A modified broth dilution assay for antibiotic sensitivity testing of Mycobacterium avium-intracellulare using paraffin slide cultures

Mycobacterium avium-intracellulare (MAI) can utilize paraffin wax as the sole carbon source in basal media. Paraffin slide culture (Para SL/C) has been employed for isolation and speciation of MAI derived from clinical sources. We have evaluated an adaptation of this method for antimicrobial sensitivity testing. Sixteen clinical isolates of MAI were tested against ciprofloxacin amikacin, and azithromycin by Para SL/C and compared with sensitivities obtained with a conventional troth microtiter procedure. The system can be performed rapidly over a median time interval of 6–8 days. The MIC was defined as the lowest concentration of antimicrobial agent necessary to inhibit growth on paraffin wax coated slides. With Para SL/C, the MIC values were determined at the time when the corresponding control tubes showed confluent growth. The procedure was reproducible with all of the agents tested. The MIC₅₀ and MIC₉₀ values obtained from the Para SL/C assay and from serial broth microtiter dilutions correlated well for ciprofloxacin and amiacin. However, results of the MIC₅₀ and MIC₉₀ for azithromycin did not correlate.