Importance of improved TB diagnostics in addressing the extensively drug-resistant TB crisis

Whole-Genome Sequencing in Relation to Resistance of Mycobacterium Tuberculosis

Tuberculosis, a disease caused by Mycobacterium tuberculosis, represents one of the deadliest infections worldwide. The incidence of resistant forms is increasing year by year; therefore, it is necessary to involve new methods for rapid diagnostics and treatment. One of the possible solutions is the use of whole-genome sequencing (WGS).

The WGS provides an identification of complete genome of the microorganism, including all genes responsible for resistance, in comparison with other genotypic methods (eg. Xpert MTB / RIF or Hain line-probes) that are capable to detect only basic genes. WGS data are available in 1-9 days and several online software tools (TBProfiler, CASTB, Mykrobe PredictorTB) are used for their interpretation and analysis, compared to 3-8 weeks in the case of classic phenotypic evaluation.

Furthermore, WGS predicts resistance to the first-line antituberculotics with a sensitivity of 85-100% and a specificity of 85-100%.

This review elucidates the importance and summarizes the current knowledge about the possible use of WGS in diagnosis and treatment of resistant forms of tuberculosis elucidates.

WGS of M. tuberculosis brings new possibilities for rapid and accurate diagnostics of resistant forms of tuberculosis. Introducing WGS into routine practice can help to reduce the spread of resistant forms of tuberculosis as well as to increase the success rate of the treatment, especially through an appropriate combination of antituberculotics ATs. Introduction of WGS into routine diagnostics can, in spite of the financial difficulty, significantly improve patient care.

The implications of whole-genome sequencing in the control of tuberculosis

The availability of whole-genome sequencing (WGS) as a tool for the diagnosis and clinical management of tuberculosis (TB) offers considerable promise in the fight against this stubborn epidemic. However, like other new technologies, the best application of WGS remains to be determined, for both conceptual and technical reasons. In this review, we consider the potential value of WGS in the clinical laboratory for the detection of Mycobacterium tuberculosis and the prediction of antibiotic resistance. We also discuss issues pertaining to data generation, interpretation and dissemination, given that WGS has to date been generally performed in research labs where results are not necessarily packaged in a clinician-friendly format. Although WGS is far more accessible now than it was in the past, the transition from a research tool to study TB into a clinical test to manage this disease may require further fine-tuning. Improvements will likely come through iterative efforts that involve both the laboratories ready to move TB into the genomic era and the front-line clinical/public health staff who will be interpreting the results to inform management decisions.

Rapid, comprehensive, and affordable mycobacterial diagnosis with whole-genome sequencing: a prospective study

Background

Slow and cumbersome laboratory diagnostics for Mycobacterium tuberculosis complex (MTBC) risk delayed treatment and poor patient outcomes. Whole-genome sequencing (WGS) could potentially provide a rapid and comprehensive diagnostic solution. In this prospective study, we compare real-time WGS with routine MTBC diagnostic workflows.

Methods

We compared sequencing mycobacteria from all newly positive liquid cultures with routine laboratory diagnostic workflows across eight laboratories in Europe and North America for diagnostic accuracy, processing times, and cost between Sept 6, 2013, and April 14, 2014. We sequenced specimens once using local Illumina MiSeq platforms and processed data centrally using a semi-automated bioinformatics pipeline. We identified species or complex using gene presence or absence, predicted drug susceptibilities from resistance-conferring mutations identified from reference-mapped MTBC genomes, and calculated genetic distance to previously sequenced UK MTBC isolates to detect outbreaks. WGS data processing and analysis was done by staff masked to routine reference laboratory and clinical results. We also did a microcosting analysis to assess the financial viability of WGS-based diagnostics.

Findings

Compared with routine results, WGS predicted species with 93% (95% CI 90–96; 322 of 345 specimens; 356 mycobacteria specimens submitted) accuracy and drug susceptibility also with 93% (91–95; 628 of 672 specimens; 168 MTBC specimens identified) accuracy, with one sequencing attempt. WGS linked 15 (16% [95% CI 10–26]) of 91 UK patients to an outbreak. WGS diagnosed a case of multidrug-resistant tuberculosis before routine diagnosis was completed and discovered a new multidrug-resistant tuberculosis cluster. Full WGS diagnostics could be generated in a median of 9 days (IQR 6–10), a median of 21 days (IQR 14–32) faster than final reference laboratory reports were produced (median of 31 days [IQR 21–44]), at a cost of £481 per culture-positive specimen, whereas routine diagnosis costs £518, equating to a WGS-based diagnosis cost that is 7% cheaper annually than are present diagnostic workflows.

