Comparison of different drug susceptibility test methods to detect rifampin heteroresistance in Mycobacterium tuberculosis

Ultrasensitive Detection of Multidrug-Resistant Mycobacterium tuberculosis Using SuperSelective Primer-Based Real-Time PCR Assays

The emergence of drug-resistant tuberculosis is a significant global health issue. The presence of heteroresistant Mycobacterium tuberculosis is critical to developing fully drug-resistant tuberculosis cases. The currently available molecular techniques may detect one copy of mutant bacterial genomic DNA in the presence of about 1-1000 copies of wild-type M. tuberculosis DNA. To improve the limit of heteroresistance detection, we developed SuperSelective primer-based real-time PCR assays, which, by their unique assay design, enable selective and exponential amplification of selected point mutations in the presence of abundant wild-type DNA. We designed SuperSelective primers to detect genetic mutations associated with M. tuberculosis resistance to the anti-tuberculosis drugs isoniazid and rifampin. We evaluated the efficiency of our assay in detecting heteroresistant M. tuberculosis strains using genomic DNA isolated from laboratory strains and clinical isolates from the sputum of tuberculosis patients. Results show that our assays detected heteroresistant mutations with a specificity of 100% in a background of up to 10⁴ copies of wild-type M. tuberculosis genomic DNA, corresponding to a detection limit of 0.01%. Therefore, the SuperSelective primer-based RT-PCR assay is an ultrasensitive tool that can efficiently diagnose heteroresistant tuberculosis in clinical specimens and contributes to understanding the drug resistance mechanisms. This approach can improve the management of antimicrobial resistance in tuberculosis and other infectious diseases.

Reevaluating Rifampicin Breakpoint Concentrations for Mycobacterium tuberculosis Isolates with Disputed rpoB Mutations and Discordant Susceptibility Phenotypes

In this study, rifampicin resistance breakpoints based on MICs of disrupted rpoB mutants of Mycobacterium tuberculosis (MTB) were explored using the Mycobacteria Growth Indicator Tube (MGIT) system and microplate alamarBlue assay (MABA). Sixty-one MTB isolates with disputed low-level rifampicin resistance-associated rpoB mutations and 40 RIF-susceptible wild-type isolates were included. Among the 61 resistant isolates, 25 (41.0%) had MICs ≥2.0 mg/L via MABA, while 16 (26.2%) were identified as RIF resistant via MGIT. Epidemiological cut-off (ECOFF) values obtained using MABA and MGIT were 0.25 and 0.125 mg/L, respectively. Based on 0.125 mg/L as a tentative critical concentration (CC), MABA RIF resistance-detection sensitivity was 93.4%, prompting the reduction of the MGIT CC to 0.125 mg/L, given that only a single isolate (1.6%) with the borderline mutation would be misclassified as susceptible to RIF based on this CC. Based on DNA sequencing of RRDR as the gold standard, the diagnostic accuracy of MGIT (99.0%) was significantly higher than that of MABA (91.1%). MICs of Leu⁵¹¹Pro mutant isolates were negatively correlated with time to liquid culture positivity (TTP) in our analysis (R = 0.957, P < 0.01). In conclusion, our results demonstrated missed detection of a high proportion of rifampicin-resistant isolates based on the WHO-endorsed CC. Such missed detections would be avoided by reducing the optimal MGIT RIF CC to 0.125 mg/L. In addition, MGIT based on reduced CC outperformed MABA in detecting borderline RIF resistance, with MABA MIC results obtained for isolates with the same mutation correlating with MTB growth rate.

IMPORTANCE Tuberculosis (TB) is still one of the world's leading infectious disease killers. The early and accurate diagnosis of RIF resistance is necessary to deliver timely and appropriate treatment for TB patients and improve their clinical outcome. Actually, a proportion of MTB isolates with disputed rpoB mutations present a diagnostic dilemma between Xpert and phenotypical drug susceptibility testing (pDST). Recently, WHO reported a pragmatic approach by lowering critical concentration (CC) to boost sensitivity of resistance detection of pDST. Therefore, a detailed analysis of the association between RIF susceptibility and disrupted mutations within rpoB gene would lay a foundation to assess the diagnostic accuracy of pDST with lowering RIF CC. In this study, we aim to determine the MICs of MTB isolates with disrupted mutations by MGIT and microplate alamarBlue assay (MABA). We also aimed to determine the optimal breakpoints for MTB isolates with these mutations.

