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 causativeMycobacterium tuberculosisbacteria. 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 21M. tuberculosisisolates 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'sttest). Further, the time to detectable growth ofM. tuberculosiswas 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, eightM. tuberculosisisolates were cultured solely from chlorhexidine-MOD9, and twoM. tuberculosisisolates 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'sttest). These data indicate the superiority of the MOD9 medium over the standard LJ medium following chlorhexidine decontamination for the recovery ofM. tuberculosis.