Mycobacterium avium complex

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A Microbial Biorealm page on the genus Mycobacterium avium complex

Classification

Higher order taxa

Bacteria (domain); Actinobacteria (phylum); Actinobacteria (class); Actinobacteridae (subclass); Actinomycetales (order); Corynebacterineae (suborder); Mycobacteriaceae (family); Mycobacterium (genus); Mycobacterium avium complex (MAC) (species group).

Species

Includes: Mycobacterium avium

Mycobacterium intracellulare

Also known by: Mycobacterium avium intracelluare (MAI)

NCBI: Taxonomy

Description and significance

Mycobacterium avium complex (MAC) contains 28 serovars of two species of mycobacteria: Mycobacterium avium and Mycobacterium intracellulare. These species are rod-shaped and non-motile. They are slow-growing species that cause opportunistic infections to animals, and immunosuppressed humans. MAC is prevalent in the environment. Their ubiquitous nature results in them being able to live under many different conditions. They are found everywhere from fresh to saltwater, from inside a host to outside under varying pH and temperature(7). This complex commonly formed biofilms in places abundant with water, food, and soil. They are notorious for being highly resistant to many antibiotics as well as disinfectant and bleach, including Chlorine (10). The biofilms were detected by the use of crystal violet staining and optic and electron microscopy (3). These detections show that MAC colonies are usually distinguished by a smooth, wet surface.

Colonies of MAC exist in three forms that are reversible and spontaneous: smooth transparent, smooth opaque, and rough opaque. Smooth transparent colonies are usually found from clinical isolates. They are more virulent and more resistant to drugs, whereas smooth and rough opaque are from environmental isolates and are more benign.

Biofilms have many negative effects on humans. The colonies of these different bacterial cells inside our bodies protect them from being attack by our immune system. They can develop in our bodies from the surfaces of medical implants such as urinary catheter or in the cracks of our teeth to form plaque. These bacterias also exist in water and oil pipelines, which slow and even clogged the flow of fluid (6). MAC biofilms are also believed to be essential to the survival of virulent strains of the two species M. avium and M. intracellulare. This is because, sometimes, the nonvirulent strains detach themselves from their colonies and wanders off, whereas virulent cells tend to remain attach to their bacterial colonies. Although, the impact of biofilms is clearly evident, the specific mechanism of how MAC biofilm form is still unclear. All that is known is that its formation is dependent on the quantity of ions such as calcium, zinc (II), and magnesium(3). Regardless of the limited amount of information, studies are being done to identify genes of M. avium that are essential in their biofilm formation, with the hope that this information will prevent bacterial colonization (3). Therefore, it is important to have the genome for this complex sequenced, especially since biofilms are becoming a problem to us.

Genome structure

Mycobacterium avium was completely sequenced on 11/29/2006 at the J. Craig Venter Institute. Its entire genome consists of one DNA molecule (one chromosome) of 5,475,491 nucleotides long, with a GC (guanine cytosine) content of 68% and 32% for AT (adenine thymine). Guanine and cytosine are paired with each other by three hydrogen bonds, whereas adenine and thymine have 2 hydrogen bonds that bring them together. Because of the larger number of hydrogen bonds, guanine and cytosine are pulled stronger together, and it would take more energy and a higher melting temperature to separate the two. To determine the DNA genome within a cell, scientists use restriction endonuclease to cleave DNA into many small fragments. The endonucleases usually cleave at an AT rich region, because two hydrogen bonds are weaker and require less energy to pull apart than three bonds. In the Mycobacterium avium, approximately 88% of its genome are coding regions for 5120 proteins. Commonly found in MAC are extrachromosomal DNA in the form of self-replicating plasmids. Different strains have different number of plasmids. Some have one, others have two or more. Studies are being done to determine the significance of plasmids in a M. avium strain (6).

By isolating M. avium from an AIDS patient in the mid-1980s, studies showed that M. avium has a 13.5% polymorphism rate, much greater than M. tuberculosis. Therefore, the genome of each strain varies greatly. Because of its variability, results from genotyping methods such as restriction fragment length polymorphism (RFLP) are limited to the strains that come from the same geographical area as the samples (11).

Cell structure and metabolism

Mycobacterium avium are acid fast bacillus. They are characterized by a cell wall covalently attached to the long chain of hydrocarbons called mycolic acid in the inner leaflet. This complex between the peptidoglycan and mycolic acids creates the waxy hydrophobic surface of the cell, which greatly restricts the transport of many compounds into and out of the cell, and eventually slows down growth. Because of its high hydrophobicity due to the waxy outer layer, many soluble antibiotic drugs cannot cross the membrane and attack the pathogen. Therefore, Mycobacterium avium are extremely resistant to many chemotherapeutic agents as well as many cleaning products, leaving only a limited number of drugs that these bacterias are fairly susceptible to. When taking drugs to treat MAC infection, it is usually accompanied by a special agent or detergent to break down the waxy layer and allowing the drug to penetrate into the cell. Mycobacteria also have a lipopolysaccharide (LPS) anchored into the plasma membrane of the cell with the carbohydrate chain sticking out of the cell (6). The carbohydrate chains are hydrophilic, because they are negatively charged. Since the majority of the outside of the cell is made of lipid, LPS is one of the few places that attracts hydrophilic molecules. The outer leaflet of the membrane has phospholipids and glycolipids called glycopeptidolipids. These glycopeptidolipids are highly antigenic and aid the pathogens in suppressing the immune response of the host.

