Mycobacterium tuberculosis
Mycobacterium tuberculosis, also known as Koch's bacillus, is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of tuberculosis.
First discovered in 1882 by Robert Koch, M. tuberculosis has an unusual, waxy coating on its cell surface primarily due to the presence of mycolic acid. This coating makes the cells impervious to Gram staining, and as a result, M. tuberculosis can appear weakly Gram-positive. Acid-fast stains such as Ziehl–Neelsen, or fluorescent stains such as auramine are used instead to identify M. tuberculosis with a microscope. The physiology of M. tuberculosis is highly aerobic and requires high levels of oxygen. Primarily a pathogen of the mammalian respiratory system, it infects the lungs. The most frequently used diagnostic methods for tuberculosis are the tuberculin skin test, acid-fast stain, culture, and polymerase chain reaction.
The M. tuberculosis genome was sequenced in 1998.
Microbiology
M. tuberculosis requires oxygen to grow, and is nonmotile. It divides every 18–24 hours. This is extremely slow compared with other bacteria, which tend to have division times measured in minutes. It is a small bacillus that can withstand weak disinfectants and can survive in a dry state for weeks. Its unusual cell wall, rich in lipids such as mycolic acid and cord factor glycolipid, is likely responsible for its resistance to desiccation and is a key virulence factor.Microscopy
Other bacteria are commonly identified with a microscope by staining them with Gram stain. However, the mycolic acid in the cell wall of M. tuberculosis does not absorb the stain. Instead, acid-fast stains such as Ziehl–Neelsen stain, or fluorescent stains such as auramine are used. Cells are curved rod-shaped and are often seen wrapped together, due to the presence of fatty acids in the cell wall that stick together. This appearance is referred to as cording, like strands of cord that make up a rope. M. tuberculosis is characterized in tissue by caseating granulomas containing Langhans giant cells, which have a "horseshoe" pattern of nuclei.Culture
M. tuberculosis can be grown in the laboratory. Compared to other commonly studied bacteria, M. tuberculosis has a remarkably slow growth rate, doubling roughly once per day. Commonly used media include liquids such as Middlebrook 7H9 or 7H12, egg-based solid media such as Lowenstein-Jensen, and solid agar-based such as Middlebrook 7H11 or 7H10. Visible colonies require several weeks to grow on agar plates. Mycobacteria growth indicator tubes can contain a gel that emits fluorescent light if mycobacteria are grown. It is distinguished from other mycobacteria by its production of catalase and niacin. Other tests to confirm its identity include gene probes and MALDI-TOF.Morphology
Analysis of Mycobacterium tuberculosis via scanning electron microscope shows the bacteria are in length with an average diameter of. The outer membrane and plasma membrane surface areas were measured to be and, respectively. The cell, outer membrane, periplasm, plasma membrane, and cytoplasm volumes were ,,,, and, respectively. The average total ribosome number was with ribosome density about.| Feature | Magnitude |
| Length | 2.71 ± 1.05μm |
| Outer membrane surface area | 3.04 ± 1.33 μm2 |
| Cell volume | 0.293 ± 0.113 fl |
Related Mycobacterium species
M. tuberculosis is part of a genetically related group of Mycobacterium species that has at least nine members:- M. tuberculosis ''sensu stricto
- M. africanum
- M. canettii
- M. bovis
- M. caprae
- M. microti
- M. pinnipedii
- M. mungi
- M. orygis''
Pathophysiology
When in the lungs, M. tuberculosis is phagocytosed by alveolar macrophages, but they are unable to kill and digest the bacterium. Its cell wall is made of cord factor glycolipids that inhibit the fusion of the phagosome with the lysosome, which contains a host of antibacterial factors.
Specifically, M. tuberculosis blocks the bridging molecule, early endosomal autoantigen 1 ; however, this blockade does not prevent fusion of vesicles filled with nutrients. In addition, production of the diterpene isotuberculosinol prevents maturation of the phagosome. The bacteria also evades macrophage-killing by neutralizing reactive nitrogen intermediates. More recently, M. tuberculosis has been shown to secrete and cover itself in 1-tuberculosinyladenosine, a special nucleoside that acts as an antacid, allowing it to neutralize pH and induce swelling in lysosomes.
