Drug resistance in tuberculosis

Modeling and analysis of barriers in controlling TB: developing countries' perspective

Purpose

Tuberculosis (TB) continues to c-exist with humans despite many TB control programs and elimination strategies. This depicts that some barriers are not allowing achieving the desired results. The current study aims to focus on identification and ranking of such barriers to facilitate TB control programs in developing countries.

Design/methodology/approach

In the present study, 13 barriers that can influence success rate of TB elimination strategies have been recognized with an in-depth assessment of related literature and opinions of specialists from medical industry and academic world. The interpretive structural modeling (ISM) and decision-making trial and evaluation laboratory (DEMATEL) techniques have been employed for the ranking of barriers.

Findings

Based on driving power of barriers, the study coined that underinvestment is a major barrier followed by poor implementation of government policies and programs, poverty and poor primary health care infrastructure.

Research limitations/implications

The findings may guide healthcare service providers and researchers in analyzing the barriers and understanding the necessity of further advancements to decrease the count of already existing and incident cases.

Practical implications

Policy- and decision-makers may utilize the information on dependence and driving power of barriers for better planning and effective execution of TB control strategies.

Originality/value

Although a lot of literature is available on different barriers that are affecting success of TB strategies, the current study analyzes all the key barriers collectively for the prioritization of barriers.

Antituberculosis Targeted Drug Delivery as a Potential Future Treatment Approach

Mycobacterium tuberculosis (Mtb) is the microorganism that causes tuberculosis. This infectious disease has been around for centuries, with the earliest record of Mtb around three million years ago. The discovery of the antituberculosis agents in the 20th century has managed to improve the recovery rate and reduce the death rate tremendously. However, the conventional antituberculosis therapy is complicated by the development of resistant strains and adverse drug reactions experienced by the patients. Research has been conducted continuously to discover new, safe, and effective antituberculosis drugs. In the last 50 years, only two molecules were approved despite laborious work and costly research. The repurposing of drugs is also being done with few drugs; antibiotics, particularly, were found to have antituberculosis activity. Besides the discovery work, enhancing the delivery of currently available antituberculosis drugs is also being researched. Targeted drug delivery may be a potentially useful approach to be developed into clinically accepted treatment modalities. Active targeting utilizes a specifically designed targeting agent to deliver a chemically conjugated drug(s) towards Mtb. Passive targeting is very widely explored, with the development of multiple types of nanoparticles from organic and inorganic materials. The nanoparticles will be engulfed by macrophages and this will eliminate the Mtb that is present in the macrophages, or the encapsulated drug may be released at the sites of infections that may be in the form of intra- and extrapulmonary tuberculosis. This article provided an overview on the history of tuberculosis and the currently available treatment options, followed by discussions on the discovery of new antituberculosis drugs and active and passive targeting approaches against Mycobacterium tuberculosis.

Mutations inside rifampicin-resistance determining region of rpoB gene associated with rifampicin-resistance in Mycobacterium tuberculosis

BACKGROUND

Rifampicin (RIF) plays a pivotal role in the treatment of tuberculosis due to its bactericidal effects. Because the action of RIF is on rpoB gene encoding RNA polymerase β subunit, 95% of RIF resistant mutations are present in rpoB gene. The majority of the mutations in rpoB gene are found within an 81bp RIF-resistance determining region (RRDR).

METHODOLOGY

Literatures on RIF resistant mutations published between 2010 and 2016 were thoroughly reviewed.

RESULTS

The most commonly mutated codons in RRDR of rpoB gene are 531, 526 and 516. The possibilities of absence of mutation in RRDR of rpoB gene in MDR-TB isolates in few studies was due to existence of other rare rpoB mutations outside RRDR or different mechanism of rifampicin resistance.

CONCLUSION

Molecular methods which can identify extensive mutations associated with multiple anti-tuberculous drugs are in urgent need so that the research on drug resistant mutations should be extended.

