Novel mutations within the embB gene in ethambutol-susceptible clinical isolates of Mycobacterium tuberculosis

Structural dynamics insights into the M306L, M306V, and D1024N mutations in Mycobacterium tuberculosis inducing resistance to ethambutol

Resistance to anti-tuberculosis drugs, especially ethambutol (EMB), has been widely reported worldwide. EMB resistance is caused by mutations in the embB gene, which encodes the arabinosyl transferase enzyme. This study aimed to detect mutations in the embB gene of Mycobacterium tuberculosis from Papua and to evaluate their impact on the effectiveness of EMB. We analyzed 20 samples of M. tuberculosis culture that had undergone whole-genome sequencing, of which 19 samples were of sufficient quality for further bioinformatics analysis. Mutation analysis was performed using TBProfiler, which identified M306L, M306V, D1024N, and E378A mutations. In sample TB035, the M306L mutation was present along with E378A. The binding affinity of EMB to arabinosyl transferase was calculated using AutoDock Vina. The molecular docking results revealed that all mutants demonstrated an increased binding affinity to EMB compared to the native protein (-0.948 kcal/mol). The presence of the M306L mutation, when coexisting with E378A, resulted in a slight increase in binding affinity compared to the M306L mutation alone. The molecular dynamics simulation results indicated that the M306L, M306L + E378A, M306V, and E378A mutants decreased protein stability. Conversely, the D1024N mutant exhibited stability comparable to the native protein. In conclusion, this study suggests that the M306L, M306L + E378A, M306V, and E378A mutations may contribute to EMB resistance, while the D1024N mutation may be consistent with continued susceptibility to EMB.

Frequency of Codon 306 Mutations in embB Gene of Mycobacterium tuberculosis Resistant to Ethambutol: A Systematic Review and Meta-Analysis

BACKGROUND

Ethambutol (EMB) resistance is a major concern in patients with tuberculosis (TB). The aim of this study was to determine the frequency rate of mutations in the embB306 gene of Mycobacterium tuberculosis (M. tuberculosis) resistant to EMB, based on a systematic review and meta-analysis.

METHODS

Thirty-seven original articles (1997-2015) that have been published in valid databases were considered for this research. The articles were systematically reviewed for the prevalence and rate of mutations in embB306 in EMB-resistant M. tuberculosis. Data were analyzed using meta-analysis and random effects models (CI 95%, P < 0.10).

RESULTS

With a 6,931 sample size in 37 original articles, the lowest rate was related to EMB resistance that was observed in 2014 with 0.05 (95% CI: 0.04-0.07) and the highest prevalence rate was 0.84 (95% CI: 0.68-1.01), observed in 1997. Lowest and highest prevalence rates of embB306 gene mutation in M. tuberculosis were 0.03 (95% CI: 0.01-0.07) in 2014 and 0.78 (95% CI: 0.71-1.84) in 2005, in the USA, respectively.

CONCLUSIONS

The present study revealed the prevalence and association of mutations in the embB306 gene of M. tuberculosis with resistance to EMB. Detecting EMB-resistant M. tuberculosis can help in controlling and correcting the administration of drugs for patients with TB.

Mycobacterial Cell Wall: A Source of Successful Targets for Old and New Drugs

Eighty years after the introduction of the first antituberculosis (TB) drug, the treatment of drug-susceptible TB remains very cumbersome, requiring the use of four drugs (isoniazid, rifampicin, ethambutol and pyrazinamide) for two months followed by four months on isoniazid and rifampicin. Two of the drugs used in this “short”-course, six-month chemotherapy, isoniazid and ethambutol, target the mycobacterial cell wall. Disruption of the cell wall structure can enhance the entry of other TB drugs, resulting in a more potent chemotherapy. More importantly, inhibition of cell wall components can lead to mycobacterial cell death. The complexity of the mycobacterial cell wall offers numerous opportunities to develop drugs to eradicate Mycobacterium tuberculosis, the causative agent of TB. In the past 20 years, researchers from industrial and academic laboratories have tested new molecules to find the best candidates that will change the face of TB treatment: drugs that will shorten TB treatment and be efficacious against active and latent, as well as drug-resistant TB. Two of these new TB drugs block components of the mycobacterial cell wall and have reached phase 3 clinical trial. This article reviews TB drugs targeting the mycobacterial cell wall in use clinically and those in clinical development.

Evaluation of Whole-Genome Sequence Method to Diagnose Resistance of 13 Anti-tuberculosis Drugs and Characterize Resistance Genes in Clinical Multi-Drug Resistance Mycobacterium tuberculosis Isolates From China

Background: Whole-genome sequencing (WGS) is a viable and financially feasible tool for timely and comprehensive diagnosis of drug resistance in developed countries. With the increase in the incidence of multidrug-resistant tuberculosis (MDR-TB), second-line anti-TB drugs are gaining importance. However, genetic resistance to second-line anti-TB drugs based on WGS has not been fully studied.

Methods: We randomly selected 100 MDR-TB and 10 non-MDR-TB isolates from a hospital in Zhejiang Province, China. Drug susceptibility tests against 13 anti-TB drugs were performed, and 34 drug resistance-related genes were analyzed using WGS in all isolates. For each drug, the accuracy, sensitivity, specificity, and positive and negative predictive values of WGS were compared with those of the conventional drug susceptibility test.

Results: The overall sensitivity and specificity for WGS were respectively, 99.0 and 100.0% for isoniazid (INH), 99.0 and 100.0% for rifampicin (RIF), 94.8 and 65.3% for ethambutol (EMB), 86.2 and 84.4% for pyrazinamide (PZA), 95.6 and 95.6% for levofloxacin (LFX), 89.5 and 65.3% for moxifloxacin (MFX), 91.3 and 95.1% for streptomycin (SM), 90.9 and 99.0% for kanamycin, 90.9 and 100.0% for amikacin, 88.9 and 98.0% for capreomycin, 87.0 and 85.1% for prothionamide (PTO), 85.7 and 99.0% for para-aminosalicylic acid (PAS), and 66.7 and 95.9% for clofazimine (CLO).

Conclusions: WGS is a promising approach to predict resistance to INH, RIF, PZA, LFX, SM, second-line injectable drugs (SLIDs), and PTO with satisfactory accuracy, sensitivity, and specificity of over 85.0%. The specificity of WGS in diagnosing resistance to EMB, and high-level resistance to MFX (2.0 mg/L) needs to be improved.