Rarity of rpoB Mutations Outside the Rifampicin Resistance-Determining Region of Mycobacterium tuberculosis Isolates from Patients Responding Poorly to First-Line Tuberculosis Regimens in Beijing, China: A Retrospective Study

Background

Early and accurate diagnosis of rifampicin (RIF)-resistant Mycobacterium tuberculosis (MTB) is essential for controlling community spread of drug-resistant tuberculosis (TB). In order to discover mutations residing outside the rifampicin resistance-determining region (RRDR) of the MTB rpoB gene, we conducted this retrospective study.

Methods

We retrospectively screened patient records to obtain Xpert MTB/RIF assay results for patients who received care at the Beijing Chest Hospital from 2016 to 2019 in order to identify subjects who met study selection criteria. Stored frozen patient isolates were cultured, harvested, and then subjected to drug susceptibility testing. Concurrently, entire rpoB gene DNA of each isolate was amplified and then sequenced to reveal rpoB mutations.

Results

Overall, 104 RIF-susceptible tuberculosis patients who were tested using the Xpert MTB/RIF assay also had poor first-line regimen treatment responses. Isolates obtained from these cases included 101 MTB isolates that possessed wild-type rpoB allelic profiles, as demonstrated using Sanger sequencing. However, sequences from the other three isolates confirmed that rpoB of one isolate harbored a mutation encoding the amino acid substitution Ile491Phe and that rpoB genes of two isolates contained a mutation encoding the amino acid substitution Ser450Leu.

Conclusion

Our data demonstrated that mutations found outside the RRDR of MTB rpoB are rare in Beijing, China, indicating that World Health Organization-approved molecular diagnostics are generally suitable for diagnosing RIF resistance.

Dependence of Xpert MTB/RIF Accuracy for Detecting Rifampin Resistance in Bronchoalveolar Lavage Fluid on Bacterial Load: A Retrospective Study in Beijing, China

INTRODUCTION

We assessed the effect of Mycobacterium tuberculosis (MTB) bacterial load on Xpert MTB/RIF accuracy for detection of rifampicin (RIF)-resistant MTB in bronchoalveolar lavage fluid (BALF) specimens obtained at a national tuberculosis (TB) specialized hospital in Beijing, China.

METHODS

A retrospective study was conducted at Beijing Chest Hospital. Patients with symptoms suggestive of pulmonary TB who provided BALF specimens for routine MTB detection between June 2019 and July 2020 were enrolled in the study. Chi-square test and Student's t-test were used to compare results across groups stratified according to BALF bacterial load.

RESULTS

In total, 1125 patients with positive Xpert results who were enrolled in final analysis, 263 provided BALF specimens that tested positive for RIF-resistant MTB via Xpert MTB/RIF. The RIF-resistance rate of specimens with very low MTB bacterial load was 30.9%, a resistance rate significantly greater than rates obtained for groups with high (25.0%), medium (17.3%) and low (19.2%) MTB loads (P<0.01). Notably, false-positive results obtained for the very low bacterial load group led to markedly reduced positive predictive value of Xpert MTB/RIF to provide correct RIF-resistance predictions for that group (67.1%, 95% CI: 56.1%-78.1%5) relative to the predictive value obtained for all other groups combined (about 90%, P<0.05). Sanger sequencing data obtained for 20 (32.8%) MTB isolates deemed RIF-resistant via Xpert (Probe E) lacked rpoB RRDR mutations. Meanwhile, of another group of 23 isolates deemed RIF-susceptible via DST but RIF-resistant via Xpert MTB/RIF, 20 isolate sequences (87.0%) lacked rpoB RRDR mutations, while sequences of the remaining 3 isolates harbored single rpoB RRDR mutations predicted to cause amino acid substitutions.

