Middlebrook 7h11 reduces invalid results and turnaround time of phenotypic drug-susceptibility testing of M. tuberculosis

Abstract

Background: Phenotypic drug-susceptibility testing (pDST), which relies on growth inhibition in the drug-containing media, remains a challenge for fastidious Mycobacterium tuberculosis complex (MTBc) isolates due to insufficient growth on the growth controls (GC). Middlebrook 7H11 (M7H11) medium contains casein hydrolysate, which may favor the growth of such strains. Method: In this study, we tested whether M7H11 reduces invalid results due to insufficient growth on the GCs and the turnaround time (TAT) of pDST for MTBc compared to Middlebrook 7H10 (M7H10) without affecting the accuracy of the pDST results and how it differs between rifampicin- and isoniazid-susceptible non multi-drug resistant (non-MDR), MDR and MDR with additional resistance to fluoroquinolones (Pre-XDR) MTBc isolates. We compared the proportions of invalid pDST results due to lack of growth on the GCs, TATs of valid parallel drug-susceptibility testings as an indicator of speed of MTBc growth, and colony-forming unit (CFU) count on the most diluted GC of the parallel pDSTs after equal incubation periods as an indicator of growth abundance on M7H11 and M7H10. We also analyzed the agreement between the pDST results of the same drug or drugs in the same drug class, tested in parallel on both media. Results: For MDR and pre-XDR isolates, relative to M7H10, M7H11 significantly reduced the occurrence of invalid pDST results due to insufficient growth on the GCs (odds ratio [OR] = ∞ [95% confidence interval (CI) 1.9–∞], P = 0.004 for MDR, OR = ∞ [95% CI 3.3–∞], P = 0.0001 for pre-XDR) and the TAT of pDSTs (OR = 17 [95% CI 2.6–710.4], P = 0.0001 for MDR, OR = 9.3 [95% CI 4.0–26.5], P 0.98). Conclusion: Our study findings suggest that M7H11 is preferred over M7H10 for pDSTs of MTBc isolates.

Keywords

• mycobacterium tuberculosis
• drug-susceptibility testing
• proportion method