillin-sensitive strains may be attacked successfully by concentrations of cephalosporines that are thought to be also effective in vivo. Since in agardiffusion-tests methicillin-resistant strains of staphylococcus aureus are recognizable as easily as are otherwise merely penicillinase-producing ones (5) by using a paper disk loaded with 6 microgram benzyl-penicillin and since infections due to other grampositive organisms than staphylococci are no indication for treatment with cephalosporines there is no need to test any other betalactam-antibiotic than benzyl-penicillin with gram-positive organisms.