Multi-drug resistant tuberculosis is an infectious disease caused by determined strains of Mycobacterium tuberculosis that are resistant to at least both isoniazid and rifampin, the two most commonly used antimicrobials to treat tuberculosis. Multi-drug resistance severely limits available treatment options.
Clinical presentation of patients suffering from MDR-TB or XDR-TB does not differ substantially from that of individuals who develop TB due to an infection with susceptible strains. In brief, symptoms characteristic of TB comprise fever, chills, night sweats, loss of appetite and weight loss, as well as complaints indicating compromise of the respiratory tract. Patients frequently claim breathing difficulties, productive cough and hemoptysis. However, TB is a multi-systemic disease and eventually, mycobacteria may compromise the gastrointestinal tract,urinary tract, the skin, the heart and the central nervous system.
The hallmark of MDR-TB is resistance to common treatment regimens. First-line treatment comprises the combined application of rifampicin, isoniazid, pyrazinamide, and ethambutol for two months followed by isoniazid and rifampicin for another four months. This approach to therapy is associated with rapid decreases of bacterial loads, relief of symptoms and better chances of success - unless the etiological agent is resistant to several of those drugs. Accordingly, disease exacerbation is common if MDR-TB patients are subjected to standard therapy. In general, patients who fail to respond to the intensive phase of standard therapy are highly suspicious of MDR-TB.
About one in ten individuals who contracted MDR-TB is immunodeficient, most commonly due to HIV infection. In some geographic regions, prevalence of HIV among MDR-TB patients is even higher . HIV patients present with lethargy, fever, lymphadenopathy, multiple, and recurrent opportunistic infections. Thus, non-specific symptoms of TB resemble those of HIV, which may lead to a co-morbidity being overlooked.
Ideally, isolation of mycobacteria is followed by susceptibility tests in all cases of suspected TB, but such continuous surveillance of resistances to tuberculostatics is implemented only in a few countries. In any case, the following anamnestic data increase the likelihood of a TB patient to be infected with drug-resistant strains and thus emphasize the need for susceptibility evaluation.
Unfortunately, standard procedures to ascertain the susceptibility of an isolate to isoniazid, rifampicin and other antibiotics are tedious and slow. After culture of mycobacteria to confirm TB, pathogens are maintained in liquid or solid media containing critical concentrations of antimicrobials. Subsequent colony counts will reveal whether the respective strain is susceptible to tested drugs, but results are only available after several weeks of bacterial culture. Liquid media favor the growth of mycobacteria, but are more expensive .
In recent years, molecular biological confirmation of TB and prediction of drug resistance has therefore been a subject of intense research. Different test procedures have been developed, e.g., the GenoType MTBDRplus and INNO-LiPA Rif.TB line probe assays and the Xpert MTB/RIF assay . All these assays detect mutations of the rpoB gene related to rifampin resistance with acceptable sensitivity and specificity; sequence anomalies conferring resistance to isoniazid may only be revealed by the GenoType MTBDRplus assay. The GenoType MTBDRsl assay has been derived from the latter and may be applied to assess susceptibility to fluoroquinolones and second-line drugs in cases of suspected XDR-TB.
Ideally, the therapy of an individual patient is tailored based on the results of susceptibility tests. It is particularly important to distinguish MDR-TB (resistance to isoniazid and rifampicin) from XDR-TB (additional resistance to fluorquinolones and second-line therapeutics). However, test results may not be readily available and a more generalized approach to MDR-TB therapy needs to be followed in the meantime. In that case, at least one drug of each of the first four groups described above should be administered. If reasonable, pyrazinamide, one fluorquinolone, one injectable drug, and cycloserine or ethionamide are combined. So-called group 5 drugs are only prescribed if a regimen cannot be completed because of lack of effectivity of majority of the medications. Treatment regimens extend over 18 to 24 months, with possible reduction to a more restricted treatment after 6 months.
MDR-TB and XDR-TB are generally associated with a poor outcome, especially in immunocompromised patients who develop detrimental complications in particularly short periods of time. According to a recent study conducted in South Africa, one-month survival rates are 60% and 49% in HIV patients diagnosed with MDR-TB and XDR-TB, respectively . One year after diagnosis, only 29% and 17% of those patients remain alive. In general, treatment success rates have been reported to be 86% and 60% for uncomplicated and MDR-TB, respectively  . Presumably, these rates vary depending on the availability of medical care, appropriate diagnosis and availability of an alternative medication. Worldwide, 190,000 deaths have been ascribed to infections with multi-drug resistant strains of Mycobacterium spp. in 2014.
The vast majority of TB cases is caused by infection with M. tuberculosis, but M. bovis and M. africanum may also trigger the disease. Mycobacteria may develop resistances to antimicrobial compounds, possibly by horizontal gene transfer ., but the molecular mechanisms underlying drug resistance are only partially understood. To this end, further research is urgently needed since the identification of genes involved in multiple drug resistance is of major importance not only for epidemiologists, but also in order to design diagnostic assays that allow for an early distinction between uncomplicated TB and MDR-TB. According to current knowledge, resistance to isoniazid is primarily caused by mutations of the pathogen's catalase-peroxidase-coding katG gene, while rifampin resistance is conferred by determined variants of the gene encoding for RNA polymerase β-subunit (rpoB gene) . Furthermore, mutations of the inhA promoter, ndh and oxyR-ahpC have been related to drug resistance of M. tuberculosis. An increased expression of drug efflux pumps may partially explain the phenotype displayed by resistant strains . Moreover, XDR-TB is defined as TB due to infection with mycobacteria displaying additional resistance to "any fluoroquinolone and any of the second-line anti-TB injectable drugs (amikacin, kanamycin or capreomycin)" ., and gyrA and gyrB mutations have been related to fluoroquinolone resistance . For further information, the interested reader is referred to an excellent review detailing the genetic basis of multiple drug resistance in mycobacteria .