Lack of association of novel mutation Asp397Gly in aftB gene with ethambutol resistance in clinical isolates of Mycobacterium tuberculosis

To discover additional genotypic indicators for ethambutol (EMB) resistant M. tuberculosis, we studied polymorphisms in arabinofuranosyl transferase encoding genes aftA (Rv3792), aftB (Rv3805) and aftC (Rv2673) in 38 EMB resistant and 34 EMB susceptible isolates from India and a repository established by the World Health Organization (WHO) Special Programme for Research and Training in Tropical Disease (TDR) by DNA sequencing. The results were correlated with the minimum inhibitory concentration (MIC) of EMB and mutations in embB (Rv3795). The most common non-synonymous polymorphism identified in aftB was Asp397Gly in 12/38 (31.6%) EMB resistant and 3/34 (8.8%) EMB susceptible isolates. Interestingly, 10/12 (83.3%) EMB resistant isolates with aftB Asp397Gly mutation also carried embB306, embB402 or embB497 mutations. Association of Asp397Gly polymorphism with EMB resistance was statistically significant (p 0.0216). However, overexpression of the mutant aftB in M. tuberculosis H37Rv did not exhibit any change in the MIC. Whole genome sequencing of a panel of Indian isolates and SNP cluster grouping (SCG) of TDR strains revealed an association between aftB mutation Asp397Gly and Beijing genotype or SCG2, a cluster group representing the Beijing genotype. To conclude, though aftBAsp397Gly mutation is not associated with EMB resistance, this mutation may be a phylogenetic marker for the Beijing clade.

A Chip for Detecting Tuberculosis Drug Resistance Based on Polymerase Chain Reaction (PCR)-Magnetic Bead Molecule Platform

Objective: A Tag Array chip was used to detect plasmids of different template concentration, and then analyzed for sensitivity and specificity. Drug resistance genes from tuberculosis clinical specimens were detected, giving comparative phenotypic resistance results to explore the feasibility and value of clinical applications.

Methods: Twenty-four strains of Mycobacterium Tuberculosis (MTB) having sequence differences in extracted plasmids of mutant strains. The plasmid was diluted into different concentrations, and then was performed to analyze the sensitivity and specificity of the chip system. A total of 427 clinical specimens (including spinal tuberculosis and pulmonary tuberculosis) were collected from patients who came from seven hospitals. Design, optimization and preparation of the chip detection system, sequencing and phenotypic drug susceptibility results were analyzed to evaluate the sensitivity and specificity of the gene chip.

Results: In the template, concentrations of 1 × 10³ copies/μL and above were consistent with sequencing results in the mutant. The sensitivity and specificity in spine Tuberculosis specimen of rifampicin (RFP) were 94.40 and 92.86%; isoniazide (INH) were 92.37 and 87.50%; ethambutol (EMB) were 61.36 and 89.29%; fluoroquinolones (FQS) were 79.41 and 92.86%; streptomycin (SM) were 90.18 and 89.29%; second line drugs (SLD) were 77.61 and 83.93%. In Pulmonary Tuberculosis specimen, the sensitivity and specificity respectively were RFP: 92.74%; 93.75%; INH: 91.26%; 87.50%; EMB: 54.17%; 89.58%; FQS: 84.87%; 93.75%; SM: 86.73%; 85.42%; SLD: 80.9%; 91.67%. The RFP, INH, FQs and SM resistance genes was highly sensitive and specific: however, for detection of amikacin (AMK), capreomycin (CPM), kanamycin (KM), specificity was higher, but sensitivity was lower. Sensitivity for the detection of a mutation in the eis promoter region could be improved.

Conclusion: Tag Array chip can achieve rapid, accurate detection of tuberculosis resistance, which has important clinical significance and feasibility.

Drug resistance mechanisms and drug susceptibility testing for tuberculosis

Tuberculosis (TB) caused by Mycobacterium tuberculosis (MTB) is the deadliest infectious disease and the associated global threat has worsened with the emergence of drug resistance, in particular multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB). Although the World Health Organization (WHO) End-TB Strategy advocates for universal access to antimicrobial susceptibility testing, this is not widely available and/or it is still underused. The majority of drug resistance in clinical MTB strains is attributed to chromosomal mutations. Resistance-related mutations could also exert certain fitness cost to the drug-resistant MTB strains and growth fitness could be restored by the presence of compensatory mutations. Understanding these underlying mechanisms could provide an important insight into TB pathogenesis and predict the future trend of MDR-TB global pandemic. This review covers the mechanisms of resistance in MTB and provides a comprehensive overview of current phenotypic and molecular approaches for drug susceptibility testing, with particular attention to the methods endorsed and recommended by the WHO.

Molecular Diagnostics for Mycobacterium tuberculosis Infection

Resistance to antimycobacterial drugs is a major barrier to effective treatment of Mycobacterium tuberculosis infection. Molecular diagnostic techniques based on the association between specific gene mutations and phenotypic resistance to certain drugs offer the opportunity to rapidly ascertain whether drug resistance is present and to alter treatment before further resistance develops. Current barriers to successful implementation of rapid diagnostics include imperfect knowledge regarding the full spectrum of mutations associated with resistance, limited utilization of molecular diagnostics where they are most needed, and the requirement for specialized laboratory facilities to perform molecular testing. Further understanding of genotypic-phenotypic correlates of resistance and streamlined implementation platforms will be necessary to optimize the public health impact of molecular resistance testing for M. tuberculosis.

Rapid detection of ethambutol-resistant Mycobacterium tuberculosis in clinical specimens by real-time polymerase chain reaction hybridisation probe method

Background

Early diagnosis of drug resistance (DR) to ethambutol (EMB) in tuberculosis (TB) remains a challenge. Simple and reliable method (s) are needed for rapid detection of DR Mycobacterium tuberculosis (MTB) in clinical specimens.

Objectives

The aim of this study was to design fluorescence resonance energy transfer hybridisation probe-based real-time polymerase chain reaction (PCR) method for the early detection of EMB-resistant MTB direct from clinical sputa.

Materials and Methods

Primers and probes were designed against 306 codon of embB gene which is commonly associated with EMB resistance. A comparative study was done between Lowenstein-Jenson (L-J) proportion and hybridisation probe-based real-time PCR method for susceptibility testing. DNA sequencing was used in nine representative isolates to validate the efficiency of real-time PCR method to detect emb306 mutation of MTB.

Results

A total of 52 clinical sputum samples and corresponding culture isolates (from category II pulmonary TB cases) were included in this study. Out of 52 MTB isolates, 32 and 20 were resistant and susceptible to EMB, respectively, as determined by L-J proportion method. Real-time PCR showed 95% specificity, 75% sensitivity and 82.69% accuracy when compared with L-J proportion method. A 100% of concordance was observed by validating the real-time PCR results with DNA sequencing.

Conclusions

Our real-time PCR hybridisation probe method promises for rapid detection of EMB-resistant MTB directly from clinical specimens. However, future studies and modifications of method by incorporating other potential loci along with targeted mutation (emb306) are still required to increase the sensitivity of method.

