Effects of carbon dioxide upon the in vitro activity of erythromycin

The in vitro activity of erythromycin against clinical isolates of Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pneumoniae, Streptococcus pyogenes and Haemophilus influenzae was examined by agar dilution and agar diffusion methods. The plates were incubated in air alone or in 8% CO2 and air. The minimal inhibitory concentrations (MICs) measured in air alone were lower for most of the isolates, compared to those found in 8% CO2. The greatest differences in MIC values were found for H. influenzae; the MIC 50% was 0.5 mg/l in air and 4 mg/l in 8% CO2. Sensitivity testing by the agar diffusion method (ICS) showed considerable differences between results obtained in air and in 8% CO2; the inhibition zones were generally smaller in CO2. The most marked reduction in zone sizes after incubation in 8% CO2 was seen with the H. influenzae isolates; 15 out of 43 isolates moved from the "sensitive" to "moderately sensitive" group. Sensitivity determination of aerobic bacteria for erythromycin should be performed in air alone in the routine laboratory.