Field evaluation of the direct detection of multidrug resistant Mycobacterium tuberculosis by nitrate reductase assay on 7H11 agar

The Effect of Inoculum Size on Antimicrobial Susceptibility Testing of Mycobacterium tuberculosis

Phenotypic drug susceptibility testing (DST) requires a standardized amount of inoculum to produce reproducible susceptibility results. The most critical step in the application of DST in Mycobacterium tuberculosis isolates is the preparation of the bacterial inoculum. In this study, the effect of bacterial inoculum prepared in various McFarland turbidities on primary antituberculosis drug susceptibility of M. tuberculosis strains was investigated. Five standard ATCC strains (ATCC 27294 [H37Rv], ATCC 35822 [izoniazid-resistant], ATCC 35838 [rifampicin-resistant], ATCC 35820 [streptomycin-resistant], ATCC 35837 [ethambutol-resistant]) were tested. Inoculums of McFarland standard of 0.5, 1, 2, 3, and 1:100 dilutions of 1 McFarland standard of each strain were used. The effect of inoculum size on DST results was determined by the proportion method in Lowenstein-Jensen (LJ) medium and nitrate reductase assay (NRA) in the LJ medium. In both test methods, the increase in inoculum size did not affect the DST results of the strains. On the contrary, DST results were obtained more rapidly as a result of the use of dense inoculum. DST results obtained in all McFarland turbidities were found to be 100% compatible with the recommended amount of inoculum, 1:100 dilution of 1 McFarland standard (inoculum size of gold standard method). In conclusion, the use of a high amount of inoculum did not change the drug susceptibility profile of tuberculosis bacilli. Minimizing manipulations during the inoculum preparation phase of susceptibility testing, this outcome will decrease the need for equipment and make the test application easier, particularly in developing countries. IMPORTANCE During DST application, it can be challenging to evenly homogenize TB cell clumps with lipid-rich cell walls. These experiments must be carried out under Biosafety Level-3 (BSL-3) laboratory conditions with personal protective equipment and taking safety precautions because the procedures applied at this stage cause the formation of bacillus-laden aerosols and carry a serious risk of transmission. Considering this situation, this stage is important given that it is not possible to establish a BSL-3 laboratory in poor and developing countries. Reducing the manipulations to be applied during the preparation of bacterial turbidity will minimize the risk of aerosol formation. Perhaps there will be no need to do these steps for susceptibility tests in these countries or even in developed countries.

Evaluation of nitrate reductase assay in 7H11 agar for diagnosis of multi-drug-resistant tuberculosis in eastern Nepal

Background

Emergence of multi-drug-resistant tuberculosis is a serious challenge for successful global tuberculosis control. Early diagnosis of drug-resistant tuberculosis by direct nitrate reductase assay (NRA) aids in appropriate treatment and reduction in disease transmission, particularly in countries with high tuberculosis burden. The aim of this study was to evaluate the performance of NRA for direct detection of resistance to rifampicin and isoniazid in Mycobacterium tuberculosis in laboratories with limited resources.

Methods

Fifty-eight new smear-positive sputum samples were processed as per the guidelines of revised national tuberculosis control program, India. The performance of NRA on middlebrook 7H11 agar was evaluated for detection of rifampicin and isoniazid resistance directly on smear-positive sputum specimens, and the results were compared with conventional proportion method. Sensitivity and specificity of the test were compared with the gold standard proportion method. Mc Nemar chi-square test was used to find out the significant difference between two methods.

Results

Direct NRA for detection of rifampicin resistance was 85.7% sensitive and 100% specific, whereas sensitivity and specificity of isoniazid resistance were 87.5% and 100%, respectively. Agreement between NRA and proportion method was 98% for both the drugs. The mean days of drug susceptibility testing results were 19.3 days for NRA and 72 days for conventional proportion method. The results of NRA were available in 21 days for 83% of the samples.

Conclusions

Direct NRA on middlebrook 7H11 medium is a highly sensitive, reliable, and significantly faster method to perform drug susceptibility testing. It has the potential to be implemented for rapid detection of multi-drug-resistant tuberculosis against insufficient resources.

Systematic review and meta-analysis of the nitrate reductase assay for drug susceptibility testing of Mycobacterium tuberculosis and the detection limits in liquid medium

Recently, the need for rapid, reliable, and low-cost drug susceptibility testing (DST) methods has increased due to the emergence of multidrug-resistant Mycobacterium tuberculosis. Colorimetric methods of DST provide results more quickly than standard culture methods and are inexpensive than molecular methods. Thus, colorimetric methods, such as the nitrate reductase assay (NRA), are being recommended. We searched Medline PubMed for reports on the NRA for DST of M. tuberculosis written in English and published within the last five years. We selected 20 reports on six major anti-TB drugs and conducted a meta-analysis using Meta-Disc software. The pooled sensitivities for isoniazid, rifampicin, streptomycin, ethambutol, ofloxacin, and kanamycin were 95.4%, 96.4%, 91.5%, 93.1%, 99.3%, and 88.4%, and the pooled specificities were 98.5%, 99.2%, 92.9%, 97.8%, 97.4%, and 99.4%, respectively. The area under the summary receiver operator curve for all drugs was 0.9723-0.9952. The time to results (TTR) for the direct and indirect NRAs was 7-28days and 6-15days, respectively. Quality assessments were conducted using the quality of diagnostic accuracy studies tool (QUADAS-2) items, and most reports showed good performance. However, ethambutol, streptomycin, and kanamycin showed relatively low sensitivity. We performed a quantitative NRA in liquid media at various inoculum concentrations. The TTR at 4.94×10⁶, 1.67×10⁴, and 2.27×10²CFU/mL was 4, 14, and 14days, respectively. The minimum absorbance and nitrite concentration for positive samples were 0.8 and 168μM, respectively. We propose a quantitative standard to determine sample positivity to address the problems with the current standard NRA which is much less expensive than the conventional assay conducted on solid medium.