Rapid detection of drug-resistant Mycobacterium tuberculosis by Modified MODS assay suitable for resource-poor settings

Abstract

Background

Cross contamination and biosafety are concerns with the microscopic observation drug susceptibility assay. To address these issues, we modified the MODS technique in the current study.

Methodology/Principal findings

Two hundred and seventy-five samples were processed on LJ media and drug susceptibility was performed by the Indirect agar proportion method. A modified MODS test was done in tissue culture bottles. GenoType MTBDR plus assay was performed to detect the resistance and mutational pattern associated with the resistances. Sensitivity, specificity, positive predictive value, and negative predictive value for the detection of tuberculosis by modified MODS were 97.44%, 80.00%, 97.44%, and 80.00% respectively. The perfect agreement was seen between modified MODS and the Indirect agar proportion method for drug susceptibility testing of isoniazid (kappa = 0.923) and rifampicin (kappa = 1). The contamination rate, cost and TAT for modified MODS were less as compared to the solid media.

In the case of MDR-TB isolates S531L (66.66%) was the most prevalent mutation in the rpoB gene followed by S315T2 mutation (58.33%) and T8C (41.66%) in katG and inhA gene respectively. In hetero-resistant strains, C-15T mutation (37.50%) was the most common followed by A-16G (12.50%) in the inhA gene. In INH mono-resistant strains only two mutations were observed i.e., S-315T1(50%) and C-15T (50%) in the katG and inhA genes respectively.

Conclusions/Significance

Modified MODS proved to be cost-effective and user-friendly, with minimal risk to the handler and no cross-contamination between samples were observed. Hence, it can be used in low-income countries for early detection of tuberculosis and its resistance.

Author summary

Tuberculosis is a major health problem, particularly in developing countries. Globally efforts are being made to control this disease. However, the emergence of multi-drug resistance (MDR) is a major threat to the tuberculosis control program. Moreover, the available methods for the detection of tuberculosis and susceptibility testing are time-consuming and labour-intensive. All these methods require pre-culture before drug susceptibility testing hence the turnaround time is prolonged. The microscopic observation drug susceptibility method is a liquid-based culture method which simultaneously detects tuberculosis and its susceptibility.

Originally developed microscopic observation drug susceptibility assay is done in 24 well tissue culture plates, but problems of false positivity which likely represent cross-contamination from another positive specimen or positive controls plated on each MODS plate is the limitation of this assay. This method also poses the risk of exposure to infection to the person performing the assay. To overcome these issues, in the present study, we introduced modified MODS using screw-capped tissue culture bottles instead of tissue culture plates. The advantage of this modification is that samples and controls are processed in separate bottles which are screw-caped hence reducing chances of cross-contamination and risk of exposure to infection. The mutational pattern of drug-resistant strains is not known in this region. GenoType MTBDR plus assay was performed to detect the resistance and mutational pattern associated with the resistances.