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''Mycobacterium'' is a
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of over 190 species in the phylum Actinomycetota, assigned its own family,
Mycobacteriaceae ''Mycobacteriaceae'' is a family of bacteria in the phylum Actinomycetota The ''Actinomycetota'' (or ''Actinobacteria'') are a phylum of all gram-positive bacteria. They can be terrestrial or aquatic. They are of great economic importance t ...
. This genus includes
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...
s known to cause serious diseases in mammals, including
tuberculosis Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, ...
('' M. tuberculosis'') and
leprosy Leprosy, also known as Hansen's disease (HD), is a long-term infection by the bacteria '' Mycobacterium leprae'' or '' Mycobacterium lepromatosis''. Infection can lead to damage of the nerves, respiratory tract, skin, and eyes. This nerve d ...
(''
M. leprae ''Mycobacterium leprae'' (also known as the leprosy bacillus or Hansen's bacillus), is one of the two species of bacteria that cause Hansen’s disease (leprosy), a chronic but curable infectious disease that damages the peripheral nerves a ...
'') in humans. The Greek prefix ''myco-'' means 'fungus', alluding to this genus' mold-like colony surfaces. Since this genus has
cell walls A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
with Gram-positive and
Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...
features, acid-fast staining is used to emphasize their resistance to acids, compared to other cell types.


Metabolism and Morphology

Mycobacteria are aerobic with 0.2-0.6 µm wide and 1.0-10 µm long rod shapes. They are generally non-motile, except for the species '' Mycobacterium marinum'', which has been shown to be motile within
macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer ce ...
s. Mycobacteria possess capsules and most do not form endospores. ''M. marinum'' and perhaps ''M. bovis'' have been shown to
sporulate In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...
; however, this has been contested by further research. The distinguishing characteristic of all ''Mycobacterium'' species is a thick,
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, ...
, and mycolic acid-rich cell wall made of peptidoglycan and arabinogalactan, with these unique components offering targets for new tuberculosis drugs. Many ''Mycobacterium'' species readily grow with minimal nutrients, using
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
and/or
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
s as nitrogen sources and
glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
as a carbon source in the presence of mineral salts. Temperatures for optimal growth vary between species and media conditions, ranging from 25-45°C. Most ''Mycobacterium'' species, including most clinically relevant species, can be cultured in
blood agar An agar plate is a Petri dish that contains a growth medium solidified with agar, used to culture microorganisms. Sometimes selective compounds are added to influence growth, such as antibiotics. Individual microorganisms placed on the plate wi ...
. However, some species grow very slowly due to extremely long reproductive cycles, such as ''
M. leprae ''Mycobacterium leprae'' (also known as the leprosy bacillus or Hansen's bacillus), is one of the two species of bacteria that cause Hansen’s disease (leprosy), a chronic but curable infectious disease that damages the peripheral nerves a ...
'' requiring 12 days per division cycle compared to 20 minutes for some ''E. coli'' strains.


Ecology

Whereas ''Mycobacterium tuberculosis'' and ''M. leprae'' are pathogenic, most mycobacteria do not cause disease unless they enter skin lesions of those with pulmonary and/or immune dysfunction, despite being widespread across aquatic and terrestrial environments. Through
biofilm A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular po ...
formation, cell wall resistance to
chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
, and association with amoebas, mycobacteria can survive a variety of environmental stressors. The agar media used for most
water testing Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a s ...
does not support the growth of mycobacteria, allowing it to go undetected in municipal and hospital systems.


Pathogenicity

''
Mycobacterium tuberculosis ''Mycobacterium tuberculosis'' (M. tb) 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 it ...
'' can remain latent in human hosts, decades after an initial infection, to continue infecting others. It has been estimated that a third of the world population has latent tuberculosis (TB). ''M. tuberculosis'' has many virulence factors, which can be divided across lipid and fatty acid metabolism, cell envelope proteins,
macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer ce ...
inhibitors,
kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...
proteins, proteases, metal-transporter proteins, and gene expression regulators. Mycobacteria have cell walls with peptidoglycan, arabinogalactan, and mycolic acid; a waxy outer mycomembrane of mycolic acid; and an outermost capsule of glucans and secreted proteins for virulence. It constantly remodels these layers to survive in stressful environments and avoid host immune defenses. This unique structure makes penicillins ineffective, instead requiring a multi-drug antibiotic treatment of isoniazid to inhibit mycolic acid synthesis,
rifampicin Rifampicin, also known as rifampin, is an ansamycin antibiotic used to treat several types of bacterial infections, including tuberculosis (TB), ''Mycobacterium avium'' complex, leprosy, and Legionnaires’ disease. It is almost always used t ...
to interfere with transcription, ethambutol to hinder arabinogalactan synthesis, and pyrazinamide to impede Coenzyme A synthesis.


Genomics

Comparative analyses of mycobacterial genomes have identified several conserved
indel Indel is a molecular biology term for an insertion or deletion of bases in the genome of an organism. It is classified among small genetic variations, measuring from 1 to 10 000 base pairs in length, including insertion and deletion events that ...
s and signature proteins that are uniquely found in all sequenced species from the genus ''Mycobacterium''. Additionally, 14 proteins are found only in the species from the genera ''Mycobacterium'' and ''
Nocardia ''Nocardia'' is a genus of weakly staining Gram-positive, catalase-positive, rod-shaped bacteria. It forms partially acid-fast beaded branching filaments (acting as fungi, but being truly bacteria). It contains a total of 85 species. Some sp ...
'', suggesting that these two genera are closely related. The genomes of some mycobacteria are quite large, such as ''M. vulneris'' encoding 6,653 proteins, larger than the ~6000 proteins of
eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...
.


