[A simple method for differentiating methicillin-resistant, penicillinase-positive and -negative staphylococci by...

Coagulase-positive staphylococci (41 methicillin-resistant strains, 28 penicillinase-positive and 12 penicillinase-negative strains) were tested against most types of penicillines commercially available on the Austrian market using both broth-dilution test (incubated during 48 hours at 35 degrees C) and agardiffusion tests (incubated during 24 hours at 30, 35 and 37 degrees C) employing Mueller-Hinton-broth and -agar, respectively, in order to find out the most convenient way of detecting methicillin-resistant strains. Consecutively, the conclusions drawn from these experiments were verified for tests on Isosensitest-agar (Oxoid). It was demonstrated that methicillin-resistant strains could be detected easily with discs of methicillin and oxacillin at 30 degrees C (Fig. 1). At 35 degrees C this was nearly as easily possible for methicillin but oxacillin discs had to be used at amounts of 1 microgram instead of 5 microgram. Excepting penicillin G-discs with the other penicillines differing numbers of methicillin-resistant strains would have been missed at 37 degrees C (Fig. 1, 2). Only with discs containing 6 microgram penicillin G methicillin-resistant strains were unequivocally identifiable in the agardiffusion test at all 3 incubation-temperatures (Fig. 3), the largest inhibition zone diameter being 12 mm. Penicillinase-positive but methicillin-sensitive strains always produced larger inhibition zones up to 30 mm. From these strains again penicillinase-negative strains were equally well distinguishable by much larger inhibition zones. So, the conclusion was drawn that on Mueller-Hinton agar one disc loaded with 6 microgram of penicillin G allows proper distinction not only of methicillin-resistant and -sensitive but also of penicillinase-positive and -negative strains of staphylococci. On Isosensitest-agar this is true for an incubation-temperature of 35 but not 37 degrees C (Tab. 1).