Little information is known about the process of biosynthesis and breakdown for this complex of organisms. What is known is that these organisms undergo aerobic respiration, requiring them to find a way to get oxygen, either through their hosts or in the environments. M. avium is also a chemoorganotroph, so it obtains energy from organic compounds. Mainly, they use palmitic and oleic acids as their main carbon and energy source. Palmitate and oleate are long chain fatty acids that could later be incorporated into the hydrophobic surface of the cell that is characteristic of acid-fast mycobacteria (6).

Pathology

Mycobacterium avium complex disease occurs in persons with defects in their cellular immunity, such as those suffering from AIDS, whose CD4 cell counts are well below 50 cells per microliter. This leads to disseminated infections. The modes of transmission are usually through ingestion or inhalation which goes through the respiratory and GI tract. These bacteria cross the bronchial and intestinal tissue to get into our bloodstream. Once in our blood, they spread throughout the body. They highly varied in the extent of virulence and their impact on the host. Once inside the host, they bind to phagocytes, which initiates phagocytosis as well as the release of cytokines and reactive oxygen radicals. The phagosomes engulfed the invading bacteria and fused with vesicles containing enzymes and agents to kill the microorganism. However, since MAC can tolerate all kinds of pH and environments, they are also able survive the enzymes inside the vesicles and are able to multiply and overpowering the phagocytic cell and killing it.

The symptoms associated with MAC for those with HIV positive includes fever, night sweat, weight loss, abdominal pain, fatigue, diarrhea, and anemia- all occurring excessively. For people who are HIV negative, the greatest risk with MAC is lymadenopathy or it could lead to pulmonary disease. MAC infections cause mycobacterial lymphadenitis in children under 12. These bacterias have unique, antigenic lipids called glycopeptidolipids (GPLs) which are located on the surface of the cell. The lipids are able to suppress the immune response of the host and can produce cytokines that will change the way the host reacts to pathogens.

In terms of treatment, MAC is resistant to many chemotherapeutic agents. The limited list of antimycobacterial drugs that they are susceptible to are clarithromycin, azithromycin, ethambutol, clofazimine, and rifamycins (rifabutin). It is treated for a minimal period of 12 months with multi-drug therapy to prevent bacterial resistance to one particular drug and to tear down the lipid covering of the cell so the drug can enter (8). However, studies are still undergoing to come up with new drugs as well as to find the regimen of drugs that will be most effective for eradicating the bacterias (9).

Application to Biotechnology

Does this organism produce any useful compounds or enzymes? What are they and how are they used?

Current Research

MAC is an intracellular pathogen that lies quietly in our tissues. Because it only affects us when our immune system is down, MAC can be difficult to detect. However, the development of in situ hybridization (ISH) assays allows us to detect MAC in culture, sputum, and tissue by using probes from rRNA oligonucleotides. These rRNA probes are very specific for M. intracellulare and M. avium. The probes target rRNA, which exists in cells at high number of copies. Therefore, the number of potential targets is high. Through this technique, scientists were able to detect and identify MAC organisms within animals and humans. The probes are highly sensitive for their specific rRNA, so it can detect as little as single cells. Scientists believe that these probes hold much promise for the future by their rapid detection and their differentiation among the species (12).

Newly synthesized antimicrobial drug called benzoxazinorifamycins, a derivative of rifamycin, is found to be effective at killing slow growing, pathogenic mycobacteria. It deems almost ineffective for rapidly growing mycobacteria. Studies showed that this newly synthesized drug is much more efficient at treating M. intracellulare infection in mice as compared to old ones such as RMP. Benzoxazinorifamycins also exhibit lower toxicities in mice. More detailed studies of the drugs’ therapeutic activities in mice are currently being administered (13).

