The effect of carbon dioxide on the in vitro activity of erythromycin and RU-28965 against anaerobic bacteria.

The in vitro activity of erythromycin and RU-28965 (a novel macrolide antimicrobial with improved pharmacokinetics) was determined against a variety of anaerobic bacteria in anaerobic atmospheres with and without added carbon dioxide. Minimum inhibitory concentrations (MIC) were determined using an antimicrobial incorporation technique in Wilkins-Chalgren agar (Oxoid, UK) containing saponinlysed horse blood to a final concentration of 10%. The inoculum used was approximately 10(4) colony forming units (cfu) contained in 10 microliters Wilkins-Chalgren broth, which was applied to the surface of the agar plates using a multipoint inoculator. Following inoculation, plates were incubated for 48 h at 37 degrees C in an anaerobic atmosphere containing 10% carbon dioxide or in hydrogen alone. The MIC of each antimicrobial for each organism examined was determined as the lowest concentration of the antimicrobial which completely inhibited growth of the inoculum. The minimum concentrations required to inhibit the growth of 50% (MIC50) and 90% (MIC90) of the bacteria examined were also determined. The MICs of erythromycin and RU-28965 for isolates of the Bacteroides fragilis group, B. bivius and Fusobacterium spp. were generally 10-100 times greater when determined in the presence of carbon dioxide than when determined in hydrogen alone. The MICs of erythromycin and RU-28965 for B. melaninogenicus, Peptococcus spp., Peptostreptococcus spp., Clostridium perfringens, Cl. difficile and Propionibacterium acnes were less affected by the presence of carbon dioxide.