CONCLUSION

Xpert MTB/RIF assay performed alarmingly poorly when used to detect RIF-resistant MTB in BALF specimens with very low bacterial loads. A high rate of Xpert probe E hybridization failure was the main driver of false-positive RIF-resistant results.

Whole-genome sequence analysis and comparisons between drug-resistance mutations and minimum inhibitory concentrations of Mycobacterium tuberculosis isolates causing M/XDR-TB

Drug resistance (DR) remains a major challenge for tuberculosis (TB) control. Whole-genome sequencing (WGS) provides the highest genetic resolution for genotypic drug-susceptibility tests (DST). We compared DST profiles of 60 Mycobacterium tuberculosis isolates which were drug resistant according to agar proportion tests (one poly DR-TB, 34 multidrug-resistant TB and 25 extensively drug-resistant TB). We additionally performed minimum inhibitory concentration (MIC) tests using Sensititre MYCOTBI plates (MYCOTB) and a WGS-based DST. Agreement between WGS-based DST and MYCOTB was high for all drugs except ethambutol (65%) and ethionamide (62%). Isolates harboring the -15 c/t inhA promoter mutation had a significantly lower MIC for isoniazid than did isolates with the katG Ser315Thr mutation (p < 0.001). Similar patterns were seen for ethambutol (embB Gly406Asp vs. embB Met306Ile), streptomycin (gid Gly73Ala vs. rpsL Lys43Arg), moxifloxacin (gyrA Ala90Val vs. gyrA Asp94Gly) and rifabutin (rpoB Asp435Phe/Tyr/Val vs. rpoB Ser450Leu). For genotypic heteroresistance, isolates with lower proportion of mapped read tended to has lower MIC of anti-TB drugs than those with higher proportion. These results emphasize the high applicability of WGS for determination of DR-TB and the association of particular mutations with MIC levels.

Detection of low-frequency resistance-mediating SNPs in next-generation sequencing data of Mycobacterium tuberculosis complex strains with binoSNP

Accurate drug resistance detection is key for guiding effective tuberculosis treatment. While genotypic resistance can be rapidly detected by molecular methods, their application is challenged by mixed mycobacterial populations comprising both susceptible and resistant cells (heteroresistance). For this, next-generation sequencing (NGS) based approaches promise the determination of variants even at low frequencies. However, accurate methods for a valid detection of low-frequency variants in NGS data are currently lacking. To tackle this problem, we developed the variant detection tool binoSNP which allows the determination of low-frequency single nucleotide polymorphisms (SNPs) in NGS datasets from Mycobacterium tuberculosis complex (MTBC) strains. By taking a reference-mapped file as input, binoSNP evaluates each genomic position of interest using a binomial test procedure. binoSNP was validated using in-silico, in-vitro, and serial patient isolates datasets comprising varying genomic coverage depths (100-500×) and SNP allele frequencies (1-30%). Overall, the detection limit for low-frequency SNPs depends on the combination of coverage depth and allele frequency of the resistance-associated mutation. binoSNP allows for valid detection of resistance associated SNPs at a 1% frequency with a coverage ≥400×. In conclusion, binoSNP provides a valid approach to detect low-frequency resistance-mediating SNPs in NGS data from clinical MTBC strains. It can be implemented in automated, end-user friendly analysis tools for NGS data and is a step forward towards individualized TB therapy.

Assessment of current diagnostic algorithm for detection of mixed infection with Mycobacterium tuberculosis and nontuberculous mycobacteria

BACKGROUND

The increasing pulmonary diseases are reported to be affected by mixed infection of Mycobacterium tuberculosis (MTB) and nontuberculous mycobacteria (NTM). In this study, our objective was to assess the efficiency of mycobacterial culture plus DNA sequencing to detect the mixed infections with MTB and various NTM organisms. We also aimed to investigate how efficiently GeneXpert detected MTB in mixed infections with NTM in in vitro models.

METHODS

A serial of mixed infection samples was generated by combining suspensions of MTB and five NTM bacteria, respectively. The mixed suspensions were further detected with GeneXpert and liquid culture plus DNA sequencing.

