Rapid determination of MICs of 15 antichlamydial agents by using an enzyme immunoassay (Chlamydiazyme)

An enzyme immunoassay (EIA), Chlamydiazyme (Abbott Laboratories), was evaluated for its determination of MICs of 15 antimicrobial agents against Chlamydia trachomatis (MRC-1, LB, TRIC/GB/MRC-1 Gf [ATCC VR-1]). The inoculum size, incubation time, and enhancers were defined for the assessment of chlamydial antigen synthesis in HeLa 229 cells seeded as monolayers in 96-well plates. MICs were determined and defined as the lowest antibiotic concentrations required to inhibit, after 24 or 48 h of incubation, antigen production as determined by the EIA. The MICs (after 48 h) were similar to those determined by the peroxidase-antiperoxidase staining of inclusions. MIC determinations after 24 h were suitable for screening the activities of quinolones, but less so for measuring the susceptibility of C. trachomatis to macrolides and tetracyclines. MIC determination by EIA was rapid, appropriate for standardization, and less cumbersome than determination by quantification of inclusions.