Mycobacterium is a genus of Actinobacteria, given its own family, the Mycobacteriaceae.
The genus includes pathogens known to cause serious diseases in mammals, including tuberculosis
and leprosy. The Greek prefix myco means fungus, alluding to the way mycobacteria have been
observed to grow in a mold-like fashion on the surface of liquids when cultured.
Microbiologic characteristics
Mycobacteria are aerobic and nonmotile bacteria that are characteristically acid-alcohol-fast.
Mycobacteria do not contain endospores or capsules and are usually considered Gram-positive.
While mycobacteria do not seem to fit the Gram-positive category from an empirical standpoint,
they are classified as an acid-fast Gram-positive bacterium due to their lack of an outer cell
membrane. Mycobacterium marinum and perhaps M. bovis have been shown to sporulate; however,
this has been contested by further research. The distinguishing characteristic of all Mycobacterium
species is that the cell wall is thicker than in many other bacteria, which is hydrophobic,
waxy, and rich in mycolic acids/mycolates. The cell wall consists of the hydrophobic
mycolate layer and a peptidoglycan layer held together by a polysaccharide, arabinogalactan.
The cell wall makes a substantial contribution to the hardiness of this genus. The biosynthetic
pathways of cell wall components are potential targets for new drugs for tuberculosis.
Many Mycobacterium species adapt readily to growth on very simple substrates, using ammonia
or amino acids as nitrogen sources and glycerol as a carbon source in the presence of mineral
salts. Optimum growth temperatures vary widely according to the species and range from 25 °C
to over 50 °C. Some species can be very difficult to culture,
sometimes taking over two years to develop in culture. Further, some species also have
extremely long reproductive cycles — M. leprae, may take more than 20 days to proceed
through one division cycle, making laboratory culture a slow process. In addition, the availability
of genetic manipulation techniques still lags far behind that of other bacterial species.
A natural division occurs between slowly– and rapidly–growing species. Mycobacteria
that form colonies clearly visible to the naked eye within seven days on subculture
are termed rapid growers, while those requiring longer periods are termed slow growers. Mycobacteria
cells are straight or slightly curved rods between 0.2 and 0.6 µm wide by 1.0 and 10 µm
long. Pigmentation
Some mycobacteria produce carotenoid pigments without light. Others require photoactivation
for pigment production. Photochromogens
Produce nonpigmented colonies when grown in the dark and pigmented colonies only after
exposure to light and reincubation. Ex: M. kansasii, M. marinum, M. simiae.
Scotochromogens Produce deep yellow to orange colonies when
grown in the presence of either the light or the dark.
Ex: M. scrofulaceum, M. gordonae, M. xenopi, M. szulgai.
Non-chromogens Nonpigmented in the light and dark or have
only a pale yellow, buff or tan pigment that does not intensify after light exposure.
Ex: M. tuberculosis, M. avium-intra-cellulare, M. bovis, M. ulcerans
Ex: M. fortuitum, M. chelonae
Staining characteristics Mycobacteria are classical acid-fast organisms.
Stains used in evaluation of tissue specimens or microbiological specimens include Fite's
stain, Ziehl-Neelsen stain, and Kinyoun stain. Mycobacteria appear phenotypically most closely
related to members of Nocardia, Rhodococcus and Corynebacterium.
Ecological characteristics Mycobacteria are widespread organisms, typically
living in water and food sources. Some, however, including the tuberculosis and the leprosy
organisms, appear to be obligate parasites and are not found as free-living members of
the genus. Pathogenicity
Mycobacteria can colonize their hosts without the hosts showing any adverse signs. For example,
billions of people around the world have asymptomatic infections of M. tuberculosis.
Mycobacterial infections are notoriously difficult to treat. The organisms are hardy due to their
cell wall, which is neither truly Gram negative nor positive. In addition, they are naturally
resistant to a number of antibiotics that disrupt cell-wall biosynthesis, such as penicillin.
Due to their unique cell wall, they can survive long exposure to acids, alkalis, detergents,
oxidative bursts, lysis by complement, and many antibiotics. Most mycobacteria are susceptible
to the antibiotics clarithromycin and rifamycin, but antibiotic-resistant strains have emerged.
As with other bacterial pathogens, surface and secreted proteins of M. tuberculosis contribute
significantly to the virulence of this organism. There is an increasing list of extracytoplasmic
proteins proven to have a function in the virulence of M. tuberculosis.
Medical classification Mycobacteria can be classified into several
major groups for purpose of diagnosis and treatment: M. tuberculosis complex, which
can cause tuberculosis: M. tuberculosis, M. bovis, M. africanum, and M. microti; M. leprae,
which causes Hansen's disease or leprosy; Nontuberculous mycobacteria are all the other
mycobacteria, which can cause pulmonary disease resembling tuberculosis, lymphadenitis, skin
disease, or disseminated disease. Mycosides
Mycosides are phenolic alcohols that were shown to be components of Mycobacterium glycolipids
that are termed glycosides of phenolphthiocerol dimycocerosate There are 18 and 20 carbon
atoms in mycosides A, and B, respectively. Genomics
Comparative analyses of mycobacterial genomes have identified several conserved indels 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, suggesting that these two genera are closely related.