RESULTS

Overall, the GeneXpert assay exhibited promising capability to identify the presence of MTB at different proportions ranging from 1% to 99%. For the liquid culture, the subsequent DNA sequencing only detected the presence of NTM bacteria in the mixed samples, which the proportion of NTM ranged from 1% to 99%, including M. intracellulare, M. kansasii, M. abscessus, and M. fortuitum. For M. avium, DNA sequencing was able to identify the mixtures as M. avium infection in suspensions with no less than 10% M. avium bacteria, whereas only MTB was found in the other suspensions with less M. avium bacteria.

CONCLUSIONS

Our data demonstrate that the current diagnostic algorithm cannot yield a precise detection of mixed infections with MTB and NTM bacteria. The GeneXpert assay only identify MTB in the mixed samples, while the subculture plus DNA sequencing prefers to identify the NTM species with the higher growth rate. Further targeted molecular analysis by specific capture of multiple loci of mycobacterial species from specimens is urgently required to solve this diagnostic dilemma.

Genetic Determinants and Prediction of Antibiotic Resistance Phenotypes in Helicobacter pylori

Helicobacter pylori is a major human pathogen. Diagnosis of H. pylori infection and determination of its antibiotic susceptibility still mainly rely on culture and phenotypic drug susceptibility testing (DST) that is time-consuming and laborious. Whole genome sequencing (WGS) has recently emerged in medical microbiology as a diagnostic tool for reliable drug resistance prediction in bacterial pathogens. The aim of this study was to compare phenotypic DST results with the predictions based on the presence of genetic determinants identified in the H. pylori genome using WGS. Phenotypic resistance to clarithromycin, metronidazole, tetracycline, levofloxacin, and rifampicin was determined in 140 clinical H. pylori isolates by E-Test^®, and the occurrence of certain single nucleotide polymorphisms (SNPs) in target genes was determined by WGS. Overall, there was a high congruence of >99% between phenotypic DST results for clarithromycin, levofloxacin, and rifampicin and SNPs identified in the 23S rRNA, gyrA, and rpoB gene. However, it was not possible to infer a resistance phenotype for metronidazole based on the occurrence of distinct SNPs in frxA and rdxA. All 140 H. pylori isolates analysed in this study were susceptible to tetracycline, which was in accordance with the absence of double or triple nucleotide substitutions in the 16S rRNA gene.

Molecular Characterization of Prothionamide-Resistant Mycobacterium tuberculosis Isolates in Southern China

Prothionamide (PTH) has been widely used in the treatment of tuberculosis (TB), especially multidrug resistant tuberculosis (MDR-TB), while data regarding prevalence of resistance-causing mutation is limited. In this study, we aimed to investigate the molecular characteristics of PTH-resistant MTB isolates, and also analyzed the risk factors for PTH resistance among Mycobacterium tuberculosis (MTB) isolates in southern China. A total of 282 MTB isolates were enrolled in from Guangzhou Chest Hospital. Among these isolates, 46 (16.3%) were resistant to PTH. Statistical analysis revealed that PTH resistance was more likely to be associated with resistance to levofloxacin (LFX; OR: 2.18, 95% CI: 1.02–4.63; P = 0.04). Of the 46 PTH-resistant MTB isolates, 37 (80.4%) isolates harbored 19 different mutation types, including 10 (21.7%) isolates with double nucleotide substitutions and 27 (58.7%) with single nucleotide substitution. The mutations in ethA (51.4%, 19/37) were most frequently observed among PTH-resistant isolates, followed by 16 (43.2%) in the promoter of inhA and 6 (16.2%) in inhA. In addition, no significant difference was found in the distribution of isolates with different mutation types between Beijing and non-Beijing genotypes (P > 0.05). In conclusion, our data demonstrate that high diversity of genetic mutations conferring PTH resistance is identified among MTB isolates from southern China. Mutations in inhA, ethA, mshA, and ndh genes confer increased resistance of MTB to PTH. Ancient Beijing genotype strains have higher proportion of drug resistance compared with modern Beijing strains. In addition, PTH resistance is more likely to be observed in the LFX-resistant MTB isolates.