Mutations within embCAB Are Associated with Variable Level of Ethambutol Resistance in Mycobacterium tuberculosis Isolates from China

The EmbCAB proteins have been considered a target for ethambutol (EMB). Mutations in embCAB are known to confer most EMB resistance. However, the knowledge about the effects of embCAB mutations on the EMB resistance level and about the role of mutation-mutation interactions is limited in China. Here, we sequenced embCAB among 125 Mycobacterium tuberculosis isolates from China and quantified their EMB MICs by testing growth at 10 concentrations. Furthermore, a multivariate regression model was established to assess the effects of both individual mutations and multiple mutations. Our results revealed that in China, 82.6% of EMB-resistant isolates (71/86 isolates) harbored at least one mutation within embCAB Most of the mutations were located in the embB and embA upstream region. Several individual mutations and multiple mutations within this region contributed to the different levels of EMB resistance. Their effects were statistically significant. Additionally, there was an association between high-level EMB resistance and multiple mutations.

Drug resistance and Mycobacterium tuberculosis strain diversity in TB/HIV co-infected patients in Ho Chi Minh city, Vietnam

BACKGROUND

Mycobacterium tuberculosis strain diversity and drug resistance among people living with human immunodeficiency virus (HIV) in Vietnam have not been described previously.

METHODS

We examined M. tuberculosis isolates from TB/HIV co-infected patients in Ho Chi Minh City, Vietnam. Drug susceptibility testing (DST), spoligotyping and 24-locus Mycobacterial Interspersed Repetitive Unit (MIRU-24 typing) were performed, and the rpoB, katG, inhA and inhA promoter, rpsL, rrs and embB genes were sequenced in all drug resistant isolates identified.

RESULTS

In total, 84/200 (42.0%) strains demonstrated "any drug resistance"; 17 (8.5%) were multi-drug resistant (MDR). Streptomycin resistance was present in 80 (40.0%) isolates; 95.2% (80/84) with "any drug resistance" and 100% with MDR. No rifampicin monoresistance was detected. Of the rifampicin resistant strains 16/18 (88.9%) had mutations in the 81-bp Rifampicin Resistance Defining Region (RRDR) of the rpoB gene. Isoniazid resistance was mostly associated with Ser315Thr mutations in the katG gene (15/17; 88.2%). Beijing (49.0%) and East African Indian (EAI) lineage strains (35.0%; 56/70 EAI-5) were most common.

CONCLUSION

TB/HIV co-infection in Vietnam was associated with high rates of TB drug resistance, although we were unable to differentiate new from retreatment cases.

Molecular detection of Isoniazid, Rifampin and Ethambutol resistance to M. tuberculosis and M. bovis in multidrug resistant tuberculosis (MDR-TB) patients in Pakistan

The various aspects of MDR-TB, type of pathogen, different drug sensitive methods and mutation (s) in specific genes were determined. The histone-like protein (hupB) gene of M. tuberculosis was targeted by using primer sets: N & S and M & S (produced 645 bp & 318 bp fragment respectively). The most significant risk factors were the poverty and male gender of ages 11-25 years. All samples were detected as M. tuberculosis. By Drug Proportion method, the highest percentage (37%) was found resistant to only Rifampin. By MGIT method, the highest percentage (82.2%) was found resistant with the triple combination (Rifampin-RIF + Isoniazid-INH + Ethambutol-EMB) of the drugs. The highest mutations (76.92%) were found in gene rpoB (codon 531) in MDR TB patients. By, MAS-PCR, the highest percentage (34%) were found resistant to combination (INH + RIF) of the drugs. Minimum samples were resistant to RIF and RIF + INH drugs by MGIT, while proportionate results were observed from MAS-PCR and DP. Moreover, by MAS-PCR mutation in gene embB (306) caused EMB resistance (51.64%). We found that M. tuberculosis was the main cause of MDR-TB. Our findings may further be used for an early diagnosis of multi-drug resistant tuberculosis.

Association between embB Codon 306 Mutations, Phenotypic Resistance Profiles, and Genotypic Characterization in Clinical Mycobacterium tuberculosis Isolates from Hebei, China

Ethambutol (EMB) is an essential first-line drug for tuberculosis (TB) treatment. Nucleotide substitutions at embB codon 306 (embB306) have been proposed to be a potential marker for EMB resistance and a predictor of broad drug resistance in clinical Mycobacterium tuberculosis isolates. However, discordant findings about the association between embB306 mutations and EMB resistance were reported. Hebei Province is located in the Beijing-Tianjin-Hebei integration region in China; however, little information about the genetic diversity of the embB locus in this area is available. In this study, we sequenced the region surrounding embB306 (codons 207 to 445) in 62 ethambutol-resistant (EMBr) isolates, 214 ethambutol-susceptible isolates resistant to other first-line drugs (EMBs isolates), and 100 pan-sensitive isolates. Our data indicated that none of the pan-sensitive isolates showed mutations at embB306 and 63 drug-resistant isolates harbored embB306 substitutions, with these substitutions being found in 56.5% (35/62) of EMBr isolates and 13.1% (28/214) of EMBs isolates. A significant association between the embB306 mutation and resistance to isoniazid, rifampin, EMB, and multiple drugs was observed, and the rate of mutation of embB306 increased with increasing numbers of first-line drugs to which the isolates were resistant. The embB306 mutation is not the sole causative factor for EMB resistance, and the poor sensitivity limits its utility as a marker for drug-resistant TB. However, it may be a potential marker for broad drug resistance, especially for multidrug resistance. The mycobacterial interspersed repetitive unit-variable-number tandem-repeat profiles may serve as markers for predicting the embB306 substitutions that may occur in drug-resistant M. tuberculosis isolates under antimicrobial selection pressure.

Prevalence and molecular characteristics of drug-resistant Mycobacterium tuberculosis in Beijing, China: 2006 versus 2012

Background

As the epidemic of MDR-TB and XDR-TB becomes increasingly severe, it is important to determine the clinical characteristics and molecular epidemiology of MDR-TB and XDR-TB. Recently, many studies have shown that clinical features and molecular characteristics of drug-resistant strains vary in different geographical areas, however, further information is needed to assess the dynamic evolution of drug-resistant TB. Comparative studies between different time periods are necessary to elucidate the development of drug-resistant TB.

Results

A total of 255 and 537 strains were collected from Beijing Chest Hospital in 2006 and in 2012, respectively. Drug-resistance rates and mutations associated with resistance to first-line anti-tuberculosis (TB) drugs were compared. The overall rate of drug resistance among strains of TB in 2012 was 54.4 %, significantly higher than that in 2006 (34.9 %, P < 0.001). Rates of resistance to each first-line drug (isoniazid, rifampicin, streptomycin and ethambutol) and to second-line drug ofloxacin increased significantly from 2006 to 2012. The overall MDR rate also increased significantly from 2006 (14.9 %) to 2012 (27.0 %). The rate of MDR increased significantly between these two time periods in previously treated cases (P = 0.023) but not in new cases (P = 0.073), and the rate of XDR was similar in new cases at the two time periods, but was marginally higher in 2012 in previously treated cases (P = 0.056). Previous treatment was found to be a risk factor for drug-resistant TB, especially for MDR-TB. In addition, the proportion of drug resistant isolates in which katG, the mabA-inhA promoter, oxyR-ahpC intergenic region, rpoB, rpsL, and embB were mutated was similar in 2006 and 2012, however patterns of mutation in these loci were more diverse in 2012 compared to 2006.