MDR-TB (including XDR-TB) has been estimated to account for almost 5% of TB cases registered annually, and this corresponds to about 480,000 cases worldwide . Epidemiologists are not only alarmed by the absolute increase of MDR-TB incidence, but also by the wide distribution of resistant strains: MDR-TB has been reported in more than 100 countries to date, with most cases being diagnosed in China and India . Particularly high ratios of MDR-TB (>10%) have been observed in Azerbaijan, Estonia, Kazakhstan, Latvia, Republic of Moldova, Russian Federation, Tajikistan, Ukraine and Uzbekistan. However, spread of mycobacteria displaying multi-drug resistance is not restricted to low-income countries. Reliable data provided by the authorities of the United Kingdom indicate an that the number of MDR-TB cases almost tripled from 2000 to 2012 .
It is generally accepted that development of drug resistances is an evolutionary process that allows pathogens to cope with selection pressure. There is a high correlation between the incidence of MDR-TB and previous exposure to anti-TB medications . Unfortunately, considerable shares of TB patients require re-treatment with first-line drugs despite compliance with their physician's recommendations. Also, TB treatment regimens comprise several months of medication, and insufficient compliance with the length of therapy may promote the selection of resistant subpopulations of bacteria. In this context, high community rates of MDR-TB and close contact to infected persons have been identified as additional risk factors for developing MDR-TB, and those conditions are often mutually dependent.
Upon cessation of selection pressure, pathogens tend to lose genes conferring drug resistance or they are enabled to avoid host defense mechanisms. This is due to an overall loss of fitness, and this process has long since been used in the development of attenuated strains for vaccine production. However, M. tuberculosis has been shown to have the capacity to regain fitness even though it conserves resistance genes. This may be possible owing to additional compensatory mutations. Such strains do not lag behind non-resistant isolates regarding pathogenicity and virulence, but are insensitive to common tuberculostatics. It has been warned that this may inevitably lead to the development of total drug resistant strains .
As has been indicated above, prior treatment against TB is one of the main risk factors of MDR-TB and XDR-TB. This observation emphasizes the need for adherence to treatment recommendations, avoidance of underdosage and premature cessation of drug therapy, as well as patient compliance. In fact, patients with a history of lack of compliance should take their drugs under supervision. All patients should be repeatedly advised on the importance of the continuation of therapy, even if symptoms start to disappear and their general condition improves. It has been proposed to apply shorter regimens to subgroups of MDR-TB patients. If the patient is required to comply with therapy for only 9 months, they are more likely to comply with medical recommendations. However, evidence regarding the effectivity of this approach is still scarce and further studies are needed to validate this recommendation .
Moreover, general measures to prevent transmission of mycobacteria may be taken, e.g., vaccination and quarantine of affected individuals.
Tuberculosis (TB) is a major threat to human health worldwide. According to official data provided by the World Health Organization, almost 10 million people fall ill to TB each year, with more than 10% of those individuals being infected with the human immunodeficiency virus (HIV). The discovery of antimicrobials effective against Mycobacterium tuberculosis and other Mycobacterial species that may induce TB - allowed for a considerable reduction of morbidity and mortality associated with the disease. In this context, isoniazid and rifampin are most commonly applied to treat TB patients and are administered in a combined therapeutic regimen. However, up to 5% of TB cases are triggered by strains resistant to isoniazid and rifampin, and about 10% of those cases are caused by pathogens displaying even more complex resistance patterns . These are referred to as multi-drug resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB). The prognosis of both MDR-TB and XDR-TB is poor, and it has been estimated that these severe infectious diseases accounted for approximately 190,000 deaths in 2014. Few years ago, only 150,000 deaths have been ascribed to infections with resistant strains worldwide . These data indicate an increasing prevalence of multi-drug resistance in Mycobacterium spp., and this development is very alarming due to few remaining treatment options.
Tuberculosis (TB) is a major health concern worldwide with about 10 million new cases registered every year. TB is an infection that is triggered by a bacteria pertaining to the genus Mycobacterium. In most cases, the causative agent is Mycobacterium tuberculosis. In order to overcome TB, patients are subjected to prolonged therapies comprising the daily intake of four antibiotics, namely isoniazid, rifampin, pyrazinamide and ethambutol. For distinct reasons, bacteria may develop resistances to those compounds, i.e., one or more of the aforementioned antibiotics are rendered ineffective. If mycobacteria carry resistances to the two major therapeutic medications such as isoniazid and rifampin, the patient is diagnosed with multi-drug resistant tuberculosis (MDR-TB). Affected individuals fail to respond to standard therapy. Individualized treatment regimens have to be developed to cure MDR-TB patients. This usually consist of at least four compounds applied over the course of 18-24 months. Still, morbidity and mortality associated with MDR-TB remain high, and prophylaxis is preferred over treatment. The main risk factors for developing MDR-TB are previous exposure to tuberculostatics and interruption or premature cessation of anti-TB therapy. It is thus of utmost importance to comply with the treating physician's recommendations regarding TB and MDR-TB therapies.