Development of a rapid method for determining the susceptibility of Mycobacterium tuberculosis to isoniazid using the Gen-Probe DNA Hybridization System.

A rapid test was developed for determining the susceptibility of Mycobacterium tuberculosis to isoniazid by using nucleic acid hybridization. The method was based on quantification of total mycobacterial rRNA hybridized to a 125I-labeled DNA probe in the absence and presence of various concentrations of isoniazid. The radioactive hybridized complex was isolated by adsorption to hydroxyapatite crystals and measured in a gamma counter. The susceptibilities of four reference strains and 20 clinical isolates were compared by the Gen-Probe DNA Hybridization System and the critical concentration method. Overall agreement between the two methods was excellent. Results were obtained with the DNA probe after 3 to 5 days of incubation instead of the 14 to 21 days required for the critical concentration method. These findings indicate that susceptibility testing of M. tuberculosis by nucleic acid hybridization has merit for the clinical laboratory. Additional studies are needed to determine the efficacy of the DNA probe method for determining the susceptibility of M. tuberculosis to other antimycobacterial agents and its correlation with clinically significant levels of resistance.