Cryptic Microheteroresistance Explains Mycobacterium tuberculosis Phenotypic Resistance

RATIONALE

Minority drug-resistant Mycobacterium tuberculosis subpopulations can be associated with phenotypic resistance but are poorly detected by Sanger sequencing or commercial molecular diagnostic assays.

OBJECTIVES

To determine the role of targeted next-generation sequencing in resolving these minor variant subpopulations.

METHODS

We used single molecule overlapping reads (SMOR), a targeted next-generation sequencing approach that dramatically reduces sequencing error, to analyze primary cultured isolates phenotypically resistant to rifampin, fluoroquinolones, or aminoglycosides, but for which Sanger sequencing found no resistance-associated variants (RAVs) within respective resistance-determining regions (study group). Isolates also underwent single-colony selection on antibiotic-containing agar, blinded to sequencing results. As a positive control, isolates with multiple colocalizing chromatogram peaks were also analyzed (control group).

MEASUREMENTS AND MAIN RESULTS

Among 61 primary culture isolates (25 study group and 36 control group), SMOR described 66 (49%) and 45 (33%) of 135 total heteroresistant RAVs at frequencies less than 5% and less than 1% of the total mycobacterial population, respectively. In the study group, SMOR detected minor resistant variant subpopulations in 80% (n = 20/25) of isolates with no Sanger-identified RAVs (median subpopulation size, 1.0%; interquartile range, 0.2-3.9%). Single-colony selection on drug-containing media corroborated SMOR results for 90% (n = 18/20) of RAV-containing specimens, and the absence of RAVs in 60% (n = 3/5) of isolates. Overall, Sanger sequencing was concordant with SMOR for 77% (n = 53/69) of macroheteroresistant (5-95% total population), but only 5% of microheteroresistant (<5%) subpopulations (n = 3/66) across both groups.

CONCLUSIONS

Cryptic minor variant mycobacterial subpopulations exist below the resolving capability of current drug susceptibility testing methodologies, and may explain an important proportion of false-negative resistance determinations.

Misdiagnosis of tuberculosis associated with some species of nontuberculous mycobacteria by GeneXpert MTB/RIF assay

Out of 12 nontuberculous mycobacteria (NTM) species, 5 were identified as Mycobacterium tuberculosis (MTB) by GeneXpert at a bacterial load of 10⁶. Notably, two species, including M. abscessus and M. smegmatis, were flagged as RIF-resistant MTB due to the high C(t) value of Probe E. In conclusion, our data have demonstrated that the misdiagnosis of MTB by GeneXpert assay is observed in five NTM species at a high bacterial load.

Evaluation of the MTBDRplus 2.0 assay for the detection of multidrug resistance among persons with presumptive pulmonary TB in China

We have conducted a multicenter study of the diagnostic accuracy of the MTBDRplus 2.0 assay in compared with conventional and molecular reference standard in four tuberculosis (TB)-specialized hospitals of China. A total of 5038 patients were enrolled in this study. The overall sensitivity of the assay for the diagnosis of TB was 92.7% [1723/1858, 95% confidence interval (95% CI): 91.5-93.9]. In smear-positive/culture-positive cases the sensitivity was 97.7% (995/1018, 95% CI: 96.6-98.6), whereas in smear-negative/culture-positive cases it was 86.7% (728/840, 95% CI: 84.2-88.9). The agreement rate between MTBDRplus 2.0 and Xpert MTB/RIF was 97.7% (1015/1039, 95% CI: 96.6-98.5) for smear-positive cases and 97.0% (3682/3794, 95% CI: 96.5-97.6) for smear-negative cases. As compared with phenotypic drug susceptibility testing, the MTBDRplus 2.0 correctly identified 298 of 315 patients (94.6%, 95% CI: 91.5-96.8) with rifampicin-resistance. As noted previously, isoniazid resistance is associated with many different mutations and consequently the sensitivity compared to phenotypic testing was lower (81.0%, 95% CI: 76.8-84.7). In conclusion, this assay is a rapid, accurate test in terms of increased sensitivity for detecting smear-negative TB patients, as well as an alternative for detecting both RIF and INH resistance in persons with presumptive TB, whereas the absence of a mutation in the specimens must be interpreted cautiously.