Drug resistance to Mycobacterium tuberculosis: from the traditional Chinese view to modern systems biology

The pathogen, Mycobacterium tuberculosis (M. tuberculosis) is a well-evolved, organized pathogen that has developed drug resistance, specifically multidrug resistance (MDR) and extensive drug resistance (XDR). This review primarily summarizes the mechanisms of drug resistance by M. tuberculosis according to the traditional Chinese view. The traditional Chinese view of drug resistance includes: the physical barrier of the cell wall; mutations relating to current anti-TB agents; drug efflux pumps; and drug stress, including the SOS response systems, the mismatch repair systems and the toxin-antitoxin systems. In addition, this review addresses the integrated systems biology of genomics, transcriptomics, proteomics, metabolomics and interactomics. Development of the various levels of systems biology has enabled determination of the anatomy of bacteria. Finally, the current review proposes that further investigation regarding the population of individuals with a high drug metabolic speed is vital to further understand drug resistance in M. tuberculosis.

Oral intake of phenylbutyrate with or without vitamin D3 upregulates the cathelicidin LL-37 in human macrophages: a dose finding study for treatment of tuberculosis

Background

We earlier showed that 4-phenylbutyrate (PB) can induce cathelicidin LL-37 expression synergistically with 1,25-dihydroxyvitamin D₃ in a lung epithelial cell line. We aimed to evaluate a therapeutic dose of PB alone or in combination with vitamin D₃ for induction of LL-37 expression in immune cells and enhancement of antimycobacterial activity in monocyte-derived macrophages (MDM).

Methods

Healthy volunteers were enrolled in an 8-days open trial with three doses of PB [250 mg (Group-I), 500 mg (Group-II) or 1000 mg (Group-III)] twice daily (b.d.) together with vitamin D₃ {5000 IU once daily (o.d.)}, PB (500 mg b.d.) (Group-IV) or vitamin D₃ (5000 IU o.d.) (Group-V), given orally for 4 days. Blood was collected on day-0, day-4 and day-8; plasma was separated, peripheral blood mononuclear cells (PBMC), non-adherent lymphocytes (NAL) and MDM were cultured. LL-37 transcript in cells and peptide concentrations in supernatant were determined by qPCR and ELISA, respectively. In plasma, 25-hydorxyvitamin D₃ levels were determined by ELISA. MDM-mediated killing of Mycobacterium tuberculosis (Mtb) (H37Rv) was performed by conventional culture method.

Results

MDM from Group-II had increased concentration of LL-37 peptide and transcript at day-4, while Group-I showed increased transcript at day-4 and day-8 compared to day-0 (p < 0.05). Both Group-I and -II exhibited higher levels of transcript on day-4 compared to Group-III and Group-V (p < 0.035). Increased induction of peptide was observed in lymphocytes from Group-II on day-4 compared to Group-I and Group-IV (p < 0.05), while Group-IV showed increased levels on day-8 compared to Group-I and Group-III (p < 0.04). Intracellular killing of Mtb on day-4 was significantly increased compared to day-0 in Group-I, -II and -V (p < 0.05).

Conclusion

The results demonstrate that 500 mg b.d. PB with 5000 IU o.d. vitamin D₃ is the optimal dose for the induction of LL-37 in macrophages and lymphocytes and intracellular killing of Mtb by macrophages. Hence, this dose has potential application in the treatment of TB and is now being used in a clinical trial of adults with active pulmonary TB (NCT01580007).

Laboratory diagnosis of tuberculosis in resource-poor countries: challenges and opportunities

With an estimated 9.4 million new cases globally, tuberculosis (TB) continues to be a major public health concern. Eighty percent of all cases worldwide occur in 22 high-burden, mainly resource-poor settings. This devastating impact of tuberculosis on vulnerable populations is also driven by its deadly synergy with HIV. Therefore, building capacity and enhancing universal access to rapid and accurate laboratory diagnostics are necessary to control TB and HIV-TB coinfections in resource-limited countries. The present review describes several new and established methods as well as the issues and challenges associated with implementing quality tuberculosis laboratory services in such countries. Recently, the WHO has endorsed some of these novel methods, and they have been made available at discounted prices for procurement by the public health sector of high-burden countries. In addition, international and national laboratory partners and donors are currently evaluating other new diagnostics that will allow further and more rapid testing in point-of-care settings. While some techniques are simple, others have complex requirements, and therefore, it is important to carefully determine how to link these new tests and incorporate them within a country's national diagnostic algorithm. Finally, the successful implementation of these methods is dependent on key partnerships in the international laboratory community and ensuring that adequate quality assurance programs are inherent in each country's laboratory network.

