A novel solid medium for culturing mycobacterium tuberculosis isolates from clinical specimens

Abstract

The laboratory diagnosis of tuberculosis usually relies on culture-based isolation of the causative Mycobacterium tuberculosis bacteria. We developed and evaluated the performance of MOD9, a new blood-free derivative of the MOD4 solid medium on which we previously reported for the isolation and culture of mycobacteria. First, inoculation of Lowenstein-Jensen medium with 21 M. tuberculosis isolates at 105, 103, and 10 CFU yielded colonies in 5.7±1.5 days, 7.6±0.8 days, and 10.8±1.7 days versus 1.5±0.4 days, 3.5±0.6 days, and 4.9±1 days in MOD9 (P<0.05, Student's t test). Further, the time to detectable growth of M. tuberculosis was measured on MOD9 and Lowenstein-Jensen media after duplicate inoculation of 250 clinical specimens decontaminated with 0.7% chlorhexidine. The contamination rate was 1.6% (4/250) on MOD9 versus 4.4% (11/250) on Lowenstein-Jensen medium (P=0.11, Fisher's exact test). Chlorhexidine-MOD9 yielded 38/250 (15.2%) isolates versus 32/250 (12.8%) isolates for the chlorhexidine-LJ (P=0.5195, Fisher's exact test). All together, eight M. tuberculosis isolates were cultured solely from chlorhexidine-MOD9, and two M. tuberculosis isolates were cultured solely from chlorhexidine-LJ. The time to detection was 9.8±3.9 (range, 5 to 18) days for chlorhexidine-MOD9 versus 17.4-5.9 (range, 10 to 35) days for chlorhexidine-LJ (P< 0.05, Student's t test). These data indicate the superiority of the MOD9 medium over the standard LJ medium following chlorhexidine decontamination for the recovery of M. tuberculosis.