Mycobacterium tuberculosis

A Microbial Biorealm page on the genus Mycobacterium tuberculosis

Classification

Higher order taxa

Domain: Bacteria; Phylum: Actinobacteria; Class: Actinobacteria; Order: Actinomycetales; family: Mycobacteriaceae; Genus: Mycobacterium

Species

The Mycobacterium tuberculosis complex (MTC) consists of Mycobacterium africanum, Mycobacterium bovis, Mycobacterium canettii, Mycobacterium microti, Mycobacterium tuberculosis.

Description and significance

Mycobacterium tuberculosis is a acid fast bacteria, which can form acid-stable complexes when certain arylmethane dyes are added. (4) All species of mycobacteria have ropelike structures of peptidoglycan that are arranged in such a way to give them properties of an acid fast bacteria. (4) Mycobacteria are abundant in soil and water, but Mycobacterium tuberculosis is mainly identified as a pathogen that lives in the host. Some species in its Mycobacterium tuberculosis complex have adapted their genetic structure specifically to infect human populations.

M. tuberculosis can be isolated in labs and stored at –80 degrees to be studied extensively, and the most commonly used strain of M. tuberculosis is the H37Rv strain. One way to study M. tuberculosis in culture is to collect samples of mononuclear cells in peripheral blood samples from a healthy human donor and challenge macrophages with the MTC. M. tuberculosis has very simple growth requirements and is able to grow slowly in harsh conditions. Their acid-fast property is the strongest when there is glycerol around. However, when glucose is the main source of nutrient, the utilization of glycerol by M. tuberculosis is inhibited. Therefore, it’s been shown that glutatmate, and not glucose, is actually the main source of nutrient for initiating growth. (4)

Since as many as 32% of the human population is affected by Tuberculosis (TB), an airborne disease caused by infection of M. tuberculosis in one way or another, and about 10% of them becomes ill per year (6), it is not hard to imagine the significance in understanding the genome of the pathogen to develop and improve strategies for treatment by developing specific drugs that target the gene products of M. tuberculosis.

Genome structure

Mycobacterium tuberculosis has circular chromosomes of about 4,200,000 nucleotides long. The G+C content is about 65%. (13)

The genome of M. tuberculosis was studied generally using the strain M. tuberculosis H37Rv. The genome contains about 4000 genes. Genes that code for lipid metabolism are a very important part of the bacterial genome, and 8% of the genome is involved in this activity. (7)

The different species of the Mycobacterium tuberculosis complex show a 95-100% DNA relatedness based on studies of DNA homology, and the sequence of the 16S rRNA gene are exactly the same for all the species. So some scientists suggest that they should be grouped as a single species while others argue that they should be grouped as varieties or subspecies of M. tuberculosis. (2)

Plasmids in M. tuberculosis are important in transferring virulence because genes on the plasmids are more easily transferred than genes located on the chromosome. One such 18kb plasmid in the M. tuberculosis H37Rv strain was proven to conduct gene transfers.

Cell structure and metabolism

M. tuberculosis has a tough cell wall that prevents passage of nutrients into and excreted from the cell, therefore giving it the characteristic of slow growth rate. The cell wall of the pathogen looks like a Gram-positive cell wall. The cell envelope contains a polypeptide layer, a peptidoglycan layer, and free lipids. In addition, there is also a complex structure of fatty acids such as mycolic acids that appear glossy. (8) The M. tuberculosis cell wall contains three classes of mycolic acids: alpha-, keto- and methoxymycolates. The cell wall also contains lipid complexes including acyl glcolipids and other complex such as free lipids and sulfolipids. There are porins in the membrane to facilitate transport. Beneath the cell wall, there are layers of arabinogalactan and peptidoglycan that lie just above the plasma membrane. (14)

The M. tuberculosis genome encodes about 190 transcriptional regulators, including 13 sigma factors, 11 two-component system and more than 140 transcription regulators. Several regulators have been found to respond to environmental distress, such as extreme cold or heat, iron starvation, and oxidative stress. (11) To survive in these harsh conditions for a prolonged period in the host, M. tuberculosis had learned to adapt to the environment by allowing or inhibiting transcription according to its surroundings.(3)

Ecology

The Mycobacterium tuberculosis forms a complex with other higher related bacteria called the M. tuberculosis complex that consists of 6 members: Mycobacterium tuberculosis and Mycobacterium africanmum, which infect humans; Mycobacterium microti, which infects vole; Mycobacterium bovis, which infects other mammalian species as well as humans; M. bovis BCG, a variant of Mycobacterium bovis and Mycobacterium canettii, a pathogen that infects humans. (7) M. tuberculosis first infected humans 10,000-15,000 years ago. (10) It has been found in early hominids originating in East-Africa. Therefore, studying the population structure of the species might provide insights about Homo sapiens' migratory and demographic history.

