Editors note: This page has been reviewed for currency and accuracy. Updates appear in blue. (3/2010)
Mycobacterium paratuberculosis has been reclassified as a subspecies of the Mycobacterium Avium Complex (MAC) and is now correctly called Mycobacterium avium subspecies paratuberculosis, commonly abbreviated MAP. A partial phylogenetic tree of the relationship among the slow-growing pathogenic mycobacteria based on 16S rDNA sequencing and the more detailed genetic analysis of the MAC by Turenne et al (J. Bacteriol. 190: 2479-2487, 2008) is shown in the adjacent figure.
Genetic studies reveal that MAP is a clone of closely related organisms derived from the parent Mycobacterium avium subspecies hominissuis. Despite genetic similarity, many observable (phenotypic) characteristics distinguish MAP from other members of the MAC. Some of MAP’s unique characteristics are:
MAP and other members of the Mycobacterium Avium Complex are less closely related genetically to pathogenic mycobacteria in the TB complex: Mycobacterium tuberculosis, the cause of tuberculosis in humans, and Mycobacterium bovis, the cause of tuberculosis in cattle and other animals. MAP also is not closely related to the cause of leprosy in humans, Mycobacterium leprae. MAP, and the disease it causes in animals (Johne's disease), does however share certain biological characteristics in common with these mycobacterial pathogens. Scientists often draw parallels among these mycobacterial organisms to try and understand their basic mechanisms for causing disease.
MAP bacteria are not thought to be free-living (able to grow and multiply) in the environment. Because of its inability to produce mycobactin (a chemical needed to transport iron), MAP can grow only inside animal cells where it "steals" iron from its host's cells, most often immune cells called macrophages. Thus, it is an obligate parasitic pathogen of mammals. This means infected animals are the only place in nature where growth and multiplication of MAP can occur. If found in soil or water samples, it can be assumed that MAP is simply persisting in those places (not multiplying) after being deposited there through fecal contamination from an infected animal.
MAP is extremely hardy and may remain in contaminated soil and water for a year or more. From an anthropomorphic perspective MAP, compensates for its inability gather iron and replicate in the environment (due to lack of mycobactin) by producing an extra tough outer coat enabling prolonged survival while it waits to be eaten by a susceptible host, enter host macrophages, and resume replication.
The environmental distribution of MAP is markedly different from that most other members of the Mycobacterium Avium Complex (MAC), in particular the ubiquitous M. avium subspecies hominissuis (MAH). MAH is commonly found in lakes, streams and domestic water supplies. Certain acidic soil types, notably peat bogs, contain higher than average numbers of MAH. MAP only occurs in environments contaminated by feces of infected animals. A tenuous association between the occurrence of Johne's disease and geographical regions with acidic soils has been reported (see discussion under "survival in soil" on this website). The strength of this association and the biological basis of this association remain to be determined.
has a broad host range. Ruminants (animals with a four-chambered stomach that
chew their cud) are the type of animal most commonly infected. These include:
cattle, sheep, goats, deer, elk, antelope, bison, camels, llamas, and alpacas
(these latter three species are technically called pseudo-ruminants as they have
a three-chambered stomach). There are also infrequent reports of MAP
infections (but not subsequent disease) in non-ruminant species such as carnivorous or omnivorous predators (weasels, , fox, stoat, crows, coyotes, raccoons, etc.). Infection in prey animals (mice, rabbits, vole, rat, sparrows, etc.) have been noted; disease is rare in these species except for some cases of rabbits (Stevenson et al.).
Like other mycobacteria, MAP has the capacity to thrive inside white blood cells
known as macrophages. As part of the immune system, macrophages are capable of
destroying a wide variety of bacterial pathogens. Mycobacteria, however, are one
of the few types of bacteria that not only can survive the antibacterial effects
of macrophages, but actually grow and multiply inside them. Bacteria able to avoid
being killed and instead replicate inside macrophages and cause disease are referred
to as facultative intracellular bacterial pathogens. This picture shows MAP (red) inside macrophages (blue).
MAP is a small (0.5 x 1.5 micron) rod-shaped bacterium, roughly the size of the common intestinal bacterium called E. coli, that grows in clumps of 10 to 100 bacterial cells. It can be seen using a light microscope with 40x or greater power objectives. When stained by the Gram stain, it is blue and so called Gram-positive. When stained by acid-fast stains like the Ziehl-Neelsen or Kinyoun's stain, MAP stains red and so is called acid-fast positive. Transmission electron microscopy reveals the waxy rough cell wall of M. paratuberculosis with its trilaminar structure. The intracellular vacuoles or inclusions common to mycobacteria can also be seen.
The cell wall of mycobacteria is composed of a thick waxy mixture of lipids and polysaccharides. The cell wall of MAP, although not well studied, seems similar in most respects to that of other mycobacteria. One feature is notable, however. While most strains of M. avium produce a surface glycolipid that allows strains to be serotyped (i.e., distinguished using antibodies specific for each glycolipid subtype),MAP strains lack such glycolipid antigens on their surface.
Biochemical tests used to distinguish among other species of mycobacteria are not used to identify MAP. The tests are taxing to perform due to the extremely slow growth rate of the organism, and test results vary among strains of MAP. However, in a seminal publication, Dr. Marie-Françoise Thorel (see photo) and colleagues described biochemical tests and numerical taxonomy methods to differentiate among subtypes of MAP-like bacteria. This work led to the adoption of the new name Mycobacterium avium subspecies paratuberculosis (MAP) (International Journal of Systematic Bacteriology 40:254-260, 1990).
The DNA of MAP is >99% identical with that of M. avium, hence the change in naming as described above. The K-10 strain of MAP was fully sequenced http://cmr.jcvi.org/cgi-bin/CMR/GenomePage.cgi?org=ntma03. It has a high proportion of guanine and cytosine (69%) and an abundance of insertion sequences, with IS900 being the first to be discovered in any species mycobacteria and now generally recognized as unique to MAP. Genetic probes used for detection of MAP in clinical specimens or identification of MAP in cultures are often based on detection of IS900. Other commonly used genetic markers unique to MAP are Mav2 and HspX. Another insertion sequence, named IS901, defines mycobacterial isolates as M. avium subsp. avium. How these insertion elements affect the biology and pathogenic capacity of MAP or MAA is not understood. Evidence suggests, however, that they play a major role.