A mutation in Mycobacterium tuberculosis rpoB gene confers rifampin resistance in three HIV-TB cases

Rifampin is a key component of standard short-course first-line therapy against Mycobacterium tuberculosis (MTB). Rifampin monoresistant MTB, previously a rare phenomenon, is now being reported at increasing rates worldwide. We report a mutation in the rpoB region leading to low level rifampin monoresistance in a cluster of HIV-positive patients. All rifampin monoresistant isolates identified from 2004 to 2006 underwent susceptibility confirmation, sequencing of rpoB and genotyping. Three patients were found to have a previously undocumented 3 base pair insertion at codon 525 in the rpoB region. The earliest initial case was infected with fully susceptible MTB. Disease relapse occurred 7 months later with a genotypically identical MTB isolate, showing acquired rifampin monoresistance. MTB isolates from 2 subsequent patients showed primary rifampin monoresistance with an identical genotype to the index case. Patients with rifampin monoresistant MTB tend to have poorer outcomes than those with fully susceptible strains. Risk factors for the development of rifampin monoresistance include co-morbid HIV infection and previously treated tuberculosis. HIV infection has been associated with malabsorption of anti-tuberculous medications leading to sub-therapeutic levels of administered drugs. These factors may have played a role in the development of this previously undocumented mutation.

Boza, a natural source of probiotic lactic acid bacteria

AIMS

To evaluate the probiotic properties of strains isolated from boza, a traditional beverage produced from cereals.

METHODS AND RESULTS

The strains survived low pH conditions (pH 3.0), grew well at pH 9.0 and were not inhibited by the presence of 0.3% (w/v) oxbile. Cytotoxicity levels of the bacteriocins, expressed as CC(50), ranged from 38 to 3776 microg ml(-1). Bacteriocin bacST284BZ revealed high activity (EC(50) = 735 microg ml(-1)) against herpes simplex virus type 1. Growth of Mycobacterium tuberculosis was 69% repressed after 5 days in the presence of bacST194BZ. Various levels of auto-cell aggregation and co-aggregation with Listeria innocua LMG 13568 were observed. Adhesion of the probiotic strains to HT-29 cells ranged from 18 to 22%.

CONCLUSIONS

Boza is a rich source of probiotic lactic acid bacteria. All strains survived conditions simulating the gastrointestinal tract and produced bacteriocins active against a number of pathogens. Adherence to HT-29 and Caco-2 cells was within the range reported for Lactobacillus rhamnosus GG, a well-known probiotic. In addition, the high hydrophobicity readings recorded define the strains as good probiotics.

SIGNIFICANCE AND IMPACT OF THE STUDY

Boza contains a number of different probiotic lactic acid bacteria and could be marketed as a functional food product.

Spinal tuberculosis: CT and MRI feature

BACKGROUND

Spinal tuberculosis (TB) is perhaps the most clinically important extrapulmonary form of the disease. Early recognition is therefore necessary to minimize residual spinal deformity and/or permanent neurological deficit. We defined the CT and MRI image morphology of spinal TB and correlated the imaging features of these two modalities.

METHODS

CT (29 patients) and MRI (11 patients) images were retrospectively analyzed in 30 patients with proved spinal TB. CT and MRI findings were compared in cases with both imaging tests (10 cases). The parameters assessed were the type and extent of bone and soft tissue involvement.

RESULTS

The majority of the 30 patients were males (n=18) in the 30-49 year age group (43%). The most common clinical presentation was backache (73.3%) followed by fever (63.3%) and malaise (36.6%). The lumbar spine was the commonest site of the disease (43.3%) followed by the thoracic region (36.6%). A fragmentary type of bone destruction was the most frequent CT feature of the disease (48.2%) followed by the lytic type (24.1%). Intervertebral disc destruction (72%) and paravertebral mass/abscess (65.5%) were other features. Of the 11 patients who had an MRI, contiguous vertebral disease with disc destruction was seen in 10 cases. In 4 patients, there was distant vertebral disease in addition to the disease at the symptomatic site.

CONCLUSIONS

MRI offers excellent visualization of the bone and soft tissue components of spinal tuberculosis and helps to identify disease at distant asymptomatic sites. CT is useful in assessing bone destruction, but is less accurate in defining the epidural extension of the disease and therefore its effect on neural structures. MR imaging clearly demonstrated the extent of soft tissue disease and its effect on the theca/cord and foramen in cases with doubtful CT findings.