Conclusions

Our data suggests that the prevalence of drug resistant TB remains high in Beijing, China, and that increasing rates of resistance in M. tuberculosis to all anti-TB drugs should be considered when choosing an optimal anti-TB regimen. Moreover, acquired multi-drug resistance may play a primary role in the MDR-TB epidemic in Beijing, China. Consequently, this highlights the importance of an earlier start to effective and supervised treatment in order to reduce the burden of retreatment.

Mutations at embB306 codon and their association with multidrug resistant M. tuberculosis clinical isolates

PURPOSE

The presence of embB306 mutation in ethambutol (EMB)-susceptible (EMBs) clinical isolates questions the significance of these mutations in conferring resistance to EMB. The present study was carried out to determine the occurrence of embB306 mutation in EMB-resistant (EMBr) and EMBs strains of M. tuberculosis. One hundred and four multidrug-resistant tuberculosis (MDR-TB) strains were also included to establish the relevance of excessive use of rifampicin (RIF) and isoniazid (INH) in occurrence of embB306 mutations in EMBs M. tuberculosis isolates.

MATERIALS AND METHODS

Deoxyribonucleic acid (DNA) from M. tuberculosis clinical strains was isolated by cetyltrimethylammonium bromide (CTAB) method. Phenotypic and genotypic drug susceptibility testing (DST) was performed on 354 M. tuberculosis isolates by using standard proportion method and multiplex-allele-specific polymerase chain reaction assay, respectively.

RESULTS

The overall frequency of embB306 mutations in EMBr isolates was found to be five times higher than its occurrence in EMB-susceptible isolates (50% vs 10%). Further, the association between embB306 mutation and EMB-resistance was observed to be statistically significant (P = 0.000).

CONCLUSION

The embB306 is not only the main causative mutation of EMB resistance, but is a sensitive applicant marker for EMB-resistance study.

Mutation at embB codon 306, a potential marker for the identification of multidrug resistance associated with ethambutol in Mycobacterium tuberculosis

Ethambutol inhibits arabinogalactan and lipoarabinomannan biosynthesis in mycobacteria. The occurrence of mutations in embB codon 306 in ethambutol-susceptible isolates and their absence in resistant isolates has raised questions regarding the utility of this codon as a potential marker for resistance against ethambutol. The characterization of mutations on embB 306 will contribute to a better understanding of the mechanisms of resistance to this drug; therefore, the purpose of this study was to investigate the association between embB 306 mutations and first-line drug resistance profiles in tuberculosis isolates. We sequenced the region surrounding the embB 306 codon in 175 tuberculosis clinical isolates, divided according to drug sensitivity, in three groups: 110 were resistant to at least one first-line drug, of which 61 were resistant to ethambutol (EMB(r)), 49 were sensitive to ethambutol (EMB(s)) but were resistant to another drug, and 65 were pansensitive isolates (P(s)). The associations between embB 306 mutations and phenotypic resistance to all first-line drugs were determined, and their validity and safety as a diagnostic marker were assessed. One of the P(s) isolates (1/65), one of the EMB(s) isolates (1/49), and 20 of the EMB(r) isolates (20/61) presented with an embB 306 mutation. Four different single-nucleotide polymorphisms (SNPs) at embB 306 were associated with simultaneous resistance to ethambutol, isoniazid, and rifampin (odds ratio [OR], 17.7; confidence interval [CI], 5.6 to 56.1) and showed a positive predictive value of 82%, with a specificity of 97% for diagnosing multidrug resistance associated with ethambutol, indicating its potential as a molecular marker for several drugs.

Molecular Analysis of the embCAB Locus and embR Gene Involved in Ethambutol Resistance in Clinical Isolates of Mycobacterium tuberculosis in France

Modification of codon 306 in embB is regarded as the main mechanism leading to ethambutol (ETB) resistance in clinical isolates of Mycobacterium tuberculosis. However, numerous mutations elsewhere in the embCAB locus and in embR, a putative transcriptional activator of this locus, have been reported to be involved in ETB resistance. Here, we investigated the diversity of nucleotide variations observed in embCAB and embR in M. tuberculosis complex isolates from France. These regions were sequenced in 71 ETB-resistant (ETB-R) and 60 ETB-susceptible (ETB-S) clinical isolates of known phylogenetic lineages. The 131 isolates had 12 mutations corresponding to phylogenetic markers. Among the 60 ETB-S isolates, only 3 (5%) had nonsynonymous mutations that were not phylogenetic markers. Among the 71 ETB-R isolates, 98% had mutations in embCAB that likely contribute to ETB resistance: 70% had mutations located in embB codon 306, 406, or 497; 13% had mutations located outside these three positions between codons 296 and 426; and 15% had mutations corresponding to mutations in the embC-embA intergenic region. We found a strong association between resistance to ETB and the presence of mutations in embB and the embC-embA intergenic region (P < 0.001). In contrast, the mutations detected in embC and embA were not involved in ETB resistance, and no mutation was detected in embR. These results strongly suggest that the sensitivity of diagnostic assays for detecting ETB resistance based on testing of embB codon 306 can be increased by testing of the embB region between codons 296 and 497 and by including the embC-embA intergenic region between positions -8 and -21.

Analysis of embCAB mutations associated with ethambutol resistance in multidrug-resistant mycobacterium tuberculosis isolates from China

Ethambutol (EMB) plays a pivotal role in the chemotherapy of drug-resistant tuberculosis (TB), including multidrug-resistant tuberculosis (MDR-TB). Resistance to EMB is considered to be caused by mutations in the embCAB operon (embC, embA, and embB). In this study, we analyzed the embCAB mutations among 139 MDR-TB isolates from China and found a possible association between embCAB operon mutation and EMB resistance. Our data indicate that 56.8% of MDR-TB isolates are resistant to EMB, and 82.2% of EMB-resistant isolates belong to the Beijing family. Overall, 110 (79.1%) MDR-TB isolates had at least one mutation in the embCAB operon. The majority of mutations were present in the embB gene and the embA upstream region, which also displayed significant correlations with EMB resistance. The most common mutations occurred at codon 306 in embB (embB306), followed by embB406, embA(-16), and embB497. Mutations at embB306 were associated with EMB resistance. DNA sequencing of embB306-497 was the best strategy for detecting EMB resistance, with 89.9% sensitivity, 58.3% specificity, and 76.3% accuracy. Additionally, embB306 had limited value as a candidate predictor for EMB resistance among MDR-TB infections in China.