Prothionamide susceptibility testing of Mycobacterium tuberculosis using the resazurin microtitre assay and the BACTECMGIT 960 system

Resazurin microtitre assay (RMA) has been successfully used to detect minimal inhibitory concentrations (MICs) of both first-line and several second-line drugs in drug susceptibility testing (DST) of Mycobacterium tuberculosis (MTB). In this study, we firstly compared prothionamide (PTH) susceptibility testing of Mycobacterium tuberculosis (MTB) using resazurin microtitre assay (RMA) and MGIT. Overall, the sensitivity and specificity of RMA for detecting PTH susceptibility was 96.5% [95% confidence interval (CI): 91.7-100.0] and 93.2% (95% CI: 89.6-96.8) respectively. In addition, the median time to positivity was significantly shorter for RMA than for the automated MGIT 960 (RMA, 8 days [range: 8-8 days] vs MGIT, 10.1 days, [range: 5.0-13.0]; P < 0.01). Concordance rate for MICs between RMA and MGIT for PTH-resistant group was 64.3% (95% CI: 46.5-82.0), which was significantly lower than that of PTH-susceptible group (85.9%, 95% CI: 78.8-93.0; P= 0.01). In conclusion, our data demonstrated that RMA can be used as an acceptable alternative for determination of PTH susceptibility with shorter turn-around time. When compared with MGIT 960, RMA method was prone to produce higher MICs for PTH-resistant MTB strains.

Factors Associated with Missed Detection of Mycobacterium tuberculosis by Automated BACTEC MGIT 960 System

Despite the demonstration of excellent performance, mycobacterial growth in BACTEC MGIT 960 can go undetected. The aim of this study was to investigate the prevalence of “false-negative” culture sample in Beijing and the potential factors associated with the detection failures by MGIT 960. Of the 577 sputum samples tested, 141 (24.4%) were culture-positive for mycobacteria, of which 133 (94.3%) were automatically determined by MGIT 960 system and 8 (5.7%) were positive for visual growth (false negative by MGIT). Statistical analysis showed that positive grade of specimen had no influence on the false-negative rate by MGIT 960 system (χ² = 2.207, P = 0.820). In addition, the mean time to detection (TTD) was 241.4 (range: 224–261) hours for false-negative group and 186.8 (range: 173–199) hours for positive group. The difference in TTD between false-negative and positive groups was statistically significant (P < 0.01). In conclusion, our data demonstrate that the automatic MGIT missed a small portion of bacteriological mycobacterial patients. In addition, the poor growth rate rather than the low grade of AFB smear is associated with the detection failure by MGIT. Our findings highlight the notion that manual inspection for all instrument-negative MGIT tubes will bring about considerable benefit to patients and clinicians.

A Comparison of the Sensititre MycoTB Plate, the Bactec MGIT 960, and a Microarray-Based Molecular Assay for the Detection of Drug Resistance in Clinical Mycobacterium tuberculosis Isolates in Moscow, Russia

Background

The goal of this study was to compare the consistency of three assays for the determination of the drug resistance of Mycobacterium tuberculosis (MTB) strains with various resistance profiles isolated from the Moscow region.

Methods

A total of 144 MTB clinical isolates with a strong bias toward drug resistance were examined using Bactec MGIT 960, Sensititre MycoTB, and a microarray-based molecular assay TB-TEST to detect substitutions in the rpoB, katG, inhA, ahpC, gyrA, gyrB, rrs, eis, and embB genes that are associated with resistance to rifampin, isoniazid, fluoroquinolones, second-line injectable drugs and ethambutol.

Results

The average correlation for the identification of resistant and susceptible isolates using the three methods was approximately 94%. An association of mutations detected with variable resistance levels was shown. We propose a change in the breakpoint minimal inhibitory concentration for kanamycin to less than 5 μg/ml in the Sensititre MycoTB system. A pairwise comparison of the minimal inhibitory concentrations (MICs) of two different drugs revealed an increased correlation in the first-line drug group and a partial correlation in the second-line drug group, reflecting the history of the preferential simultaneous use of drugs from these groups. An increased correlation with the MICs was also observed for drugs sharing common resistance mechanisms.