In M. tuberculosis infections, PPM1A levels were found to be upregulated, and this, in turn, would impact the normal apoptotic response of macrophages to clear pathogens, as PPM1A is involved in the intrinsic and extrinsic apoptotic pathways. Hence, when PPM1A levels were increased, the expression of it inhibits the two apoptotic pathways. With kinome analysis, the JNK/AP-1 signalling pathway was found to be a downstream effector that PPM1A has a part to play in, and the apoptotic pathway in macrophages are controlled in this manner. As a result of having apoptosis being suppressed, it provides M. tuberculosis with a safe replicative niche, and so the bacteria are able to maintain a latent state for a prolonged time.
Granulomas, organized aggregates of immune cells, are a hallmark feature of tuberculosis infection. Granulomas play dual roles during infection: they regulate the immune response and minimize tissue damage, but also can aid in the expansion of infection.
The ability to construct M. tuberculosis mutants and test individual gene products for specific functions has significantly advanced the understanding of its pathogenesis and virulence factors. Many secreted and exported proteins are known to be important in pathogenesis. For example, one such virulence factor is cord factor, which serves to increase survival within its host. Resistant strains of M. tuberculosis have developed resistance to more than one TB drug, due to mutations in their genes. In addition, pre-existing first-line TB drugs such as rifampicin and streptomycin have decreased efficiency in clearing intracellular M. tuberculosis due to their inability to effectively penetrate the macrophage niche.
JNK plays a key role in the control of apoptotic pathways—intrinsic and extrinsic. In addition, it is also found to be a substrate of PPM1A activity, hence the phosphorylation of JNK would cause apoptosis to occur. Since PPM1A levels are elevated during M. tuberculosis infections, by inhibiting the PPM1A signalling pathways, it could potentially be a therapeutic method to kill M. tuberculosis-infected macrophages by restoring its normal apoptotic function in defence of pathogens. By targeting the PPM1A-JNK signalling axis pathway, then, it could eliminate M. tuberculosis-infected macrophages.
The ability to restore macrophage apoptosis to M. tuberculosis-infected ones could improve the current tuberculosis chemotherapy treatment, as TB drugs can gain better access to the bacteria in the niche. thus decreasing the treatment times for M. tuberculosis infections.
Symptoms of M. tuberculosis include coughing that lasts for more than three weeks, hemoptysis, chest pain when breathing or coughing, weight loss, fatigue, fever, night sweats, chills, and loss of appetite. M. tuberculosis also has the potential of spreading to other parts of the body. This can cause blood in urine if the kidneys are affected, and back pain if the spine is affected.
Strain variation
Typing of strains is useful in the investigation of tuberculosis outbreaks, because it gives the investigator evidence for or against transmission from person to person. Consider the situation where person A has tuberculosis and believes he acquired it from person B. If the bacteria isolated from each person belong to different types, then transmission from B to A is definitively disproven; however, if the bacteria are the same strain, then this supports the hypothesis that B infected A.Until the early 2000s, M. tuberculosis strains were typed by pulsed field gel electrophoresis. This has now been superseded by variable numbers of tandem repeats, which is technically easier to perform and allows better discrimination between strains. This method makes use of the presence of repeated DNA sequences within the M. tuberculosis genome.
Three generations of VNTR typing for M. tuberculosis are noted. The first scheme, called exact tandem repeat, used only five loci, but the resolution afforded by these five loci was not as good as PFGE. The second scheme, called mycobacterial interspersed repetitive unit, had discrimination as good as PFGE. The third generation added a further nine loci to bring the total to 24. This provides a degree of resolution greater than PFGE and is currently the standard for typing M. tuberculosis. However, with regard to archaeological remains, additional evidence may be required because of possible contamination from related soil bacteria.
Antibiotic resistance in M. tuberculosis typically occurs due to either the accumulation of mutations in the genes targeted by the antibiotic or a change in titration of the drug. M. tuberculosis is considered to be multidrug-resistant if it has developed drug resistance to both rifampicin and isoniazid, which are the most important antibiotics used in treatment. Additionally, extensively drug-resistant M. tuberculosis is characterized by resistance to both isoniazid and rifampin, plus any fluoroquinolone and at least one of three injectable second-line drugs.