Agreement between Phenotypically Detected Linezolid Resistance and Mutations in rrl and rplC Genes of Mycobacterium tuberculosis Isolates Using Nanopore Sequencing.

BACKGROUND: Phenotypic drug susceptibility testing (DST) is considered the gold standard for detecting linezolid (LZD) resistance in Mycobacterium tuberculosis (MTB), but it is time-consuming. Nanopore sequencing offers a potentially faster alternative approach. This study evaluated the agreement between phenotypically detected LZD resistance and mutations in the rrl and rplC genes of MTB isolates using nanopore sequencing. METHODS: Consecutive drug-resistant MTB isolates from pulmonary samples collected in 2021 underwent liquid culture (LC) DST for LZD. All resistant isolates and an equal number of susceptible isolates were subjected to targeted sequencing of the rrl and rplC genes using nanopore technology. RESULTS: Sequencing identified a C154R mutation in the rplC gene in only one LZD-resistant isolate. No mutations were detected in the rrl gene. The agreement between sequencing and LC-DST for detecting LZD resistance was poor (Cohen's kappa: 0.03571, 95% confidence interval [CI]: -0.034-0.105). Additionally, no significant association was found between LZD resistance and clinical or microbiological outcomes at 6-month follow-up. CONCLUSION: This study revealed a considerable discrepancy between phenotypic and genotypic detection of LZD resistance in MTB. Further research is needed to better understand the genetic mechanisms underlying LZD resistance and to develop reliable molecular diagnostics for rapid resistance detection.

Efficacy of TrueNat cartridge extracted DNA for detecting drug resistant tuberculosis by line probe assay.

BACKGROUND: Currently for diagnosing Mycobacterium tuberculosis and its drug resistance, two sputum samples are required. One of them is subjected to TrueNat™ and if positive the other sample is subjected to line probe assay (LPA). This study was done to evaluate whether TrueNat extracted DNA can be directly used for performing LPA in a diagnostic laboratory setting to decrease patient turn-around time. METHODS: Total 45 smear positive sputum samples were subjected to TrueNat™ MTB detection and first and second line (FL and SL) LPA testing in parallel. DNA extracted by Trueprep® Cartridge was also tested by LPA and results were compared. Further, TrueNat extracted DNA from 20 samples was divided into 6 aliquots each, two of which were stored at 4 °C, 37 °C and 55 °C (under humidification) each. One aliquot from stored DNA at each temperature was used for FL & SL LPA on day three and the other on day eight. The blots thus obtained were compared with those of conventional LPA at day 1. RESULTS: For FL-LPA, TrueNat extracted DNA gave valid results for all 45 (100%) samples but conventionally extracted DNA could give results for 44 (97.8%) samples. Likewise, for SL-LPA, valid results were obtained for 40 (88.9%) and 35 (77.8%) samples respectively using TrueNat extracted DNA and conventionally extracted DNA respectively. All samples with invalid LPA results had Ct values ≥ 28 by TrueNat PCR. LPA results were obtained for all the 20 samples using stored DNA at all temperatures and duration. CONCLUSIONS: TrueNat extracted DNA can be used for performing LPA under field conditions for selected samples.

High frequency of isoniazid and fluroquinolone resistance among patients with rifampicin sensitive tuberculosis.

This study was done to determine frequency of isoniazid (INH) and fluoroquinolones FQ resistance among rifampicin sensitive strains of Mycobacterium tuberculosis and to study their mutation patterns. Retrospective analysis was done for samples with M. tuberculosis detected by Cartridge based NAAT (CBNAAT). They were tested sequentially by first line (FL) and second line - line probe assay (SL-LPA) depending on their drug resistance pattern and following diagnostic algorithm. Total 9722 (74.1 %) of 13124 NAAT positive samples were sensitive for rifampicin. On FL-LPA, 833 (8.6 %) were resistant to INH and of which 110 (13.2 %) were also resistant to FQ by SL-LPA. Most common mutations observed for INH resistance were katG S315T1 mutation in 615 (97.3 %) strains, inhA C15T mutation in 174 (86.6 %) strains and for FQ resistance were gyrA D94G mutation in 46 (41.8 %) strains. Heteroresistance, inferred mutations, combination of mutations and unique mutations were also observed in all genes.

Agreement between CBNAAT, liquid culture and line probe assay for detection of Mycobacterium tuberculosis and anti-tubercular drug resistance in extrapulmonary samples.

PURPOSE: Cartridge based nucleic acid amplification test (CBNAAT) has been endorsed by the WHO as the screening test for diagnosing extrapulmonary tuberculosis (EPTB). In the present study we report the agreement between CBNAAT (Xpert MTB/RIF), liquid culture (LC) and line probe assay (LPA) for diagnosis of Mycobacterium tuberculosis and detection of drug resistance among EPTB cases. METHODS: The EP samples were subjected to CBNAAT (Xpert MTB/RIF, Cepheid, USA) and wherever possible, to LC (MGIT 960, Becton Dickinson, USA) followed sequentially by first line and second line-LPA (FL-LPA, SL-LPA, Hain Lifescience, Germany) on the isolates. RESULTS: Total 566/4080 (13.9%) EP samples were detected positive for M. tuberculosis on CBNAAT. Aspirates from lymph nodes were most often positive (11/30; 36.6%), followed by pus (240/873; 27.5%) and CSF samples (166/104; 15.8%). The detection of M. tuberculosis was more in adults than children except in tissue biopsy samples. Rifampicin resistance was also higher among adults except CSF in which resistance was more in children. Total 185 of 566 (32.7%) CBNAAT positive and 770 of 3510 (21.9%) CBNAAT negative samples could be cultured of which 110/185 (59.4%) and 33/770 (4.3%) respectively turned positive. FL-LPA and SL-LPA of 143 culture isolates showed that 27 isolates had drug resistance, of which 3 (2.1%) were XDR, 11 (7.7%) were Pre-XDR (FQ) and 13 (9.1%) were MDR. Of these 27 resistant isolates, 12 were negative by CBNAAT and two were mislabeled as Rifampicin sensitive or indeterminate based on the unique RpoB gene mutation patterns on LPA. The positive and negative agreements between LC and CBNAAT for detection of M. tuberculosis were 67.1% and 92.7% respectively and between LPA and CBNAAT for rifampicin resistance detection were 98.9% and 92.9% respectively. CONCLUSIONS: For EPTB, CBNAAT should be accompanied with LC wherever possible irrespective of the CBNAAT result.