Influence of Mycobacterium bovis bacillus Calmette Guérin on in vitro induction of CD1 molecules in human adherent mononuclear cells

Nonpeptide antigens (including glycolipids of microbial origin) can be presented to T cells by CD1 molecules expressed on monocyte-derived dendritic cells. These HLA unrestricted responses appear to play a role in host immunity against Mycobacterium tuberculosis and other pathogenic bacteria. It is known that vaccination with Mycobacterium bovis bacillus Calmette-Guérin (BCG) has limited efficacy in many clinical settings, although the reasons for its inadequacy remain unclear. Here we have investigated the influence of BCG on the induction of CD1b on human monocytes by granulocyte-macrophage colony-stimulating factor (GM-CSF), which is believed to be the principal inducer of this antigen-presenting molecule. Although BCG alone led to a slight induction of CD1b expression, this agent reduced markedly the ability of GM-CSF to induce high levels of CD1b that were typically observed in uninfected cells. Inhibition of CD1b expression in BCG-infected monocytes was apparent at both the mRNA transcript and CD1b protein levels. Down-regulation of CD1b expression by BCG was mediated, at least in part, by one or more soluble factors and could not be reversed with high concentrations of GM-CSF or a variety of other cytokines. The present results suggest that BCG could diminish the efficiency of CD1-restricted T-cell responses against nonpeptide mycobacterial antigens by reducing CD1 expression on antigen-presenting cells. These findings have potential implications for understanding the nature of the immune response elicited by BCG in humans and suggest potential strategies that could be important for the development of better vaccines for the prevention of tuberculosis.

Molecular characterization of rifampin-resistant isolates of Mycobacterium tuberculosis from Hungary by DNA sequencing and the line probe assay

Two regions of rpoB associated with rifampin resistance were sequenced in 29 rifampin-resistant (determined by the proportion method) isolates of Mycobacterium tuberculosis obtained from patients from three counties in Hungary. Of the 29 resistant strains, 27 had a mutation in either the 81-bp region (26 strains) or the N-terminal region (1 strain), while the other 2 strains had no mutations in either region. The locations and frequencies of the mutations differed from those previously reported. The most common mutation in this study, D516V, was found in 38% of the Hungarian strains, a frequency 2 to 10 times higher than that found in studies from other countries. These same 29 isolates were also evaluated with the Inno-LiPA Rif. TB test (LiPA), a reverse hybridization assay for the rapid detection of rifampin resistance. Although LiPA detected the presence of an rpoB mutation in 26 of the resistant isolates, the type of mutation could not be determined in 4 isolates because the mutations present were not among those included on the LiPA strip. In addition, a silent mutation in one of the rifampin-susceptible control strains was interpreted as rifampin resistant by LiPA. These findings demonstrate the importance of validating this rapid molecular test by comparison with DNA sequence results in each geographic location before incorporating the test into routine diagnostic work.

Laboratory diagnosis of mycobacterial infections: new tools and lessons learned

Even in the 21st century, tuberculosis continues to be a problem. Although the number of cases continues gradually to decrease in the United States, cases get more difficult to treat, specifically those that are multiple-drug resistant. Infection of one-third of the world's population ensures that tuberculosis will not disappear in the near future. In light of this, it will be useful to know the goals for the health care system and how these goals may be accomplished. Laboratory testing in the mycobacteriology field is experiencing more changes today than ever before. Determining what assays will be most useful to the clinician is a challenge, and acceptance of the new technology by the medical community an even greater one. Clinicians must use the best available resources to determine the most appropriate care for their patients and work together with the laboratory to ensure that the communication channels are open. This review focuses on current state-of-the-art resources useful for accurate and rapid laboratory diagnosis of mycobacterial infections.

ATP stimulates human macrophages to kill intracellular virulent Mycobacterium tuberculosis via calcium-dependent phagosome-lysosome fusion

Advances in therapy for tuberculosis will require greater understanding of the molecular mechanisms of pathogenesis and the human immune response in this disease. Exposure of Mycobacterium tuberculosis-infected human macrophages to extracellular ATP (ATP(e)) results in bacterial killing, but the molecular mechanisms remain incompletely characterized. In this study, we demonstrate that ATP(e)-induced bactericidal activity toward virulent M. tuberculosis requires an increase in cytosolic Ca(2+) in infected macrophages. Based on our previous work with primary infection of human macrophages, we hypothesized that the Ca(2+) dependence of ATP-induced killing of intracellular M. tuberculosis was linked to promotion of phagosome-lysosome fusion. Using confocal laser-scanning microscopy, we demonstrate that ATP(e) induces fusion of the M. tuberculosis-containing phagosome with lysosomes, defined by accumulation of three lysosomal proteins and an acidophilic dye. Stimulation of phagosome-lysosome fusion by ATP(e) exhibited distinct requirements for both Ca(2+) and phospholipase D and was highly correlated with killing of intracellular bacilli. Thus, key signal transduction pathways are conserved between two distinct models of human macrophage antituberculous activity: primary infection of naive macrophages and physiologic stimulation of macrophages stably infected with M. tuberculosis.