Diagnostic accuracy of a molecular drug susceptibility testing method for the antituberculosis drug ethambutol: a systematic review and meta-analysis

Ethambutol (EMB) is a first-line antituberculosis drug; however, drug resistance to EMB has been increasing. Molecular drug susceptibility testing (DST), based on the embB gene, has recently been used for rapid identification of EMB resistance. The aim of this meta-analysis was to establish the accuracy of molecular assay for detecting drug resistance to EMB. PubMed, Embase, and Web of Science were searched according to a written protocol and explicit study selection criteria. Measures of diagnostic accuracy were pooled using a random effects model. A total of 34 studies were included in the meta-analysis. The respective pooled sensitivities and specificities were 0.57 and 0.93 for PCR-DNA sequencing that targeted the embB 306 codon, 0.76 and 0.89 for PCR-DNA sequencing that targeted the embB 306, 406, and 497 codons, 0.64 and 0.70 for detecting Mycobacterium tuberculosis isolates, 0.55 and 0.78 for detecting M. tuberculosis sputum specimens using the GenoType MTBDRsl test, 0.57 and 0.87 for pyrosequencing, and 0.35 and 0.98 for PCR-restriction fragment length polymorphism. The respective pooled sensitivities and specificities were 0.55 and 0.92 when using a lower EMB concentration as the reference standard, 0.67 and 0.73 when using a higher EMB concentration as the reference standard, and 0.60 and 1.0 when using multiple reference standards. PCR-DNA sequencing using multiple sites of the embB gene as detection targets, including embB 306, 406, and 497, can be a rapid method for preliminarily screening for EMB resistance, but it does not fully replace phenotypic DST. Of the reference DST methods examined, the agreement rates were the best using MGIT 960 for molecular DST and using the proportion method on Middlebrook 7H10 media.

Mutations in the embB gene and their association with ethambutol resistance in multidrug-resistant Mycobacterium tuberculosis clinical isolates from Poland

Ethambutol (EMB) continues to be used as part of a standard drug regimen for the treatment of tuberculosis (TB). Mutations in the embB gene and those within its conserved EMB resistance determining region (ERDR) in particular have repeatedly been associated with resistance to EMB in Mycobacterium tuberculosis. The aim of this study was to examine the mutational "hot spots" in the embB gene, including the ERDR, among multidrug-resistant (MDR) M. tuberculosis clinical isolates and to find a possible association between embB mutations and resistance to EMB. An 863-bp fragment of the embB gene was sequenced in 17 EMB-resistant and 33 EMB-susceptible MDR-TB isolates. In total, eight embB mutation types were detected in 6 distinct codons of 27 (54%) M. tuberculosis isolates. Mutations in codon 306 were most common, found in both EMB-resistant (9) and EMB-susceptible (11) isolates. Only mutations in codons 406 and 507 were found exclusively in four and one EMB-resistant isolates, respectively. Sequence analysis of the ERDR in the embB gene is not sufficient for rapid detection of EMB resistance, and the codon 306 mutations are not good predictive markers of resistance to EMB.

Characterization of the embB gene in Mycobacterium tuberculosis isolates from Barcelona and rapid detection of main mutations related to ethambutol resistance using a low-density DNA array

OBJECTIVES

Ethambutol resistance has mostly been related to mutations in the embB gene. The objective of the present study was to characterize the embB gene in a collection of ethambutol-resistant and ethambutol-susceptible isolates of Mycobacterium tuberculosis complex (MTBC) from Barcelona, and to develop a DNA microarray for the rapid detection of embB mutations in our area.

METHODS

Fifty-three ethambutol-resistant and 702 ethambutol-susceptible isolates of MTBC were sequenced in internal 982-1495 bp fragments of the embB gene. In addition, a low-cost, low-density array was designed to include the embB codons identified as being most frequently mutated in our area (LD-EMB array).

RESULTS

The global prevalence of embB mutations found among the ethambutol-resistant isolates was 77.4% (41/53). Substitutions in embB306 were the most common [53.7% (22/41)], followed by substitutions in embB406 [26.8% (11/41)]. The presence of mutations in embB406 was related to higher levels of ethambutol resistance and to multidrug resistance. Among unrelated isolates (from 24-locus MIRU-VNTR genotyping), the percentage of embB-mutated isolates was 72.9% (27/37)--59.3% (16/27) in embB306 and 25.9% (7/27) in embB406. None of the ethambutol-susceptible isolates studied showed a mutation in codon 306 or 406. The LD-EMB array showed 100% sensitivity and specificity in identifying the main embB substitutions in our area.

CONCLUSIONS

Mutations at codons 306 and 406 of embB have a relevant role in resistance to ethambutol in our area. The LD-EMB array developed in this study would appear to be a good molecular test for rapid detection of ethambutol resistance.

Characterization of mutations in multi- and extensive drug resistance among strains of Mycobacterium tuberculosis clinical isolates in Republic of Korea

In order to characterize molecular mechanisms of first- and second-line drug resistance in Mycobacterium tuberculosis and to evaluate the use of molecular markers of resistance, we analyzed 62 multidrug-resistant, 100 extensively drug-resistant, and 30 pan-susceptible isolates from Korean tuberculosis patients. Twelve genome regions associated with drug resistance, including katG, ahpC, and inhA promoter for isoniazid (INH); embB for ethambutol (EMB), rpoB for rifampin (RIF), pncA for pyrazinamide (PZA), gyrA for fluoroquinolones; rpsL, gidB, and rrs for streptomycin; rrs and eis for kanamycin (KM); rrs and tylA for capreomycin (CAP); and rrs for amikacin (AMK) were amplified simultaneously by polymerase chain reaction, and the DNA sequences were determined. We found mutations in 140 of 160 INH-resistant isolates (87.5%), 159 of 162 RIF-resistant isolates (98.15%), 127 of 143 EMB-resistant isolates (88.8%), 108 of 123 ofloxacin-resistant isolates (87.8%), and 107 of 122 PZA-resistant isolates (87.7%); 43 of 51 STM-resistant isolates (84.3%), 15 of 17 KM-resistant isolates (88.2%), and 14 of 15 (AMK and CAP)-resistant isolates (93.3%) had mutations related to specific drug resistance. In addition, the sequence analyses of the study revealed many novel mutations involving these loci. This result suggests that mutations in the rpoB531, katGSer315Thr, and C-15T in the inhA promoter region, and gyrA94, embB306, pncA159, rpsL43, and A1401G in the rrs gene could serve as useful markers for rapid detection of resistance profile in the clinical isolates of M. tuberculosis in Korea, with potentials for the new therapeutic benefits in actual clinical practice.