Conclusions

The quantitative measures of phenotypic drug resistance produced by the Sensititre MycoTB and the timely detection of mutations using the TB-TEST assay provide guidance for clinicians for the choice of the appropriate drug regimen.

Rapid Drug Susceptibility Testing of Drug-Resistant Mycobacterium tuberculosis Isolates Directly from Clinical Samples by Use of Amplicon Sequencing: a Proof-of-Concept Study

Increasingly complex drug-resistant tuberculosis (DR-TB) is a major global health concern and one of the primary reasons why TB is now the leading infectious cause of death worldwide. Rapid characterization of a DR-TB patient's complete drug resistance profile would facilitate individualized treatment in place of empirical treatment, improve treatment outcomes, prevent amplification of resistance, and reduce the transmission of DR-TB. The use of targeted next-generation sequencing (NGS) to obtain drug resistance profiles directly from patient sputum samples has the potential to enable comprehensive evidence-based treatment plans to be implemented quickly, rather than in weeks to months, which is currently needed for phenotypic drug susceptibility testing (DST) results. In this pilot study, we evaluated the performance of amplicon sequencing of Mycobacterium tuberculosis DNA from patient sputum samples using a tabletop NGS technology and automated data analysis to provide a rapid DST solution (the Next Gen-RDST assay). One hundred sixty-six out of 176 (94.3%) sputum samples from the Republic of Moldova yielded complete Next Gen-RDST assay profiles for 7 drugs of interest. We found a high level of concordance of our Next Gen-RDST assay results with phenotypic DST (97.0%) and pyrosequencing (97.8%) results from the same clinical samples. Our Next Gen-RDST assay was also able to estimate the proportion of resistant-to-wild-type alleles down to mixtures of ≤1%, which demonstrates the ability to detect very low levels of resistant variants not detected by pyrosequencing and possibly below the threshold for phenotypic growth methods. The assay as described here could be used as a clinical or surveillance tool.

Rapid diagnosis of MDR and XDR tuberculosis with the MeltPro TB assay in China

New diagnostic methods have provided a promising solution for rapid and reliable detection of drug-resistant TB strains. The aim of this study was to evaluate the performance of the MeltPro TB assay in identifying multidrug-resistant (MDR-) and extensively drug-resistant tuberculosis (XDR-TB) patients from sputum samples. The MeltPro TB assay was evaluated using sputum samples from 2057 smear-positive TB patients. Phenotypic Mycobacterial Growth Indicator Tube (MGIT) 960 drug susceptibility testing served as a reference standard. The sensitivity of the MeltPro TB assay was 94.2% for detecting resistance to rifampicin and 84.9% for detecting resistance to isoniazid. For second-line drugs, the assay showed a sensitivity of 83.3% for ofloxacin resistance, 75.0% for amikacin resistance, and 63.5% for kanamycin resistance. However, there was a significant difference for detecting kanamycin resistance between the two pilot sites in sensitivity, which was 53.2% in Guangdong and 81.5% in Shandong (P = 0.015). Overall, the MeltPro TB assay demonstrated good performance for the detection of MDR- and XDR-TB, with a sensitivity of 86.7% and 71.4%, respectively. The MeltPro TB assay is an excellent alternative for the detection of MDR- and XDR-TB cases in China, with high accuracy, short testing turn-around time, and low unit price compared with other tests.

Genotypic, Phenotypic and Clinical Validation of GeneXpert in Extra-Pulmonary and Pulmonary Tuberculosis in India

Background

Newer molecular diagnostics have brought paradigm shift in early diagnosis of tuberculosis [TB]. WHO recommended use of GeneXpert MTB/RIF [Xpert] for Extra-pulmonary [EP] TB; critics have since questioned its efficiency.

Methods

The present study was designed to assess the performance of GeneXpert in 761 extra-pulmonary and 384 pulmonary specimens from patients clinically suspected of TB and compare with Phenotypic, Genotypic and Composite reference standards [CRS].