Interpretation

We have shown that WGS has a scalable, rapid turnaround, and is a financially feasible method for full MTBC diagnostics. Continued improvements to mycobacterial processing, bioinformatics, and analysis will improve the accuracy, speed, and scope of WGS-based diagnosis.

Funding

National Institute for Health Research, Department of Health, Wellcome Trust, British Colombia Centre for Disease Control Foundation for Population and Public Health, Department of Clinical Microbiology, Trinity College Dublin.

[The new tools of microbiological diagnosis of tuberculosis]

This review focuses on the role of new tools in the "modern" microbiological diagnosis of tuberculosis. Traditional techniques of microscopy and culture remain essential to diagnostic certainty, but some innovations replace daily the older techniques such as the identification of Mycobacterium tuberculosis complex by immunochromatography or mass spectrometry MALDI-TOF type from positive cultures, or susceptibility testing in liquid medium. New tools that use molecular techniques have become important. They all have in common to optimize the fight against tuberculosis by reducing diagnostic delay. They also allow rapid detection of drug resistance. However, the techniques of gene amplification directly from clinical samples are still less sensitive than culture. Bacteriological diagnosis of tuberculosis disease therefore still relies on the complementarities of different phenotypic and molecular techniques.

Phylogenetic polymorphisms in antibiotic resistance genes of the Mycobacterium tuberculosis complex

OBJECTIVES

Sequence analysis of known antibiotic resistance genes of the Mycobacterium tuberculosis complex (MTBC) is increasingly being used to infer phenotypic resistance to a variety of antibiotics. However, a clear understanding of the genotype-phenotype relationship is required to interpret genotypic susceptibility results accurately. In this context, it is particularly important to distinguish phylogenetically informative neutral polymorphisms from true resistance-conferring mutations.

METHODS

Using a collection of 71 strains that encompasses all major MTBC genotypes, we mapped the genetic diversity in 18 genes that are known to be involved or were previously implicated in antibiotic resistance to eight current as well as two novel antibiotics. This included bedaquiline, capreomycin, ethambutol, fluoroquinolones, isoniazid, PA-824, para-aminosalicylic acid, prothionamide, rifampicin and streptomycin. Moreover, we included data from one of our prior studies that focused on two of the three known pyrazinamide resistance genes.

RESULTS

We found 58 phylogenetic polymorphisms that were markers for the genotypes M. tuberculosis Beijing, Haarlem, Latin American-Mediterranean (LAM), East African Indian (EAI), Delhi/Central Asian (CAS), Ghana, Turkey (Tur), Uganda I and II, Ural and X-type, as well as for Mycobacterium africanum genotypes West African I (WA I) and II (WA II), Mycobacterium bovis, Mycobacterium caprae, Mycobacterium pinnipedii, Mycobacterium microti and Mycobacterium canettii.

CONCLUSIONS

This study represents one of the most extensive overviews of phylogenetically informative polymorphisms in known resistance genes to date, and will serve as a resource for the design and interpretation of genotypic susceptibility assays.

Comparison between the BACTEC MGIT 960 system and the agar proportion method for susceptibility testing of multidrug resistant tuberculosis strains in a high burden setting of South Africa

Background

The increasing problem of multi-drug-resistant (MDR) tuberculosis (TB) [ie resistant to at least isoniazid (INH) and rifampicin (RIF)] is becoming a global problem. Successful treatment outcome for MDR-TB depends on reliable and accurate drug susceptibility testing of first-line and second-line anti-TB drugs.

Method

Consecutive M. tuberculosis isolates identified as MDR-TB during August 2007 to January 2008 using the BACTEC MGIT 960 systems and the agar proportion method were included in this study. Susceptibility testing of MDR-TB isolates against ethambutol (EMB) and streptomycin (STR) as well as two second-line anti-TB drugs, kanamycin (KAN) and ofloxacin (OFX) was performed using the BACTEC MGIT 960 systems at a routine diagnostic laboratory. The results were compared to those obtained by the agar proportion method.

Result

The agreement between the BACTEC MGIT 960 system and the agar proportion method was 44% for EMB, 61% for STR and 89% for both KAN and OFX. The sensitivity and specificity of the BACTEC MGIT 960 system using the agar proportion method as a gold standard was 92% and 37% for EMB, 95% and 37% for STR, 27% and 97% for KAN and 84% and 90% for OFX, respectively.

Conclusions

The BACTEC MGIT 960 system showed acceptable sensitivity for EMB, STR, and OFX; however, the BACTEC MGIT 960 system was less specific for EMB and STR and demonstrated a low sensitivity for KAN. The lower agreement found between the two methods suggests the unreliability of the BACTEC MGIT 960 system for the drugs tested. The reasons for the lower agreement between the two methods need to be investigated and further studies are needed in this setting to confirm the study finding.

