Decoding Xpert MTB/RIF Assay
As a surrogate marker for the diagnosis of multidrug-resistant tuberculosis, rifampicin is one of the most effective first-line anti-TB medications. In 1993, Telenti et al. reported a significant degree of amino acid code similarity between multiple bacterial species, including E. coli, M. tuberculosis and M. leprae and proposed that rifampicin resistance in the M. tuberculosis complex was linked to changes in the rpoB gene.
The rpoB gene, which codes for the β subunit of RNA polymerase needed for RNA transcription, is primarily linked to rifampicin mutations. The gene has 3519 bp and 1173 codons. However, mutations in an 81-bp rifampicin resistance-determining region (RRDR) of the rpoB gene, which corresponds to codons 507 to 533 (according to the Escherichia coli numbering system), account for 95% of cases.
Even though the GeneXpert results don't pinpoint the precise location of the mutation in the rpoB gene, comparing them to the results of other tests can help us determine the kind of resistance. The wild type and mutant forms linked to rifampicin resistance can be distinguished from one another with the help of these probes of GeneXpert.
Since different mutations affect bacterial physiology and survival in different ways, understanding the kinds, frequencies and geographical correlates of these mutations may affect future interventions. Rifampicin resistance must be quickly and accurately detected in order to diagnose and treat drug-resistant and multidrug-resistant (MDR) tuberculosis.
Rapid molecular diagnostics have supplanted traditional drug-sensitivity testing, opening up new research avenues that could one day aid in the development of novel probes for a variety of alleles. These new technologies have shown to be very effective and it wouldn't be incorrect to argue that they are helping nations where tuberculosis is more prevalent and providing a faster path to diagnosis and treatment.
References
1.Andre E, Goeminne L, Cabibbe A, Beckert P, Mukadi BK, Mathys V et al. Consensus numbering system for the rifampicin resistance-associated rpoB gene mutations in pathogenic mycobacteria. Clinical Microbiology and infection 2017;23(3):167-172.
2.Shea J, Halse TA, Kohlerschmidt D, Lapierre P, Modestil HA, Kearns CH, Dworkin FF, Rakeman JL, Escuyer V, Musser KA. 2021. Low-level rifampin resistance and rpoB mutations in Mycobacterium tuberculosis: an analysis of whole-genome sequencing and drug susceptibility test data in New York. J Clin Microbiol 59:e01885-20.
3.Alamgir M, Sajjad M, Baig MS, Noori MY. Mutational Frequencies in Mycobacterial rpoB gene using GeneXpert/MTB Rif Assay in Rifampicin Resistant patients at a tertiary care setting in Urban Sindh, Pakistan: Analysis from a Five-Year Period. Pak J Med Sci.2021;37(4):1151-1154.
4.Cheruvu mani, N Selvakumar, Sujatha Narayanan, PR Narayanan. Mutations in the rpoB Gene of Multidrug-Resistant Mycobacterium tuberculosis Clinical Isolates from India. Journal of Clinical Microbiology 2001;39(8):2987–2990.
5.Hameed HMA, Fang C, Liu Z, Ju Y, Han X, Gao Y et al. Characterization of Genetic Variants Associated with Rifampicin Resistance Level in Mycobacterium tuberculosis Clinical Isolates Collected in Guangzhou Chest Hospital, China. Infect Drug Resist. 2022;15:5655-5666.
Dr Kinjal Patel
Medical Microbiologist with an aptitude for holistic wellness and serving community with so far acquired knowledge. A dedicated reader in pursuit of improving human lives by transforming laboratory medicine and sharing pearls of medicine. Her life's mantra is "do what you love or love what you do".
