Resistance of group A beta-hemolytic streptococci to lincomycin and erythromycin

Ten (0.05%) of 18,628 strains of Streptococcus pyogenes isolated from clinical specimens in the 3 years 1968 to 1970 were resistant to lincomycin and erythromycin. All 10 strains were highly resistant to lincomycin, having minimal inhibitory concentration (MIC) values of 200 mug/ml. There were two degrees of resistance to erythromycin: four strains were highly resistant, having MIC values of 200 mug or more/ml; and six strains showed slight resistance, MIC values being 0.78 to 1.56 mug/ml. There was no known epidemiological relationship between any of the patients infected with the resistant strains, which belonged to a variety of T serotypes. A zonal pattern of resistance to lincomycin occurred in four strains, all of which were only slightly resistant to erythromycin. After incubation for 24 hr in a twofold dilution series of lincomycin in broth, the strains grew in 0.05 mug or less/ml and in 50 and 100 mug/ml, but not in intermediate concentrations. Tests in agar indicated that the bacterial population of one strain, but not of the other three, was homogeneous in respect to its ability to grow readily in low and high, but not in intermediate, concentrations. The zone phenomenon is of significance in the clinical laboratory, since unawareness of it might result in a highly resistant strain being regarded as susceptible to lincomycin in tube or plate MIC tests that do not include sufficiently high concentrations of lincomycin.