Mycobacterium tuberculosis (MTB) is a pathogenic bacterial species in the family Mycobacteriaceae and the causative agent of most cases of tuberculosis (TB). First discovered in 1882 by Robert Koch, M. tuberculosis has an unusual, waxy coating on its cell surface (primarily mycolic acid), which makes the cells impervious to Gram staining. Acid-fast detection techniques are used instead. The physiology of M. tuberculosis is highly aerobic and requires high levels of oxygen. Primarily a pathogen of the mammalian respiratory system, MTB infects the lungs. The most frequently used diagnostic methods for TB are the tuberculin skin test, acid-fast stain, and chest radiographs.
Keywords
Mycobacterium tuberculosis; Mtb; M. tb; 16kDa antigen; HspX
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Background
Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), is the single infectious disease that causes the largest number of deaths worldwide. The majority of the infected population presents with latent infection, of which about 5-10% may develop active TB. Mycobacterium tuberculosis can invade several organs of the human body, causing the formation of nodules or caseous necrosis in the organs, causing gradual wasting, pain, coughing up blood and other symptoms, and even death. Latent infections are usually detected by purified protein derivative tests or IFN release assays, whereas in active infections they can be detected analytically by acid-resistant staining and culture of sputum specimens.
Mycobacterium tuberculosis is a Gram-positive, acid-resistant bacillus closely related to Mycobacterium africanum and Mycobacterium bovis. The bacterium grows slowly, requiring 18-24 h to proliferate a generation at 37°C and optimal oxygen and nutrient conditions, and 3-4 weeks to form white to yellowish colonies on agar. Mtb has an impermeable cell wall composed of peptidoglycans, polysaccharides, specific glycolipids, and lipids, and this unique structural feature provides a very strong impermeable barrier to the entry of toxic compounds and drugs. Mtb is able to evade the host's immune system, thus causing disease. It reprograms macrophages after the initial infection, thus preventing them from destroying themselves. Subsequently Mtb promotes the formation of well-organized granulomas containing different immune cells to create a confined space where the host is isolated from the pathogen. In addition, Mtb shuts down its own central metabolism, terminates replication, and enters a dormant state, thereby resisting host defense mechanisms and drug treatment.
Figure 1. Mycobacterium tuberculosis infection (Source: Stewart GR, et al. 2003)
Vaccination is the most effective way to prevent TB, and the only vaccine currently approved for TB prevention is the Bacillus Calmette-Guérin (BCG) vaccine. BCG prevents severe tuberculosis in infants and young children, but there are large differences in the protective effect in adolescents and adults. TB vaccines have long been developed with the goal of stimulating the body to produce a stronger Th1-type immune response, but recent studies have found that inducing the production of high levels of IFN-γ alone does not provide a full protective effect. Accelerating the development of a new TB vaccine is therefore a top priority in the fight against TB.
Alternative Names
Mycobacterium tuberculosis 16kDa (HspX) antigen
References
1. Stewart GR, et al. Tuberculosis: a problem with persistence. Nat Rev Microbiol. 2003 Nov;1(2):97-105.
2. Huang L, et al. Mycobacterium tuberculosis: Bacterial Fitness within the Host Macrophage. Microbiol Spectr. 2019 Mar;7(2):10.1128/microbiolspec.BAI-0001-2019.
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References
Temporal and spatial Mycobacterium bovis prevalence patterns as evidenced in the All Wales Badgers Found Dead (AWBFD) survey of infection 2014-2016
In order to better understand the spatial spread of bovine tuberculosis (bTB) in Wales, an All Wales Badgers Found Dead (AWBFD) survey was carried out from 2014-2016. For Wales, as a whole, there was a significant decrease (p<0.001) in prevalence of bTB in badgers since a similar survey was carried out in 2005-2006, with a drop from 13.3% to 7.3%. The highest prevalence was observed for the High TB Area East (18.6%), which shares its border with England, and differed significantly (p<0.001) from the High TB Area West (7.4%). The lowest proportion of carcases diagnosed with the disease (0.7%) was in the Low TB Area, followed by the two Intermediate TB Areas of Wales (2.7%). The M. bovis isolates from badgers tended to be similar to the genotypes of cattle in the same area, except in the Low TB Area. The direction of any cross species transmission and the drivers for this cannot be determined from this study. The spatial variations described here support the need for regionally adapted surveillance and control measures for bovine tuberculosis in Wales.
Does Mycobacterium bovis persist in cattle in a non-replicative latent state as Mycobacterium tuberculosis in human beings?
VETERINARY MICROBIOLOGY
Authors: Garcia, Julia Sabio Y.; Bigi, Maria M.; Klepp, Laura, I; Garcia, Elizabeth A.; Blanco, Federico C.; Bigi, Fabiana
Members of the Mycobacterium tuberculosis complex (MTBC) are responsible for tuberculosis in several mammals. In this complex, Mycobacterium tuberculosis and Mycobacterium bovis, which are closely related, show host preference for humans and cattle, respectively. Although human and bovine tuberculosis are clinically similar, M. tuberculosis mostly causes latent infection in humans, whereas M. bovis frequently leads to an acute infection in cattle. This review attempts to connect the pathology in experimental animal models as well as the cellular responses to M. bovis and M. tuberculosis regarding the differences in protein expression and regulatory mechanisms of both pathogens that could explain their apparent divergent latency behaviour. The occurrence of latent bovine tuberculosis (bTB) would represent a serious complication for the eradication of the disease in cattle, with the risk of onward transmission to humans. Thus, understanding the physiological events that may lead to the state of latency in bTB could assist in the development of appropriate prevention and control tools.
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Figure 1. Mycobacterium tuberculosis infection
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