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Antimicrobial Agents and Chemotherapy, August 1999, p. 1862-1865, Vol. 43, No. 8
0066-4804/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Effect of Carbon Dioxide on Testing of Susceptibilities of Respiratory Tract Pathogens to Macrolide and Azalide Antimicrobial Agents

M. M. Johnson,1 S. L. Hill,2 and Laura J. V. Piddock1,*

Antimicrobial Agents Research Group, Division of Immunity and Infection, University of Birmingham,1 and Department of Respiratory Medicine, Queen Elizabeth Hospital,2 Birmingham, United Kingdom

Received 17 September 1998/Returned for modification 8 March 1999/Accepted 27 May 1999

The in vitro activities of erythromycin, azithromycin, and clarithromycin against 178 clinical isolates from the lower respiratory tract of patients with chronic obstructive pulmonary disease were determined by an agar dilution method. The plates were incubated in air alone or in 5% carbon dioxide. The MICs measured in air alone were lower for most isolates than those measured in 5% carbon dioxide, illustrating the "pH effect" of incubation in carbon dioxide. Testing of isolates in 5% carbon dioxide on pH-adjusted medium (pH 8.4) resulted in MICs of one or two doubling dilutions lower than those obtained on agar with a neutral pH. A bioassay of the three agents incubated in air and in 5% carbon dioxide resulted in a significant loss of activity 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.

* Corresponding author. Mailing address: Antimicrobial Agents Research Group, Division of Immunity and Infection, University of Birmingham, Birmingham B15 2TT, United Kingdom. Phone: 0121 414 6966. Fax: 0121 414 6966. E-mail: l.j.v.piddock{at}bham.ac.uk.

Antimicrobial Agents and Chemotherapy, August 1999, p. 1862-1865, Vol. 43, No. 8
0066-4804/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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