Cross-Resistance of clinical isolates of Candida albicans and Candida glabrata to over-the-counter azoles used in the treatment of vaginitis

Antifungal drug resistance in Candida spp. continues to increase in response to the widespread application of triazole therapeutics among immunosuppressed patients. Azole-based over-the-counter (OTC) antifungal agents used to treat vaginitis have the potential to exacerbate this problem by contributing to the selection of highly resistant strains of Candida in otherwise healthy women. In this study, we show that fluconazole-resistant (MIC > 64 microg/mL) blood stream isolates of Candida albicans and Candida glabrata obtained from cancer patients were cross-resistant to the root drugs miconazole, clotrimazole, and tioconazole (found in several over-the-counter products), but remained susceptible to butoconazole. We also provide evidence that spontaneous mutants of Candida glabrata selected for resistance to clotrimazole were cross-resistant to other azolebased drugs, including fluconazole. Our findings demonstrate cross-resistance of Candida strains to fluconazole and OTC azole antifungals, and support the notion that OTC drugs can promote azole resistance in Candida spp.

Bacillus Calmette-Guerin down-regulates CD1b induction by granulocyte-macrophage colony stimulating factor in human peripheral blood monocytes

Non-peptide antigens (e.g. glycolipids of microbial origin) presented by monocyte-associated CD1 molecules to T cells appear to play an important role in host immunity against tuberculosis and other pathogenic bacteria. Since vaccination with Bacillus Calmette-Guerin (BCG) has limited efficacy, the influence of viable BCG organisms on the induction of CD1b antigen by granulocyte macrophage-colony stimulating factor (GM-CSF) has been tested in adherent mononuclear cells obtained from peripheral blood of healthy donors. The results indicate that the vaccine reduces substantially CD1b induction by GM-CSF. On the other hand, BCG was found to promote a slight increase in the expression of this molecule on target cells not exposed to GM-CSF. Attempts to reverse the antagonistic effects of BCG on GM-CSF with high concentrations of GM-CSF, alone, or associated with IL-4, were unsuccessful. Moreover, mycobacteria suppression by 10 microg/ml of rifampin, did not affect BCG influence on CD1b induction. The present results suggest that mycobacterium-induced impairment of the CD1 system could play a role in the unsatisfactory results obtained with BCG vaccination.

The molecular basis of resistance to isoniazid, rifampin, and pyrazinamide in Mycobacterium tuberculosis

Multidrug-resistant (MDR) strains of Mycobacterium tuberculosis have emerged worldwide. In many countries and regions, these resistant strains constitute a serious threat to the efficacy of tuberculosis control programs. An important element in gaining control of this epidemic is developing an understanding of the molecular basis of resistance to the most important antituberculosis drugs: isoniazid, rifampin, and pyrazinamide. On the basis of this information, more exacting laboratory testing, and ultimately more appropriate and timely treatment regimens, can be developed.

Tuberculosis presenting as a perirectal mass: report of a case

The epidemic of acquired immunodeficiency syndrome has caused a worldwide resurgence of tuberculosis. A case of acute tuberculosis with anorectal involvement presenting at an urban American hospital is discussed. Although anorectal involvement by tuberculosis is not uncommon, the diagnosis is usually initially missed.