Importance of the genetic diversity within the Mycobacterium tuberculosis complex for the development of novel antibiotics and diagnostic tests of drug resistance

Despite being genetically monomorphic, the limited genetic diversity within the Mycobacterium tuberculosis complex (MTBC) has practical consequences for molecular methods for drug susceptibility testing and for the use of current antibiotics and those in clinical trials. It renders some representatives of MTBC intrinsically resistant against one or multiple antibiotics and affects the spectrum and consequences of resistance mutations selected for during treatment. Moreover, neutral or silent changes within genes responsible for drug resistance can cause false-positive results with hybridization-based assays, which have been recently introduced to replace slower phenotypic methods. We discuss the consequences of these findings and propose concrete steps to rigorously assess the genetic diversity of MTBC to support ongoing clinical trials.

Polymorphisms in isoniazid and prothionamide resistance genes of the Mycobacterium tuberculosis complex

Sequence analyses of 74 strains that encompassed major phylogenetic lineages of the Mycobacterium tuberculosis complex revealed 10 polymorphisms in mshA (Rv0486) and four polymorphisms in inhA (Rv1484) that were not responsible for isoniazid or prothionamide resistance. Instead, some of these mutations were phylogenetically informative. This genetic diversity must be taken into consideration for drug development and for the design of molecular tests for drug resistance.

Overview of errors in the reference sequence and annotation of Mycobacterium tuberculosis H37Rv, and variation amongst its isolates

Since its publication in 1998, the genome sequence of the Mycobacterium tuberculosis H37Rv laboratory strain has acted as the cornerstone for the study of tuberculosis. In this review we address some of the practical aspects that have come to light relating to the use of H37Rv throughout the past decade which are of relevance for the ongoing genomic and laboratory studies of this pathogen. These include errors in the genome reference sequence and its annotation, as well as the recently detected variation amongst isolates of H37Rv from different laboratories.

Role of the clinical mycobacteriology laboratory in diagnosis and management of tuberculosis in low-prevalence settings

Tuberculosis (TB) remains a global epidemic, despite a significant decline in reported cases in the United States between 2008 and 2009. While the exact nature of this decline is unclear, one thing remains certain: TB, including multidrug-resistant (MDR) and extensively drug-resistant (XDR) TB, is no longer restricted to developing regions of the globe. It is of vital importance that both public and private mycobacteriology laboratories maintain the ability to detect and identify Mycobacterium tuberculosis from patient specimens, as well as correctly determine the presence of antibiotic resistance. To do this effectively requires careful attention to preanalytical, analytical, and postanalytical aspects of testing. Respiratory specimens require digestion and concentration followed by fluorescence microscopy. The Centers for Disease Control and Prevention (CDC) recommends the performance of a direct nucleic acid amplification method, regardless of smear results, on specimens from patients in whom the suspicion of tuberculosis is high. Liquid-based technologies are more rapid and sensitive for the detection of M. tuberculosis in culture and nucleic acid probes, but biochemicals are preferred for identification once growth is detected. Susceptibility testing is most often done using either the agar proportion method or a commercial broth system. New genotypic and phenotypic methods of susceptibility testing include first- and second-line agents and are promising, though not yet widely available. Finally, gamma interferon release assays are preferred to the tuberculin skin test for screening certain at-risk populations, and new CDC guidelines are available that assist clinicians in their use.

Drug-resistant tuberculosis: a worldwide epidemic poses a new challenge

BACKGROUND

Although the incidence of tuberculosis (TB) in Germany is now declining, the world as a whole faces the threat of a catastrophe that will also affect the industrialized nations. The main reason, aside from TB/HIV co-infection, is the increase of resistant TB strains. The situation is already serious because of the spread of multidrug-resistant TB, i.e., TB that is resistant to the two most important antituberculous drugs, and is being further aggravated by resistance to second-line drugs as well.

METHOD

Selective review of the literature.

RESULTS

There are an estimated half a million cases of multidrug-resistant TB worldwide, and so-called extensively resistant TB (XDR-TB), with additional resistance to defined second-line drugs, is now prevalent in more than 45 countries. An accurate assessment of the situation is hampered by a widespread lack of laboratory capacity and/or proper surveillance. The problem is mainly due to inappropriate treatment, which may have many causes, but is theoretically avoidable. Aside from programmatic weaknesses, a lack of diagnostic and therapeutic tools causes difficulties in many countries.

DISCUSSION

Only rapid and internationally concerted action, combined with intensified research efforts and the support of the affected nations, will be able to prevent the development of a situation that will no longer be manageable even with 21(st)-century technology.