Once inside the human host cell, Mycobacterium tuberculosis inflicts a contagious-infectious disease called tuberculosis (TB), although the disease could either be latent or active depending on the ability of the person’s immune system to defend against the pathogen. In 1993, the World Health Organization declared TB a global public health emergency. It is estimated that one third of the world population is infected by Mycobacterium tuberculosis, which leads to 8 to 10 million new cases, and 3 million deaths each year. (10) The disease especially affects those in developing countries, those of the aging population and those who have HIV/AIDs because of their weak immune system. (10)

Because TB is an infectious disease for humans, it is important to sequence the genome of the Mycobacterium tuberculosis in order to find drugs fight against the bacteria by developing potential drug targets. Especially since Mycobacterium tuberculosis is multi-drug resistance and could cause latent infection, it is especially hard to treat and prompts scientists to research for new drug targets by looking through the Mycobacterium tuberculosis genome and gene products.

HIV and TB

Introduction

It is estimated that a third of the 36 million people on the planet with HIV/AIDS are infected with M. tuberculosis (1). This co-infection has been dubbed the "cursed duet," draining approximately 30% of the annual income of an infected household in both direct and indirect costs, becoming a social and economic disaster for families (2).

The countries with the highest TB/HIV co-infection rates continue to be in Sub-saharan Africa, where HIV is ravaging the population. One example is Zambia. It's TB rates were stable in the 1960's and 1970's, but the number of TB cases quadrupled as HIV spread across the country (3). Worldwide, the largest increase in co-infection with TB and HIV has occurred in the 25-44 year old population range (4). Since this age group is generally so active in the workforce, the consequent economic impact is huge.

A TB patient who is infected with HIV has lost over half his body weight (www.WorldLungFoundation.org)

Co-Infection

Why is co-infection so common? HIV infection is commonly listed as the highest risk-factor for becoming infected with TB, as well as greatly increasing a person's risk for developing active tuberculosis. Because HIV/AIDS depletes the immune system, opportunistic infections become serious problems. (An opportunistic infection is an infection that is kept under control in an immuno-competent person, but becomes too much for the immune system when infected with HIV.) People with healthy immune systems can become infected with TB but only about 5% develop active disease. In the population with latent TB, the infection may lie dormant for years, becoming active disease only when the body's immune system is weakened, as occurs with HIV infection (2).

Once latent TB resurfaces in an HIV-positive patient, it can move from the lungs to other organs more rapidly than in patients without HIV infection. This is known as extrapulmonary TB. As it moves into other organs and tissues, it causes irreversible damage. With time, more systems become involved with the extrapulmonary TB, weakening them in turn and allowing the body to become devastated with increasing speed. These weakened systems allow the HIV to progress more rapidly, greatly expediting both disease processes and greatly decreasing life expectancy (2).

Treatment and Related Issues

Medical treatment of TB and HIV/AIDS is extremely complicated due to several severe medication interactions, often malnutrition and conflicting disease processes. Treatment for TB always takes precedence over HIV/AIDS treatment due to TB's infectious nature through the air. However, with antiretroviral therapy taking a backseat to TB, the HIV infection may progress more quickly, weakening the immune system even more rapidly. This makes it easier for opportunistic infections to take over, putting more stress on the patient's body, lowering life-expectancy (5).

The anti-TB drug thiacetazone can cause serious, sometimes fatal, side effects in many HIV-positive patients (www.WorldLungFoundation.org)

A major issue affecting treatment of HIV/TB Co-infection is the potential cost of treatment. The preferred treatment for HIV is highly active combination anti-retroviral therapy. While these drugs are very effective at slowing the progression of HIV to AIDS, they are expensive, especially for people in developing countries. Adding the cost of the common antibiotics used to treat TB moves the price of medication out of reach for even more people (6). Further complicating the problem is that the co-infection and weakened immune system makes the patients more susceptible to adverse drug reactions from potentially quite toxic drugs.