Results

Comparison of GeneXpert results to CRS, demonstrated sensitivity of 100% and 90.68%, specificity of 100% and 99.62% for pulmonary and extra-pulmonary samples. On comparison with culture, sensitivity for Rifampicin [Rif] resistance detection was 87.5% and 81.82% respectively, while specificity was 100% for both pulmonary and extra-pulmonary TB. On comparison to sequencing of rpoB gene [Rif resistance determining region, RRDR], sensitivity was respectively 93.33% and 90% while specificity was 100% in both pulmonary and extra-pulmonary TB. GeneXpert assay missed 533CCG mutation in one sputum and dual mutation [517 & 519] in one pus sample, detected by sequencing. Sequencing picked dual mutation [529, 530] in a sputum sample sensitive to Rif, demonstrating, not all RRDR mutations lead to resistance.

Conclusions

Current study reports observations in a patient care setting in a high burden region, from a large collection of pulmonary and extra-pulmonary samples and puts to rest questions regarding sensitivity, specificity, detection of infrequent mutations and mutations responsible for low-level Rif resistance by GeneXpert. Improvements in the assay could offer further improvement in sensitivity of detection in different patient samples; nevertheless it may be difficult to improve sensitivity of Rif resistance detection if only one gene is targeted. Assay specificity was high both for TB detection and Rif resistance detection. Despite a few misses, the assay offers major boost to early diagnosis of TB and MDR-TB, in difficult to diagnose pauci-bacillary TB.

Rapid antibiotic-resistance predictions from genome sequence data for Staphylococcus aureus and Mycobacterium tuberculosis

The rise of antibiotic-resistant bacteria has led to an urgent need for rapid detection of drug resistance in clinical samples, and improvements in global surveillance. Here we show how de Bruijn graph representation of bacterial diversity can be used to identify species and resistance profiles of clinical isolates. We implement this method for Staphylococcus aureus and Mycobacterium tuberculosis in a software package ('Mykrobe predictor') that takes raw sequence data as input, and generates a clinician-friendly report within 3 minutes on a laptop. For S. aureus, the error rates of our method are comparable to gold-standard phenotypic methods, with sensitivity/specificity of 99.1%/99.6% across 12 antibiotics (using an independent validation set, n=470). For M. tuberculosis, our method predicts resistance with sensitivity/specificity of 82.6%/98.5% (independent validation set, n=1,609); sensitivity is lower here, probably because of limited understanding of the underlying genetic mechanisms. We give evidence that minor alleles improve detection of extremely drug-resistant strains, and demonstrate feasibility of the use of emerging single-molecule nanopore sequencing techniques for these purposes.

An investigation on the population structure of mixed infections of Mycobacterium tuberculosis in Inner Mongolia, China

OBJECTIVES

Mixed infections of Mycobacterium tuberculosis strains have attracted more attention due to their increasing frequencies worldwide, especially in the areas of high tuberculosis (TB) prevalence. In this study, we accessed the rates of mixed infections in a setting with high TB prevalence in Inner Mongolia Autonomous Region of China.

METHODS

A total of 384 M. tuberculosis isolates from the local TB hospital were subjected to mycobacterial interspersed repetitive unit-variable number tandem repeat (MIRU-VNTR) typing method. The single clones of the strains with mixed infections were separated by subculturing them on the Löwenstein-Jensen medium.

RESULTS

Of these 384 isolates, twelve strains (3.13%) were identified as mixed infections by MIRU-VNTR. Statistical analysis indicated that demographic characteristics and drug susceptibility profiles showed no statistically significant association with the mixed infections. We further subcultured the mixed infection strains and selected 30 clones from the subculture for each mixed infection. Genotyping data revealed that eight (8/12, 66.7%) strains with mixed infections had converted into single infection through subculture. The higher growth rate was associated with the increasing proportion of variant subpopulation through subculture.

CONCLUSIONS

In conclusion, by using the MIRU-VNTR method, we demonstrate that the prevalence of mixed infections in Inner Mongolia is low. Additionally, our findings reveal that the subculture changes the population structures of mixed infections, and the subpopulation with higher growth rate show better fitness, which is associated with high proportion among the population structure after subculture. This study highlights that the use of clinical specimens, rather than subcultured isolates, is preferred to estimate the prevalence of mixed infections in the specific regions.