Arabinogalactan and lipoarabinomannan biosynthesis: structure, biogenesis and their potential as drug targets

Mycobacterium tuberculosis, the etiological agent of TB, remains the leading cause of mortality from a single infectious organism. The persistence of this human pathogen is associated with its distinctive lipid-rich cell wall structure that is highly impermeable to hydrophilic chemical drugs. This highly complex and unique structure is crucial for the growth, viability and virulence of M. tuberculosis, thus representing an attractive target for vaccine and drug development. It contains a large macromolecular structure known as the mycolyl-arabinogalactan-peptidoglycan complex, as well as phosphatidyl-myo-inositol derived glycolipids with potent immunomodulatory activity, notably lipomannan and lipoarabinomannan. These cell wall components are often the targets of effective chemotherapeutic agents against TB, such as ethambutol. This review focuses on the structural details and biosynthetic pathways of both arabinogalactan and lipoarabinomannan, as well as the effects of potent drugs on these important (lipo)polysaccharides.

High-resolution melting analysis for the rapid detection of fluoroquinolone and streptomycin resistance in Mycobacterium tuberculosis

Background

Molecular methods for the detection of drug-resistant tuberculosis are potentially more rapid than conventional culture-based drug susceptibility testing, facilitating the commencement of appropriate treatment for patients with drug resistant tuberculosis. We aimed to develop and evaluate high-resolution melting (HRM) assays for the detection of mutations within gyrA, rpsL, and rrs, for the determination of fluoroquinolone and streptomycin resistance in Mycobacterium tuberculosis (MTB).

Methodology/Principal Findings

A blinded series of DNA samples extracted from a total of 92 clinical isolates of MTB were analyzed by HRM analysis, and the results were verified using DNA sequencing. The sensitivity and specificity of the HRM assays in comparison with drug susceptibility testing were 74.1% and 100.0% for the detection of fluoroquinolone resistance, and 87.5% and 100.0% for streptomycin resistance. Five isolates with low level resistance to ofloxacin had no mutations detected in gyrA, possibly due to the action of efflux pumps, or false negativity due to mixed infections. One fluoroquinolone-resistant isolate had a mutation in a region of gyrA not encompassed by our assay. Six streptomycin-resistant strains had undetectable mutations by HRM and DNA sequencing, which may be explained by the fact that not all streptomycin-resistant isolates have mutations within rpsL and rrs, and suggesting that other targets may be involved.

Conclusion

The HRM assays described here are potentially useful adjunct tests for the efficient determination of fluoroquinolone and streptomycin resistance in MTB, and could facilitate the timely administration of appropriate treatment for patients infected with drug-resistant TB.

Characteristics of embB mutations in multidrug-resistant Mycobacterium tuberculosis isolates in Henan, China

Objectives

To determine the association between embB mutations and drug resistance, and to further investigate the mechanism of embB mutations involved in the development of ethambutol and multidrug resistance in Mycobacterium tuberculosis.

Methods

One hundred and thirty-eight multidrug-resistant clinical M. tuberculosis isolates, including 86 ethambutol-resistant and 52 ethambutol-susceptible strains, were analysed to characterize mutations within the entire coding region of the embB gene. Moreover, a two-step genotyping was performed to identify the genetic lineage.

Results

In total, 27 embB mutation types were detected in 19 distinct codons. Though a strong association was observed between embB mutations and ethambutol resistance, 19.2% of embB306 mutants and 11.5% of embB406 or embB497 mutants were ethambutol susceptible. Among 39 ethambutol-resistant strains without embB306 mutations, 51.3% harboured mutations at codons 406 or 497. Particularly, three pairs of isolates with identical embB mutations and genotyping features were identified with variant ethambutol susceptibility. Among 77 isoniazid, rifampicin, streptomycin and ethambutol quadruple drug-resistant isolates, 89.6% carried embB mutations and 83.1% could be identified by detecting 10 embB mutations.

Conclusions

Our results suggest embB mutations alone are not sufficient for the development of full resistance to ethambutol in M. tuberculosis and mutations other than embB are also needed. Our study confirms the importance of mutations at embB406 and embB497 as hotspots, in addition to embB306, for detecting ethambutol resistance. Ten selected mutations of embB, covered by a short PCR product, can be used as candidate markers for the prediction of quadruple resistance to isoniazid, rifampicin, streptomycin and ethambutol.

Molecular basis and mechanisms of drug resistance in Mycobacterium tuberculosis: classical and new drugs

Tuberculosis (TB) remains one of the leading public health problems worldwide. Declared as a global emergency in 1993 by the WHO, its control is hampered by the emergence of multidrug resistance (MDR), defined as resistance to at least rifampicin and isoniazid, two key drugs in the treatment of the disease. More recently, severe forms of drug resistance such as extensively drug-resistant (XDR) TB have been described. After the discovery of several drugs with anti-TB activity, multidrug therapy became fundamental for control of the disease. Major advances in molecular biology and the availability of new information generated after sequencing the genome of Mycobacterium tuberculosis increased our knowledge of the mechanisms of resistance to the main anti-TB drugs. Better knowledge of the mechanisms of drug resistance in TB and the molecular mechanisms involved will help us to improve current techniques for rapid detection and will also stimulate the exploration of new targets for drug activity and drug development. This article presents an updated review of the mechanisms and molecular basis of drug resistance in M. tuberculosis. It also comments on the several gaps in our current knowledge of the molecular mechanisms of drug resistance to the main classical and new anti-TB drugs and briefly discusses some implications of the development of drug resistance and fitness, transmission and pathogenicity of M. tuberculosis.

embCAB sequence variation among ethambutol-resistant Mycobacterium tuberculosis isolates without embB306 mutation

OBJECTIVES

Mechanisms of resistance to ethambutol in Mycobacterium tuberculosis remain inadequately described. Although there is mounting evidence that mutations of codon 306 in embB play a key role, a significant number of phenotypically ethambutol-resistant strains do not carry mutations in this codon. Here, other mutations in the embCAB operon are suggested to be involved in resistance development.

METHODS

The entire embCAB operon ( approximately 10 kb) was analysed in 34 phenotypically ethambutol-resistant M. tuberculosis strains without mutations in embB306 and in 12 ethambutol-susceptible strains. Furthermore, 106 control strains were investigated for the presence of particular mutations only.

RESULTS

Overall, 18 non-synonymous mutations in 15 distinct codons of the embCAB operon were identified in ethambutol-resistant strains but not in ethambutol-susceptible isolates. The majority occurred in the embB gene (10 distinct codons), in a 570 bp region also encompassing embB306. Mutations in embC and embA were found rarely and in most cases in combination with polymorphisms in embB. One synonymous mutation (embA 228 bp) and two non-synonymous mutations (embCVal981Leu and embCArg738Gln) were found in ethambutol-susceptible strains as well as resistant strains and were confirmed to represent phylogenetic markers for strains of the Beijing, Haarlem and Delhi/CAS genotypes, respectively.

CONCLUSIONS

Besides mutations in embB306, mutations in embB406 and embB497 were confirmed as hot spots for genomic variation in ethambutol-resistant clinical isolates. Of all resistant strains 70.6% carry a mutation in a relatively short region in embB, which therefore represents a promising target for inclusion in molecular assays for rapid detection of ethambutol resistance.