Species

Mycobacteria have historically been categorized through phenotypic testing, such as the
Runyon classification The Timpe and Runyon classification of nontuberculous mycobacteria based on the rate of growth, production of yellow pigment and whether this pigment was produced in the dark or only after exposure to light. It was introduced by Ernest Runyon i ...
of analyzing growth rate and production of yellow/orange carotenoid pigments. Group I contains photochromogens (pigment production induced by light), Group II comprises scotochromogens (constitutive pigment production), and the non-chromogens of Groups III and IV have a pale yellow/tan pigment, regardless of light exposure. Group IV species are "rapidly-growing" mycobacteria compared to the "slowly-growing" species because samples grow into visible colonies in less than seven days. Because the International Code of Nomenclature of Prokaryotes (ICNP) currently recognizes 195 ''Mycobacterium'' species, classification and identification systems now rely on
DNA sequencing DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. T ...
and computational phylogenetics. The major disease-causing groups are the Mycobacterium tuberculosis complex (
tuberculosis Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, ...
),
Mycobacterium avium complex ''Mycobacterium avium ''complex is a group of mycobacteria comprising ''Mycobacterium intracellulare'' and ''Mycobacterium avium'' that are commonly grouped because they infect humans together; this group, in turn, is part of the group of nontub ...
(
mycobacterium avium-intracellulare infection ''Mycobacterium avium-intracellulare'' infection (MAI) is an atypical mycobacterial infection, i.e. one with nontuberculous mycobacteria or NTM, caused by ''Mycobacterium avium'' complex (MAC), which is made of two '' Mycobacterium'' species, ''M ...
), ''
M. leprae ''Mycobacterium leprae'' (also known as the leprosy bacillus or Hansen's bacillus), is one of the two species of bacteria that cause Hansen’s disease (leprosy), a chronic but curable infectious disease that damages the peripheral nerves a ...
'' and '' M. lepromatosis'' (
leprosy Leprosy, also known as Hansen's disease (HD), is a long-term infection by the bacteria '' Mycobacterium leprae'' or '' Mycobacterium lepromatosis''. Infection can lead to damage of the nerves, respiratory tract, skin, and eyes. This nerve d ...
), and Mycobacterium abscessus ( chronic lung infection). Microbiologist Enrico Tortoli has constructed a phylogentic tree of the genus' key species based on the earlier genetic sequencing of Rogall, et al. (1990), alongside new phylogentic trees based on Tortoli's 2017 sequencing of 148 ''Mycobacterium'' species:


Proposed division of the genus

Gupta ''et al.'' have proposed dividing ''Mycobacterium'' into five genera, based on an analysis of 150 species in this genus. Due to controversy over complicating clinical diagnoses and treatment, all of the renamed species have retained their original identity in the ''Mycobacterium'' genus as a valid taxonomic synonym: * ''Mycobacterium'' based on the Slowly-Growing Tuberculosis-Simiae clade * ''Mycobacteroides'' based on the Rapidly-Growing Abscessus-Chelonae clade * ''Mycolicibacillus'' based on the Slowly-Growing Triviale clade * ''Mycolicibacter'' based on the Slowly-Growing Terrae clade * ''Mycolicibacterium'' based on the Rapidly-Growing Fortuitum-Vaccae clade


Diagnosis

The two most common methods for visualizing these acid-fast bacilli as bright red against a blue background are the Ziehl-Neelsen stain and modified Kinyoun stain. Fite's stain is used to color ''M. leprae'' cells as pink against a blue background. Rapid Modified Auramine O Florescent staining has specific binding to slowly-growing mycobacteria for yellow staining against a dark background. Newer methods include Gomori-Methenamine Silver staining and Perioidic Acid Schiff staining to color ''Mycobacterium avium complex'' (MAC) cells black and pink, respectively. While some mycobacteria can take up to eight weeks to grow visible colonies from a cultured sample, most clinically relevant species will grow within the first four weeks, allowing physicians to consider alternative causes if negative readings continue past the first month.


Mycobacteriophages

Mycobacteria can be infected by mycobacteriophages, a class of viruses with high specificity for their targets. By hijacking the cellular machinery of mycobacteria to produce additional phages, such viruses can be used in phage therapy for eukaryotic hosts, as they would die alongside the mycobacteria. Since only some mycobacteriophages are capable of penetrating the ''M. tuberculosis'' membrane, the viral DNA may be delivered through artificial liposomes because bacteria uptake, transcribe, and translate foreign DNA into proteins.


Mycosides

Mycosides are glycolipids isolated from ''Mycobacterium'' species with Mycoside A found in photochromogenic strains, Mycoside B in bovine strains, and Mycoside C in avian strains. Different forms of Mycoside C have varying success as a receptor to inactivate mycobacteriophages. Replacement of the gene encoding mycocerosic acid synthase in ''M. bovis'' prevents formation of mycosides.


References


External links


Bacterial and Viral Bioinformatics Resource Center
Genomes, proteins, epitopes, and pathways of mycobacteria
Merck Manual - Mycobacteria

Mycobrowser
Genomic and proteomic database for pathogenic mycobacteria




TB Structural Genomics Consortium
{{Taxonbar, from=Q194309 Acid-fast bacilli Tuberculosis Bacteriology Bacterial diseases Bacteria genera Mycobacteria Pathogenic bacteria Infectious causes of cancer