Effect of culture medium and carbon dioxide concentration on growth of anaerobic bacteria and medium pH

Two Danish agars, Danish Blood Agar (D.B.A.) and Anaerobic Agar (A.A.), were evaluated for their ability to support growth of 47 clinically isolated anaerobic strains in 5 different CO2-concentrations ranging from 0-10% CO2. CO2 and the use of an enriched medium (A.A.) are essential for satisfactory recovery of anaerobes. No gain could be seen when raising the CO2-concentration above 5%. The surface pH of the agars was measured both on non-inoculated and inoculated plates at room temperature and anaerobic incubation in the 5 different CO2-concentrations at 37 degrees C. Temperature change from room temperature to 37 degrees C resulted in a pH decrease of 0.1 units. There was a CO2-mediated decrease in pH (approximately 0.05 units/pr. CO2%) on non-inoculated media. On inoculated plates there was a minor additional fall in pH, which increased with time of incubation, but first became significant when the plates were incubated for more than 24 h. The use of 5% CO2 and A.A. is recommended for antimicrobial susceptibility studies on solid media.