Recently, in China, scientists were trying to determine factors that might affect sputum conversion and the results of treating MAC pulmonary disease. Sputum conversion tells these scientists the response to treatment in the test subjects. With this information, the researchers can detect failure to respond to the drugs or therapy. They observed 46 patients over the course of 7 years. They checked histories of these clients and found that 31 out of 46 had lung diseases in the past. These scientists divide the study subjects into three groups: 1. those receiving anti-MAC drugs for less than five months or were given a regimen that does not contain at least 2 anti-MAC drugs (this reduce the effect of the antibiotics) 2. some patients were treated with anti-MAC drugs for more than five months 3. the third group did not receive any drugs or therapy. Not surprisingly, those given the drugs for less than five months did not have sputum conversion, but the other subjects did receive it. Follow-ups on sputum cultures of these 28 found that sputum conversion occurred in 17 patients. In the end, these scientists came up with 2 conclusions. One, MAC pulmonary disease occurs with preexisting lung disease. Two, inappropriate or inadequate drug treatment will not cause sputum conversion (14).

References

1. http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Info&id=1764&lvl=3&p=mapview&p=has_linkout&p=blast_url&p=genome_blast&lin=f&keep=1&srchmode=1&unlock
2. Freeman, R., Geier, H., Weigel, K., Do, J., Ford, T., Cangelosi, G. “Roles of Cell Wall Glycopeptidolipid in Surface Adherence and Planktonic Dispersal of Mycobacterium avium”. Applied and Environmental Microbiology. 2006. Volume 72. p. 7554-7558. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1694245
3. Carter, G., Wu, M., Drummond, D., Bermudez, L. “Characterization of Biofilm Formation By Clinical Isolates of Mycobacterium avium”. Journal of Medical Microbiology. 2003. Volume 52. p. 747-752. http://jmm.sgmjournals.org/cgi/content/full/52/9/747
4. http://www.ncbi.nlm.nih.gov/sites/entrez?Db=genome&Cmd=ShowDetailView&TermToSearch=20086
5. Netting, J. “Scientists are Beginning to Understand How Bacteria Find Strength in Numbers”. Science News. 2001. Volume 160. p. 28. http://www.sciencenews.org/articles/20010714/bob12.asp
6. Inderlied, C., Kemper, C., Bermudez, L. “The Mycobacterium avium Complex”. Clinical Microbiology Reviews. 1993. Volume 6. p. 266-310. http://www.pubmedcentral.nih.gov/pagerender.fcgi?artid=358286&pageindex=7#page
7. Chatterjee, D., Khoo, KH. “The Surface Glycopeptidolipids of Mycobacteria: Structures and Biological Properties”. Cellular and Molecular Life Science. 2001. Volume 14. p. 2018-2042. http://www.ncbi.nlm.nih.gov/sites/entrez?db=PubMed&cmd=Retrieve&list_uids=11814054
8. Koirala, J.,Harley, W. “Mycobacterium Avium-Intracellulare”. http://www.emedicine.com/med/topic1532.htm
9. Havlir, D. “Mycobacterium avium Complex: Advances in Therapy”. European Journal of Clinical Microbiology and Infectious Diseases. 1994. Volume 13. p 1435-4373. http://www.springerlink.com/content/l1h0831h47r7336h/?p=72a856958d2149be8144b1d64331f1a3&pi=9
10. Mdluli, K., Swanson, J., Fischer, E., Lee, R., Barry, C. “Mechanisms Involved in the Intrinsic Isoniazid Resistance of Mycobacterium avium”. Molecular Microbiology. 1998. Volume 27. p. 1223-1233. http://www.blackwell-synergy.com/doi/pdf/10.1046/j.1365-2958.1998.00774.x?cookieSet=1
11. Horan, K., Freeman, R., Weigel, K., Semret, M., Pfaller, S., Covert, T., Soolingen, D., Leao, S., Behr, M., Cangelosi, G. “Isolation of the Genome Sequence Strain Mycobacterium avium 104 from Multiple Patients over a 17-Year Period”. Journal of Clinical Microbiology. 2006. Volume 44. p. 783-789. http://jcm.asm.org/cgi/reprint/44/3/783.pdf
12. Amand, A., Frank,D., Groote, M., Pace, N. “Use of Specific rRNA Oligonucleotide Probes for Microscopic Detection of Mycobacterium avium Complex Organisms in Tissue”. Journal of Clinical Microbiology. 2005. Volume 43. p 1505-1514. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1081365#id2676416
13. Saito, H., Tomioka,H., Sato, K., Emori, M., Yamane, T., Yamashita, K., Hosoe, K., Hidaka, T. “In Vitro Antimycobacterial Activities of Newly Synthesized Benzoxazinorifamycins”. Antimicrobial Agents and Chemotherapy. 1991. Volume 35. p 542-547. http://www.pubmedcentral.nih.gov/pagerender.fcgi?artid=245047&pageindex=5#page
14. Ye, J.,Wu, T., Chiang, P., Lee, M. “Factors That Affect Sputum Conversion and Treatment Outcome in Patients with Mycobacterium avium-intracellulare Complex Pulmonary Disease”. Journal of Microbiology, Immunology and Infection. 2007. Volume 40. p. 342-348. http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=17712469&ordinalpos=3&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum

Edited by student of Rachel Larsen