The stigma associated with both diseases is another major issue surrounding diagnosis and treatment of both HIV and TB. The stigma of the separate and combined diseases is intertwined with disease perception. Many consider the TB of co-infection with HIV to be a new disease, since "the TB of today should be given another name because it doesn't cure" (7). In many developed countries, the TB of the past was viewed as a curable, relatively simple infection that was acquired in more mundane ways like "men smoking tobacco or marijuana," or "drinking home brewed liquor." In contrast, new TB is acquired though "hanging out in bars" and "sexual transgressions" (7). When someone is diagnosed with TB, their past behavior is often viewed as improper, immoral, with a consequent judgment passed about how the person contracted HIV/TB. These judgments can be reinforced by moral judgments from religious leaders, since "Satan is these two diseases" (7).

This problem with TB is compounded by the presence of HIV, which has been highly stigmatized since it exploded in the 1980's. The most common ways to transmit HIV are through intimate sexual contact and blood-to-blood contact from transfusions or sharing needles, which is common among injection drug users. Thus, HIV infection is commonly associated with unprotected sex and drug use.

In many parts of the world, TB and HIV are both so common and occur together so often that the distinctions between the two diseases are obscured, creating the need to create a new label for the joint stigmatization faced by patients. Further compound the TB-HIV stigma is the misinformation surrounding treatment of both diseases. People with active cases of TB are considered most contagious for the first 2-3 weeks of treatment. Close contact should be avoided during those weeks; however, misinformation often leads to exaggerated fears about contracting the disease, causing prolonged physical and social isolation (7).

One positive aspect is that concurrent testing for HIV and TB is becoming an increasingly common intervention at HIV treatment centers worldwide. Previously, neither HIV nor TB clinics commonly tested for the other disease. While the disapprobation surrounding both diseases can make dual testing difficult, it is an important step in reducing the number of co-infections. Early detection of TB/HIV infection and early treatment can help to reduce the severe effects of co-infection on the body's immune system, increasing life expectancy. The sooner a person begins treatment for TB, the less chance they have of infecting others, and the sooner they are aware of their HIV positive status, the more they are able to limit their high-risk behavior, providing less chances to infect others with HIV (6).

References

1. Colebunders, R. and M.L. Lambert 2002) "Management of co-infection with HIV and TB: Improving tuberculosis control programmes and access to highly active antiretroviral treatment is crucial" Brit Med J 324:802-803.
2. Sharma, S.K., A. Mohan, and T. Kadhiravan (2005) "HIV-TB co-infection: Epidemiology, diagnosis and management," Ind. J Med Res 212:550-567.
3. Godfrey-Faussett, P. and H. Ayles (2003) "Can we control tuberculosis in high HIV prevalence settings?," Tuberculosis 83:68-76.
4. World Health Organization (2006) "Tuberculosis and HIV/AIDS - Some Questions and Answers" http://www.searo.who.int/en/Section10/Section18/Section356/section421_1624.htm
5. Corbett, E.L., C.J. Watt, N. Walker, D. Maher, B.G. Williams, M.C. Raviglione and C. Dye (2003). "The Growing Burden of Tuberculosis: Global Trends and Interactions with the HIV Epidemic," Arch Intern Med 163:1009-1021.
6. Manosuthi, W., S. Chottanapand, S. Thongyen, A. Chaovavanich, and S. Sungkanuparph (2006) "Survival Rate and Risk Factors of Mortality Among HIV/Tuberculosis Coinfected Patients With and Without Antiretroviral Therapy," J Acq Immun Def Syndrome 431:42-46.
7. Bond, V. and L. Nyblade (2006) "Importance of Addressing the Unfolding TB-HIV Stigma in High HIV Prevalence Settings," J Comm Appl Soc Psych 16:452-461.

Application to Biotechnology

Gene for histone-like protein (hupB [Rv2986c]) of Mycobacterium tuberculosis had been used to distinguishing members of the MTB complex from other mycobacterium species and differentiating between members within the complex.

In addition, in vivo complementation in Mycobacterium tuberculosisstrain H37Ra can be used to identify genomic fragments associated with virulence. By studying the genes encoding for virulent gene products, a combination of drugs and vaccines could be developed to target the MDR and XDR characteristic of the pathogen.

Current Research

Since the pathogen-host interaction of Mycobacterium tuberculosis is still unknown, much of the current research is geared towards the understanding of the mechanism of virulence. For example, one such research showed that prokaryotic- and eukaryotic-like isoforms of the glyozxylate cycle enzyme isocitrate lyase (ICL) are jointly required for fatty acid catabolism and virulence in Mycobacterium tuberculosis. This discovery provides insight such as drugs that are glycoxylate cycle inhibitors could be used to treat tuberculosis. (12).

