Rapid PCR test measures tuberculosis bacteria directly from samples

Estimating how many tuberculosis bacteria are present in a patient’s sample matters for judging how sick they are, how well treatment is working, and how much DNA is available for sequencing. But rapid, practical tools to count Mycobacterium tuberculosis in routine clinical samples have been limited. Renu Verma and colleagues set out to build and test a direct quantification method that could work on real patient material. They developed a quantitative PCR approach that combines high-resolution melt (HRM) analysis with molecular beacon chemistry and targets the single-copy RD9 region of Mycobacterium tuberculosis. The team checked how the assay performed against laboratory DNA using 10-fold serial dilutions of H37Rv DNA and then validated it clinically. Validation samples included DNA from 100 M. tuberculosis culture isolates and 40 sputum samples taken from Xpert MTB/RIF- and culture-positive pulmonary TB patients. To probe specificity, they tested 30 non-tuberculous mycobacterium (NTM) culture isolates from patients infected with Mycobacterium abscessus (n=25) and Mycobacterium fortuitum (n=5), and DNA from saliva samples of 10 healthy controls.

The HRM-qPCR assay showed a clear analytical performance profile. It produced a linear dynamic range from 10^1 to 10^6 genome copies per reaction and a lower limit of detection of 10 copies. Standardized melt-curve analysis produced a single target-specific peak at 73.7±0.12°C across the dilution series, supporting specific amplification of the RD9 target. In clinical testing the assay detected M. tuberculosis in 100% of culture isolates and in 95.0% (38/40) of sputum samples from Xpert MTB/RIF- and culture-positive patients, with no false positives among M. tuberculosis-negative controls. Specificity was 100% in both culture isolates and sputum, with no extra melt peaks observed. The team saw no non-specific melt peaks in most NTMs, although two NTM samples did amplify and showed a Tm matching the M. tuberculosis RD9 peak (median M. tuberculosis copies = 678.5), which the authors suggest could reflect co-infection or contamination. Saliva from healthy controls showed no amplification or specific melt peaks, indicating high probe specificity.

Taken together, these findings show that a molecular beacon-based HRM-qPCR assay can rapidly and specifically quantify M. tuberculosis genome copies directly from clinical material. Because it measures genome copy number across a wide dynamic range and gives a clear, reproducible melt-curve signature, the assay could help clinicians and laboratories in several ways: monitoring treatment response by tracking bacterial load, deciding which specimens have enough DNA to send for sequencing, and assessing transmission risk in high-burden settings by identifying patients with higher genome counts. The observation of two NTM samples with matching Tm highlights that occasional ambiguous results may require follow-up to rule out co-infection or contamination. Overall, the study presents a sensitive, specific technique that could strengthen TB clinical management and laboratory workflows where rapid quantification of M. tuberculosis is needed.