Effect of extended pre-incubation with chlamydia pneumoniae and extended incubation with antimicrobial on the minimum inhibitory concentrations (MICs) of five antimicrobials

There is no single standard methodology for in vitro susceptibility testing for Chlamydia pneumoniae, but many investigators pre-incubate this organism with the cell monolayer for 1 h prior to adding antimicrobial and incubating for 72 h. The aim of this study was to determine the effect of extended C. pneumoniae pre-incubation, and extended incubation in the presence of antimicrobial, on the MICs of 5 antimicrobials. MICs were determined for 5 ATCC strains of C. pneumoniae by employing similar methods as those previously described in the literature. MICs were then determined following 1, 4, 6, 20 and 24 h C. pneumoniae pre-incubation. Finally, MICs were determined following 1 and 24 h C. pneumoniae pre-incubation, and 48 and 72 h incubation with antimicrobial and organism. Extending the incubation time in the presence of antimicrobial from 48 to 72 h had little or no effect on MICs. Similarly, pre-incubation periods of less than 20 h had little effect on MICs, but MICs increased significantly with 20 and 24 h pre-incubation.