Allelic exchange and mutant selection demonstrate that common clinical embCAB gene mutations only modestly increase resistance to ethambutol in Mycobacterium tuberculosis

Mutations within codon 306 of the Mycobacterium tuberculosis embB gene modestly increase ethambutol (EMB) MICs. To identify other causes of EMB resistance and to identify causes of high-level resistance, we generated EMB-resistant M. tuberculosis isolates in vitro and performed allelic exchange studies of embB codon 406 (embB406) and embB497 mutations. In vitro selection produced mutations already identified clinically in embB306, embB397, embB497, embB1024, and embC13, which result in EMB MICs of 8 or 14 microg/ml, 5 microg/ml, 12 microg/ml, 3 microg/ml, and 4 microg/ml, respectively, and mutations at embB320, embB324, and embB445, which have not been identified in clinical M. tuberculosis isolates and which result in EMB MICs of 8 microg/ml, 8 microg/ml, and 2 to 8 microg/ml, respectively. To definitively identify the effect of the common clinical embB497 and embB406 mutations on EMB susceptibility, we created a series of isogenic mutants, exchanging the wild-type embB497 CAG codon in EMB-susceptible M. tuberculosis strain 210 for the embB497 CGG codon and the wild-type embB406 GGC codon for either the embB406 GCC, embB406 TGC, embB406 TCC, or embB406 GAC codon. These new mutants showed 6-fold and 3- to 3.5-fold increases in the EMB MICs, respectively. In contrast to the embB306 mutants, the isogenic embB497 and embB406 mutants did not have preferential growth in the presence of isoniazid or rifampin (rifampicin) at their MICs. These results demonstrate that individual embCAB mutations confer low to moderate increases in EMB MICs. Discrepancies between the EMB MICs of laboratory mutants and clinical M. tuberculosis strains with identical mutations suggest that clinical EMB resistance is multigenic and that high-level EMB resistance requires mutations in currently unknown loci.

The arabinosyltransferase EmbC is inhibited by ethambutol in Mycobacterium tuberculosis

Ethambutol (EMB) is an antimycobacterial drug used extensively for the treatment of tuberculosis caused by Mycobacterium tuberculosis. EMB targets the biosynthesis of the cell wall, inhibiting the synthesis of both arabinogalactan and lipoarabinomannan (LAM), and is assumed to act via inhibition of three arabinosyltransferases: EmbA, EmbB, and EmbC. EmbA and EmbB are required for the synthesis of arabinogalactan, and at least one enzyme (M. tuberculosis EmbA [EmbA(Mt)]) is essential in M. tuberculosis. EmbC(Mt) is also essential for the viability of M. tuberculosis but is involved in the synthesis of LAM. We show that mutations in EmbC(Mt) that reduce its arabinosyltransferase activity result in increased sensitivity to EMB and the production of smaller LAM species in M. tuberculosis. Overexpression of EmbC(Mt) was not tolerated in M. tuberculosis, but overexpression of Mycobacterium smegmatis EmbC (EmbC(Ms)) led to EMB resistance and the production of larger LAM species in M. tuberculosis. Treatment of wild-type M. tuberculosis strains with EMB led to inhibition of LAM synthesis, resulting in the production of smaller species of LAM. In contrast, no change in LAM production was seen in EMB-resistant strains. Overexpression of EmbB(Ms) in M. tuberculosis also resulted in EMB resistance, but at a lower level than that caused by EmbC(Ms). Overexpression of EmbA(Mt) in M. tuberculosis had no effect on EMB resistance. Thus, there is a direct correlation between EmbC activity and EMB resistance, as well as between EmbC activity and the size of the LAM species produced, confirming that EmbC is one of the cellular targets of EMB action.

Mutations at embB codon 306 are an important molecular indicator of ethambutol resistance in Mycobacterium tuberculosis

Ethambutol resistance in clinical Mycobacterium tuberculosis isolates is associated primarily with missense mutations in the embB gene. However, recent reports have described the presence of embB mutations, especially those at embB codon 306, in isolates susceptible to ethambutol. To clarify the role of embB mutations in ethambutol resistance, we sequenced the ethambutol resistance-determining region in spontaneous ethambutol-resistant mutants. In our study, 66% of spontaneous mutants contained a single point mutation in embB, with 55% of these occurring at embB 306. The MIC of ethambutol for spontaneous mutants was increased two- to eightfold relative to the pansusceptible M. tuberculosis strains from which the mutants were generated. To further characterize the role of embB 306 mutations, we directly introduced mutant alleles, embB(M306V) or embB(M306I), into pansusceptible M. tuberculosis strains and conversely reverted mutant alleles in spontaneous ethambutol-resistant mutants back to those of the wild type via allelic exchange using specialized linkage transduction. We determined that the MIC of ethambutol was reduced fourfold for three of the four spontaneous ethambutol-resistant embB 306 mutants when the mutant allele was replaced with the wild-type embB allele. The MIC for one of the spontaneous mutants genetically reverted to wild-type embB was reduced by only twofold. When the wild-type embB allele was converted to the mutant allele embB(M306V), the ethambutol MIC was increased fourfold, and when the allele was changed to M306I, the ethambutol MIC increased twofold. Our data indicate that embB 306 mutations are sufficient to confer ethambutol resistance, and detection of these mutations should be considered in the development of rapid molecular tests.

Phenotypic and genotypic characterization of drug-resistant Mycobacterium tuberculosis strains

Of 142 pulmonary tuberculosis patients, 76 were considered high risk for the development of resistance, and 24 were confirmed as resistant strain carriers. Resistant isoniazid strains presented a high frequency of katG and ahpC mutations (90%) correlated with an MIC >4 microg/mL (94%). inhA mutations were not seen. rpoB mutations were identified in 78.6% of rifampicin-resistant strains, usually in codon 531 (72.7%), and 75% had an MIC >16 microg/mL. katG and rpoB mutations recognized 88.2% of multidrug-resistant strains and proved more efficient than the katG and rpoB mutations alone. Seventy percent of resistant pyrazinamide strains had pncA mutations between genes 136 and 188, 62.5% of them with an MIC >900 microg/mL. Pyrazinamidase inactivity was not an efficient resistance marker because 60% of pncA-mutated strains maintained enzymatic activity despite displaying good correlation with high resistance levels. Resistant ethambutol strains had embB mutations in codon 306, with MIC >16 microg/mL.

