The regression line for erythromycin is not valid for beta-hemolytic streptococci group A.

The antibacterial activity of erythromycin to 50 beta-hemolytic streptococci group A was assessed using disc diffusion and MIC techniques on solid media at pHs 6, 7 and 8 in CO2, aerobic and anaerobic atmospheres. The majority of the strains were only intermediately sensitive to erythromycin according to the disc diffusion method, whereas they were sensitive as judged by the MIC determination. This demonstrates that the regression line is invalid for the drug-bug combination of erythromycin and beta-hemolytic streptococci group A. It was also shown that the activity of erythromycin is reduced at a lower pH and in CO2 incubation.