Another group of scientists found that a newly identified protein with carboxyesterase activity is required for the virulence of Mycobacterium tuberculosis. They found that the gene MT2282 encodes a protein that is associated with carboxyesterase. It hydrolyzes ester bonds of the substrate. When a strain containing a mutant of this gene was used to infect mice, the mice’s life was prolonged as compared with those that were infected with the wild type strain. (5)

In addition, as mentioned earlier, very little is known regarding host-microbe interaction that happens before M. tuberculosis gets into the macrophages and how M. tuberculosis adheres to the host is still being researched. One research suggested M. tuberculosis produces tiny pili that enable them to colonize the host by adhering to the host and invading the macrophages and epithelial cells of the host. (1) The pili produced are called MTP. The study is important because MTP could be used as vaccine because MTP-mediated events are critical for TB infection. (1)

References

1. Alteri, C.J., Xicohtencatl-Cortes, J., Hess, S. Caballero-Olin, G., Giron, J.A., and Friedman, R.L. “Mycobacterium tuberculosis produces pili during human infection.” Microbiology 2007; 104(12): 5145-5150.

2. Aranaz A, Liébana E, Gomez-Mampaso E, et al. “M. Tuberculosis subsp. caprae subsp. nov.: a taxonomic study of a new member of the Mycobacterium tuberculosis complex isolated from goats in Spain. Int J Syst Bacteriol 1999; 49:1263 73.

3. Banaiee, N., Jacobs Jr, W.R., and Ernst, J.D. “Regulation of Mycobacterium tuberculosis whiB3 in the mouse lung and macrophages.” Infection and Immunity 2006; 74: 6449-6457.

4. Barksdale, L. and Kim, K. “Mycobacterium.” Bacteriological Reviews 1977; 41: 217-372.

5. Bishai, W.R., Lun, S. “Characterization of a novel cell wall-anchored protein with carboyesterase activity required for virulence in Mycobacterium tuberculosis.” The Journal of Biological Chemistry 2007; 1-22.

6. Chen, M., Gan, H., and Rernold, H.G. “A mechanism of virulence: virulent M. Tuberculosis strain H37Rv, but not attenuated H37Ra, causes significant mitochondrial inner membrane disruption in macrophages leading to necrosis.” The Journal of Immunology 2006; 176: 3707-3716.

7. Cole, S.T. “Comparative and functional genomics of the Mycobacterium tuberculosis complex.” Microbiology (2002); 148: 2919-2928.

8. Cole, S. T. , Brosch, R. , Parkhill, J. , Garnier, T. , Churcher, C. , Harris, D. , Gordon, S. V. , Eiglmeier, K. , Gas, S. , Barry, C. E. “Deciphering the biology of Mycobacterium tuberculosis from the complete genome sequence.” Nature 1998; 393(6685): 537-44.

9. Cosma, C. L., K. Klein, R. Kim, D. Beery, and L. Ramakrishnan. “Mycobacterium marinum Erp is a virulence determinant required for cell wall integrity and intracellular survival.” Infect. Immun. 2006; 74:3125-3133.

10. Ducati, R.G., Ruffino-Netto, A., Basso, L.A., Santos, D.S. “The resumption of consumption – A review on tuberculosis.” Rio de Janeiro 2006; 101: 697-714.

11. Manganelli, R., Voskuil, M.I., Schoolnik, G.K., and Smith, I. “The Mycobacterium tuberculosis ECF sigma factor sigmaE: role in global gene expression and survival in macrophages.” Mol Microbiol 2006; 41: 423– 437.

12. Munoz-Elias, E.J., McKinney, J.D. “Mycobacterium tuberculosis isocitrate lyases 1 and 2 are jointly required for in vivo growth and virulence.” Nat Med. 2005; 11: 638-644.

13. NCBI. 24 May 2007. Welcome Trust Sanger Insitute. 2 June 2007 <http://www.ncbi.nlm.nih.gov/sites/entrez?db=genomeprj&cmd=Retrieve&dopt=Overview&list_uids=224>.

14. Riley, L.W. “Of mice, men and elephants: Mycobacterium tuberculosis cell envelope lipids and pathogenesis.” American Society for Clinical Investigation 2006; 116:1475-1478.

Edited by Ying Liu of Rachel Larsen and Kit Pogliano

KMG