Species
Phenotypic tests can be used to identify and distinguish different Mycobacteria species
and strains. In older systems, mycobacteria are grouped based upon their appearance and
rate of growth. However, these are symplesiomorphies, and more recent classification is based upon
cladistics. Slowly growing
Mycobacterium tuberculosis complex Mycobacterium tuberculosis complex members
are causative agents of human and animal tuberculosis. Species in this complex include:
M. tuberculosis, the major cause of human tuberculosis
M. bovis M. bovis BCG
M. africanum M. canetti
M. caprae M. microti
M. pinnipedii
Mycobacterium avium complex Mycobacterium avium complex is a group of
species that, in a disseminated infection but not lung infection, used to be a significant
cause of death in AIDS patients. Species in this complex include:
M. avium M. avium paratuberculosis, which has been
implicated in Crohn's disease in humans and is the causative agent of Johne's disease
in cattle and sheep M. avium silvaticum
M. avium "hominissuis" M. colombiense
M. indicus pranii
Mycobacterium gordonae clade M. asiaticum
M. gordonae Mycobacterium kansasii clade
M. g M. kansasii
Mycobacterium nonchromogenicum/terrae clade M. hiberniae
M. nonchromogenicum M. terrae
M. triviale Mycolactone-producing mycobacteria
M. ulcerans, which causes the "Buruli", or "Bairnsdale, ulcer"
M. pseudoshottsii M. shottsii
Mycobacterium simiae clade M. triplex
M. genavense M. florentinum
M. lentiflavum M. palustre
M. kubicae M. parascrofulaceum
M. heidelbergense M. interjectum
M. simiae Ungrouped
M. branderi M. cookii
M. celatum M. bohemicum
M. haemophilum M. malmoense
M. szulgai M. leprae, which causes leprosy
M. lepraemurium M. lepromatosis, another cause of leprosy,
described in 2008 M. botniense
M. chimaera M. conspicuum
M. doricum M. farcinogenes
M. heckeshornense M. intracellulare
M. lacus M. marinum
M. monacense M. montefiorense
M. murale M. nebraskense
M. saskatchewanense M. scrofulaceum
M. shimoidei M. tusciae
M. xenopi M. yongonense
Intermediate growth rate M. intermedium
Rapidly growing Mycobacterium chelonae clade
M. abscessus M. chelonae
M. bolletii Mycobacterium fortuitum clade
M. fortuitum M. fortuitum subsp. acetamidolyticum
M. boenickei M. peregrinum
M. porcinum M. senegalense
M. septicum M. neworleansense
M. houstonense M. mucogenicum
M. mageritense M. brisbanense
M. cosmeticum Mycobacterium parafortuitum clade
M. parafortuitum M. austroafricanum
M. diernhoferi M. hodleri
M. neoaurum M. frederiksbergense
Mycobacterium vaccae clade M. aurum
M. vaccae CF
M. chitae M. fallax
Ungrouped M. confluentis
M. flavescens M. madagascariense
M. phlei M. smegmatis
M. goodii M. wolinskyi
M. thermoresistibile M. gadium
M. komossense M. obuense
M. sphagni M. agri
M. aichiense M. alvei
M. arupense M. brumae
M. canariasense M. chubuense
M. conceptionense M. duvalii
M. elephantis M. gilvum
M. hassiacum M. holsaticum
M. immunogenum M. massiliense
M. moriokaense M. psychrotolerans
M. pyrenivorans M. vanbaalenii
M. pulveris Ungrouped
M. arosiense M. aubagnense
M. caprae M. chlorophenolicum
M. fluoroanthenivorans M. kumamotonense
M. novocastrense M. parmense
M. phocaicum M. poriferae
M. rhodesiae M. seoulense
M. tokaiense Mycobacteriophage
Mycobacteria can be infected by Mycobacteriophage, bacterial viruses that may be used in the
future to treat tuberculosis and related diseases by phage therapy.
References
Further reading
External links Tuberculist: Genome annotation database
MTB Sysborg: Genome annotation database from the Institute of Genomics and Integrative
Biology TB Structural Genomics Consortium: Structures
of Mycobacterium tuberculosis proteins MycDB: Mycobacterium database
TBDB: Tuberculosis database Mycobacterium genomes and related information
at PATRIC, a Bioinformatics Resource Center funded by NIAID
Frequently Asked Questions about NTM Lung Disease
PRASITE: Identification of mycobacteria