Effect of carbon dioxide on in vitro susceptibility of anaerobic bacteria to erythromycin

The activity of erythromycin against 317 strains of anaerobic bacteria, including 133 strains of the Bacteroides fragilis group, was tested by the agar dilution method in an anaerobic atmosphere with two different concentrations of carbon dioxide and without CO₂. The effect of the atmosphere of incubation on the agar surface pH was also determined. All strains grew well in the GasPak (GP) environment. However, 3.5 and 30.3% of strains failed to grow in the 2 and 0% CO₂ environments, respectively. The quality of growth was best in the GP environment and poorest in the 0% CO₂ environment. Minimal inhibitory concentrations in the GP and 2% CO₂ environments were frequently the same or one dilution lower in the 0% than in the GP environment. In the 0% CO₂ atmosphere, minimal inhibitory concentrations were usually two to three dilutions lower than in the GP environment. Consequently, only 24% of B. fragilis strains were susceptible to erythromycin in the GP environment, whereas 77% were susceptible in the 0% CO₂ environment. For Fusobacterium species, 12% were susceptible to erythromycin in the GP environment, and 73% were susceptible in the 0% CO₂ environment. There was a comparable decrease in pH in all three atmospheres tested. In vitro susceptibility testing of erythromycin against anaerobic bacteria should be performed in an atmosphere containing carbon dioxide.