Effect of carbon dioxide on testing of susceptibilities of respiratory tract pathogens to macrolide and azalide antimicrobial agents

The in vitro activities of erythromycin, azithromycin, and clarithromycin a gainst 178 clinical isolates from the lower respiratory tract of patients w ith chronic obstructive pulmonary disease were determined by an agar diluti on method. The plates mere incubated in air alone or in 5% carbon dioxide. The MICs measured in air alone were lower for most isolates than those meas ured in 5% carbon dioxide, illustrating the "pH effect" of incubation in ca rbon dioxide. Testing of isolates in 5% carbon dioxide on ps-adjusted mediu m (pH 8.4) resulted in MICs of one or two doubling dilutions lower than tho se obtained on agar with a neutral pH. A bioassay of the three agents incub ated in air and in 5% carbon dioxide resulted in a significant loss of acti vity of all three agents in the carbon dioxide-enriched atmosphere. However , this loss-of-activity effect was significantly reduced when the bioassay medium was adjusted to pH 8.4 prior to incubation in 5% carbon dioxide.