Evaluation of efficiency of nested multiplex allele-specific PCR assay for detection of multidrug resistant tuberculosis directly from sputum samples

Novel mutations detected from drug resistant Mycobacterium tuberculosis isolated from North East of Thailand

The emergence of drug-resistant tuberculosis is a major global public health threat. Thailand is one of the top 14 countries with high tuberculosis and multi-drug resistant tuberculosis rates. Immediate detection of drug-resistant tuberculosis is necessary to reduce mortality and morbidity by effectively providing treatment to ameliorate the formation of resistant strains. Limited data exist of mutation profiles in Northeastern Thailand. Here, 65 drug-resistant Mycobacterium tuberculosis isolates were used to detect mutations by polymerase chain reaction (PCR) and DNA sequencing. In the katG gene, mutations were occurred in 47 (79.7%) among 59 isoniazid resistant samples. For rpoB gene, 31 (96.9%) were observed as mutations in 32 rifampicin resistant isolates. Of 47 katG mutation samples, 45 (95.7%) had mutations in katG315 codon and 2 (4.3%) showed novel mutations at katG365 with amino acid substitution of CCG-CGG (Pro-Arg). Moreover, out of 31 rpoB mutation isolates, the codon positions rpoB516, rpoB526, rpoB531 and rpoB533 were 3 (9.7%), 8 (25.8%), 11 (35.5%) and 1 (3.2%), respectively. Seven isolates of double point mutation were found [rpoB516, 526; 1 (3.2%) and rpoB516, 531; 6 (19.4%)]. In addition, 1 (3.2%) sample had triple point mutation at codon positions rpoB516, 526 and 531. Common and novel mutation codons of the rpoB and katG genes were generated. Although DNA sequencing showed high accuracy, conventional PCR could be applied as an initial marker for screening drug-resistant Mycobacterium tuberculosis isolates in limit resources region. Mutations reported here should be considered when developing new molecular diagnostic methods for implementation in Northeastern Thailand.

Revisiting the methods for detecting Mycobacterium tuberculosis: what has the new millennium brought thus far?

Tuberculosis (TB) affects around 10 million people worldwide in 2019. Approximately 3.4 % of new TB cases are multidrug-resistant. The gold standard method for detecting Mycobacterium tuberculosis, which is the aetiological agent of TB, is still based on microbiological culture procedures, followed by species identification and drug sensitivity testing. Sputum is the most commonly obtained clinical specimen from patients with pulmonary TB. Although smear microscopy is a low-cost and widely used method, its sensitivity is 50-60 %. Thus, owing to the need to improve the performance of current microbiological tests to provide prompt treatment, different methods with varied sensitivity and specificity for TB diagnosis have been developed. Here we discuss the existing methods developed over the past 20 years, including their strengths and weaknesses. In-house and commercial methods have been shown to be promising to achieve rapid diagnosis. Combining methods for mycobacterial detection systems demonstrates a correlation of 100 %. Other assays are useful for the simultaneous detection of M. tuberculosis species and drug-related mutations. Novel approaches have also been employed to rapidly identify and quantify total mycobacteria RNA, including assessments of global gene expression measured in whole blood to identify the risk of TB. Spoligotyping, mass spectrometry and next-generation sequencing are also promising technologies; however, their cost needs to be reduced so that low- and middle-income countries can access them. Because of the large impact of M. tuberculosis infection on public health, the development of new methods in the context of well-designed and -controlled clinical trials might contribute to the improvement of TB infection control.

Rapid detection of drug-resistant Mycobacterium tuberculosis directly from clinical specimens using allele-specific polymerase chain reaction assay

Background & objectives:

Rapid detection of drug resistance in Mycobacterium tuberculosis (MTB) is essential for the efficient control of tuberculosis. Hence, in this study a nested-allele-specific (NAS) PCR, nested multiple allele-specific PCR (NMAS-PCR) and multiple allele-specific (MAS) PCR assays were evaluated that enabled detection of the most common mutations responsible for isoniazid (INH) and rifampicin (RIF) resistance in MTB isolates directly from clinical specimens.

Methods:

Six pairs of primers, mutated and wild type, were used for the six targets such as codon 516, 526 and 531 of rpoB, codon 315 of katG and C15-T substitution in the promoter region of mabA-inhA using allele-specific (AS) PCR assays (NAS-PCR, NMAS-PCR and MAS-PCR). The performance of AS PCR method was compared with phenotypic drug susceptibility testing (DST).

Results:

The usefulness of AS PCR assays was evaluated with 391 clinical specimens (251 Acid fast bacilli smear positive and MTB culture positive; 93 smear negative and MTB culture positive; 47 smear positive and MTB culture negative) and 344 MTB culture positive isolates. With culture-based phenotypic DST as a reference standard, the sensitivity and specificity of the NAS-PCR, NMAS-PCR and MAS-PCR assay for drug resistance-related genetic mutation detection were 98.6 and 97.8 per cent for INH, 97.5 and 97.9 per cent for RIF and 98.9 and 100 per cent for multidrug resistance (MDR).

Interpretation & conclusions:

The performance of AS PCR assays showed that those could be less expensive and technically executable methods for rapid detection of MDR-TB directly from clinical specimens.