ATP-induced killing of virulent Mycobacterium tuberculosis within human macrophages requires phospholipase D

The global dissemination of antibiotic-resistant Mycobacterium tuberculosis has underscored the urgent need to understand the molecular mechanisms of immunity to this pathogen. Use of biological immunomodulatory compounds to enhance antituberculous therapy has been hampered by the limited efficacy of these agents toward infected human macrophages and lack of information regarding their mechanisms of activity. We tested the hypotheses that extracellular ATP (ATPe) promotes killing of virulent M. tuberculosis within human macrophages, and that activation of a specific macrophage enzyme, phospholipase D (PLD), functions in this response. ATPe treatment of infected monocyte-derived macrophages resulted in 3.5-log reduction in the viability of three different virulent strains of M. tuberculosis. Stimulation of macrophage P2X7 purinergic receptors was necessary, but not sufficient, for maximal killing by primary macrophages or human THP-1 promonocytes differentiated to a macrophage phenotype. Induction of tuberculocidal activity by ATPe was accompanied by marked stimulation of PLD activity, and two mechanistically distinct inhibitors of PLD produced dose-dependent reductions in ATPe-induced killing of intracellular bacilli. Purified PLD restored control levels of mycobacterial killing to inhibitor-treated cells, and potentiated ATPe-dependent tuberculocidal activity in control macrophages. These results demonstrate that ATPe promotes killing of virulent M. tuberculosis within infected human macrophages and strongly suggest that activation of PLD plays a key role in this process.

Value of radiologically guided fine needle aspiration cytology (FNAC) in the diagnosis of spinal tuberculosis: a study of 29 cases

FNAC is a simple diagnostic tool for the initial evaluation of various deep seated pathological lesions. This study describes the applicability and practical aspects of the technique in establishing the diagnosis of spinal tuberculosis (TB) with the aid of radiographic guidance. The study was conducted in a major teaching hospital in Kuwait between the years 1985 and 1994. Twenty-nine patients (M:F = 18:11 and age range 8-72 years) with clinically and/or radiologically suspected spinal TB were seen in the Department of Cytology, Mubarak Al Kabeer Hospital. The patients were re-examined by either computed tomography (CT) scanning (n = 19) or fluoroscopy (n = 10) to localize the lesion for FNAC. FNAC smears were routinely stained with Papanicolaou and Diff Quik stains and one smear of each case was stained with Ziehl-Neelsen (Z-N) stain for acid-fast bacilli (AFB). Aspirated purulent material or syringe washings of dry aspirates were also submitted for microbiological cultures including AFB. Radiological and cytological findings were recorded in each case. Radiological findings included: bony rarefaction and destruction (93.1%), narrowed disc space (89.7%), soft tissue calcification (65.5%) and para-vertebral abscess formation (51.7%). Cytological findings included: epithelioid cell granulomas (89.7%), granular necrotic background (82.8%) and lymphocytic infiltration (75.9%). Smears were positive for AFB in 51.7% of cases. A positive AFB culture was obtained in 82.8% of cases, including all cases with positive AFB on smear by Z-N stain. Radiologically guided FNAC with AFB culture is a simple, reliable and practical approach to diagnosing spinal TB lesions. With a high diagnostic yield, it allows immediate initiation of specific treatment, helps to avoid invasive diagnostic procedures, and decreases hospitalization time.

Conventional and genetic laboratory tests used to guide antimicrobial therapy

Detection of antimicrobial resistance is important so that clinicians can make rational decisions about optimal antimicrobial therapy for their patients. During the past decade, new types of antimicrobial resistance have emerged, some of which present new challenges for the clinical microbiology laboratory. In most cases, conventional culture-based testing methods continue to be useful. In other situations in which the organism responsible for infection grows slowly (for example, Mycobacterium tuberculosis), culture methods are technically difficult (such as for human immunodeficiency virus), or genotypes are inconsistently expressed (for instance, methicillin resistance in staphylococci), genetic susceptibility testing methods may offer special advantages. Determining serum concentrations of antimicrobial agents may be useful both to ensure adequacy of treatment and to prevent toxicity. In this review, methods are described for conventional and genetic tests used to guide antimicrobial therapy.

DNA repair in Mycobacterium tuberculosis. What have we learnt from the genome sequence?

The genome sequence of Mycobacterium tuberculosis was analysed by searching for homologues of genes known to be involved in the reversal or repair of DNA damage in Escherichia coli and related organisms. Genes necessary to perform nucleotide excision repair (NER), base excision repair (BER), recombination, and SOS repair and mutagenesis were identified. In particular, all of the genes known to be directly involved in the repair of oxidative and alkylative damage are present in M. tuberculosis. In contrast, we failed to identify homologues of genes involved in mismatch repair. This finding has potentially significant implications with respect to genome stability, strain variability at repeat loci and the emergence of chromosomally encoded drug resistance mutations.