Transfer of embB codon 306 mutations into clinical Mycobacterium tuberculosis strains alters susceptibility to ethambutol, isoniazid, and rifampin

Implicated as a major mechanism of ethambutol (EMB) resistance in clinical studies of Mycobacterium tuberculosis, mutations in codon 306 of the embB gene (embB306) have also been detected in EMB-susceptible clinical isolates. Other studies have found strong associations between embB306 mutations and multidrug resistance, but not EMB resistance. We performed allelic exchange studies in EMB-susceptible and EMB-resistant clinical M. tuberculosis isolates to identify the role of embB306 mutations in any type of drug resistance. Replacing wild-type embB306 ATG from EMB-susceptible clinical M. tuberculosis strain 210 with embB306 ATA, ATC, CTG, or GTG increased the EMB MIC from 2 microg/ml to 7, 7, 8.5, and 14 microg/ml, respectively. Replacing embB306 ATC or GTG from two high-level EMB-resistant clinical strains with wild-type ATG lowered EMB MICs from 20 microg/ml or 28 microg/ml, respectively, to 3 microg/ml. All parental and isogenic mutant strains had identical isoniazid (INH) and rifampin (RIF) MICs. However, embB306 CTG mutants had growth advantages compared to strain 210 at sub-MICs of INH or RIF in monocultures and at sub-MICs of INH in competition assays. CTG mutants were also more resistant to the additive or synergistic activities of INH, RIF, or EMB used in different combinations. These results demonstrate that embB306 mutations cause an increase in the EMB MIC, a variable degree of EMB resistance, and are necessary but not sufficient for high-level EMB resistance. The unusual growth property of embB306 mutants in other antibiotics suggests that they may be amplified during treatment in humans and that a single mutation may affect antibiotic susceptibility against multiple first-line antibiotics.

Lack of correlation between embB mutation and ethambutol MIC in Mycobacterium tuberculosis clinical isolates from China

Seventy-four Mycobacterium tuberculosis clinical isolates from China were subjected to drug susceptibility testing using ethambutol, isoniazid, rifampin, and ofloxacin. The results revealed that the presence of embB mutations did not correlate with ethambutol resistance but was associated with multiple-drug resistance, especially resistance to both ethambutol and rifampin.

Association between embB codon 306 mutations and drug resistance in Mycobacterium tuberculosis

embB306 mutants were detected in both ethambutol (EMB)-resistant and EMB-susceptible strains of Mycobacterium tuberculosis. Multidrug-resistant (MDR) strains had a higher proportion of embB306 mutants than non-MDR strains (odds ratio, 6.78; P < 0.001). The embB306 locus is a candidate marker for rapid detection of MDR and extremely drug resistant tuberculosis.

Frequency of embB codon 306 mutations in ethambutol-susceptible and -resistant clinical Mycobacterium tuberculosis isolates in Kuwait

SETTING

Recent reports of embB306 mutations in ethambutol-resistant and ethambutol-susceptible drug-resistant strains of Mycobacterium tuberculosis have questioned the significance of these mutations in conferring resistance to ethambutol (EMB).

OBJECTIVE

To determine the occurrence of embB306 mutations in all EMB-resistant and -susceptible drug-resistant M. tuberculosis strains isolated during a specified period at a single geographical location.

DESIGN

Twenty-five pansusceptible, all EMB-resistant and -susceptible M. tuberculosis strains resistant to other first-line drugs isolated in Kuwait during 2000-2003 were analyzed. The embB306 mutations were detected by PCR-restriction fragment length polymorphism and DNA sequencing. The PCR-based methods were also used for determining strain relatedness.

RESULTS

None of the pansusceptible M. tuberculosis strains contained a mutation at embB306. Fifteen of 50 (30%) EMB-resistant strains but only three of 122 (2%) EMB-susceptible isolates resistant to other first-line drugs contained a mutated embB306. The EMB-susceptible isolates with embB306 mutation were resistant to isoniazid plus other drug(s). The isolates carrying similar mutations were genotypically distinct strains.

CONCLUSIONS

The frequency of embB306 mutations in EMB-resistant strains compared to EMB-susceptible M. tuberculosis isolates resistant to other drugs was 15 times higher. Association of embB306 mutations in EMB-susceptible strains with isoniazid resistance and inherent problems associated with phenotypic EMB susceptibility testing suggest that these strains may actually be EMB-resistant.

Genotypic Analysis of Multidrug-Resistant Mycobacterium tuberculosis Isolates Recovered From Central China

DNA sequencing analysis was used to investigate genetic alterations in the rpoB, katG, and inhA regulatory region and embB in 66 Mycobacterium tuberculosis isolates recovered from Central China. Of the 36 multidrug-resistant isolates, 33 (92%) had mutations in the amplified region of rpoB. The most frequent mutation (58%, 19/36) was S531L (TCG-->TTG). At least one mutation was found in the katG and inhA regulatory region in 83% (30/36) of the multidrug-resistant isolates, and mutations at katG codon 315 were identified in 78% (28/36). Alterations at embB306 may not confer resistance to EMB, and embB306 mutants were more frequently accompanied by rpoB mutations (100%, 16/16) than by katG 315 mutations (75%, 12/16). Our results show that geographic variation in the molecular genetic mechanism is responsible for drug resistance in multidrug-resistant M. tuberculosis. This observation will facilitate the development of a rapid molecular drug resistance screening approach for drug-resistant M. tuberculosis.

Development of a quantitative assay for mycobacterial endogenous arabinase and ensuing studies of arabinase levels and arabinan metabolism in Mycobacterium smegmatis

Treatment of either Mycobacterium tuberculosis or M. smegmatis with ethambutol results both in inhibition of arabinan synthesis and in copious loss of previously formed arabinan from the cell wall. The loss of arabinan has been shown to be due to the action of an endogenous arabinase. To better understand this phenomenon, a quantitative assay for endogenous arabinase was developed. Using the assay it was determined that various subcellular fractions of M. smegmatis showed significant amounts of endogenous arabinase activity. Surprisingly, treatment with ethambutol yielded only minor changes in the amounts of endogenous arabinase activities. Endogenous arabinase was present in the cell wall, and consistently, incubation of the M. smegmatis cell wall in only buffer resulted in the release of arabinan, mimicking the effect of ethambutol on whole cells. To determine if cell wall arabinan is rapidly turned over, the arabinan was labeled in the early log phase of culture by feeding [(14)C]glucose, followed by a "chase" with nonradioactive glucose. Most of the labeled arabinan remained in the cell wall after the culture was grown to late log phase. Thus, there is active arabinase in the cell wall, but arabinan is not rapidly removed unless ethambutol is present. Purification of the endogenous arabinase, using the assay described, is ongoing to help further discern its biological function.

Significance of mutations in embB codon 306 for prediction of ethambutol resistance in clinical Mycobacterium tuberculosis isolates

We analyzed 159 Mycobacterium tuberculosis isolates (101 ethambutol [EMB]-resistant strains, 33 multidrug-resistant but not EMB-resistant strains, and 25 fully susceptible strains) for the presence of mutations in embB codon 306 (embB306). Mutations were detected only in EMB-resistant strains (n = 69; 68%), thus confirming the significance of embB306 mutations for the prediction of resistance to EMB.