Rhodococcus equi
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''Rhodococcus equi'' is a
Gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bact ...
coccobacillus A coccobacillus (plural coccobacilli), or bacilluscocco, is a type of bacterium with a shape intermediate between cocci (spherical bacteria) and bacilli (rod-shaped bacteria). Coccobacilli, then, are very short rods which may be mistaken for coc ...
bacterium. The organism is commonly found in dry and dusty soil and can be important for diseases of domesticated animals (horses and goats). The frequency of infection can reach near 60%. ''R. equi'' is an important pathogen causing
pneumonia Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severi ...
in foals. Since 2008, ''R. equi'' has been known to infect
wild boar The wild boar (''Sus scrofa''), also known as the wild swine, common wild pig, Eurasian wild pig, or simply wild pig, is a suid native to much of Eurasia and North Africa, and has been introduced to the Americas and Oceania. The species is ...
and
domestic pig The pig (''Sus domesticus''), often called swine, hog, or domestic pig when distinguishing from other members of the genus '' Sus'', is an omnivorous, domesticated, even-toed, hoofed mammal. It is variously considered a subspecies of ''Sus ...
s. ''R. equi'' can infect humans. At-risk groups are
immunocompromised Immunodeficiency, also known as immunocompromisation, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that a ...
people, such as
HIV-AIDS Human immunodeficiency virus infection and acquired immunodeficiency syndrome (HIV/AIDS) is a spectrum of conditions caused by infection with the human immunodeficiency virus (HIV), a retrovirus. Following initial infection an individual ma ...
patients or transplant recipients. ''Rhodococcus'' infection in these patients resemble clinical and pathological signs of pulmonary
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, i ...
. It is facultative intracellular.


Hosts

*
Pig The pig (''Sus domesticus''), often called swine, hog, or domestic pig when distinguishing from other members of the genus '' Sus'', is an omnivorous, domesticated, even-toed, hoofed mammal. It is variously considered a subspecies of ''Sus ...
s (wild and domestic) *
Goat The goat or domestic goat (''Capra hircus'') is a domesticated species of goat-antelope typically kept as livestock. It was domesticated from the wild goat (''C. aegagrus'') of Southwest Asia and Eastern Europe. The goat is a member of the a ...
s *
Horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...
s *
Sheep Sheep or domestic sheep (''Ovis aries'') are domesticated, ruminant mammals typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus '' Ovis'', in everyday usage it almost always refers to domesticated ...
*
Cattle Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus ''Bos''. Adult females are referred to as cows and adult ma ...
*
Human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
s *
Cat The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...
s may become infected if wound is exposed.


Virulence

The most common route of infection in horses is likely via inhalation of contaminated dust particles. Inhaled
virulent Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ...
strains of ''R. equi'' are
phagocytosed Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is c ...
by alveolar
macrophages 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 ...
. During normal
phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
, bacteria are enclosed by the
phagosome In cell biology, a phagosome is a vesicle formed around a particle engulfed by a phagocyte via phagocytosis. Professional phagocytes include macrophages, neutrophils, and dendritic cells (DCs). A phagosome is formed by the fusion of the cell ...
, which fuses with the
lysosome A lysosome () is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane pr ...
to become a
phagolysosome In biology, a phagolysosome, or endolysosome, is a cytoplasmic body formed by the fusion of a phagosome with a lysosome in a process that occurs during phagocytosis. Formation of phagolysosomes is essential for the intracellular destruction of mic ...
. The internal environment of the phagolysosome contains nucleases and
protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...
s, which are activated by the low pH of the compartment. The macrophage produces
bacteriocidal A bactericide or bacteriocide, sometimes abbreviated Bcidal, is a substance which kills bacteria. Bactericides are disinfectants, antiseptics, or antibiotics. However, material surfaces can also have bactericidal properties based solely on thei ...
compounds (e.g., oxygen radicals) following the
respiratory burst Respiratory burst (or oxidative burst) is the rapid release of the reactive oxygen species (ROS), superoxide anion () and hydrogen peroxide (), from different cell types. This is usually utilised for mammalian immunological defence, but also play ...
. However, like its close relative '' Mycobacterium tuberculosis'', ''R. equi'' prevents the fusion of the phagosome with the lysosome and acidification of the phagosome. Additionally, the respiratory burst is inhibited. This allows ''R. equi'' to multiply within the phagosome where it is shielded from the immune system by the very cell that was supposed to kill it. After about 48 hours, the macrophage is killed by necrosis, not apoptosis. Necrosis is pro-inflammatory, attracting additional phagocytic cells to the site of infection, eventually resulting in massive tissue damage.


Virulence plasmid

All strains isolated from foals and the majority of human, cattle, and pig isolates contain a large plasmid. This plasmid has been shown to be essential for infection of foals, and presumably plays a similar role for
infection An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dis ...
of other hosts, although this has not been established yet. Strains that lack the virulence plasmid are unable to proliferate in macrophages. This virulence plasmid has been characterised in detail from equine and porcine strains, although only the former has been functionally characterised. These circular plasmids consist of a conserved backbone responsible for replication and bacterial conjugation of the plasmid. This portion of the plasmid is highly conserved and found in nonpathogenic ''Rhodococci'' plasmids. In addition to the conserved region, the virulence plasmids contain a highly variable region that has undergone substantial genetic rearrangements, including inversion and deletions. This region has a different GC-content from the rest of the plasmid, and is flanked by genes associated with
mobile genetic elements Mobile genetic elements (MGEs) sometimes called selfish genetic elements are a type of genetic material that can move around within a genome, or that can be transferred from one species or replicon to another. MGEs are found in all organisms. In ...
. It is therefore assumed to be derived from a different bacterial species than the backbone of the plasmid via
lateral gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring ( reproduction). ...
.


Pathogenicity island

The variable region of the virulence plasmid contain genes that are highly expressed following phagocytosis of ''R. equi'' by macrophages. This variable region is believed to be a
pathogenicity island Pathogenicity islands (PAIs), as termed in 1990, are a distinct class of genomic islands acquired by microorganisms through horizontal gene transfer. Pathogenicity islands are found in both animal and plant pathogens. Additionally, PAIs are found ...
that contains genes essential for virulence. A hallmark of the pathogenicity island (PAI) is that many genes within it do not have homologues in other species. The most notable of these are the virulence-associated protein (''vap'') genes. All foals infected with ''R. equi'' produce high levels of antibodies specific for ''vapA'', the first ''vap'' gene to be characterised. Deletion of ''vapA'' renders the resulting strain avirulent. In addition to ''vapA'', the PAI encodes a further five full-length ''vap'' homologues, one truncated ''vap'' gene, and two ''vap'' pseudogenes. The porcine PAI contains five full-length ''vap'' genes, including the ''vapA'' homologue, ''vapB''. In addition to these unique genes, the PAI contains genes that have a known function, in particular two regulatory genes encoding the LysR-type regulator VirR and the response regulator Orf8. These two proteins have been shown to control expression of a number of PAI genes including ''vapA''. Other genes have homology to transport proteins and enzymes. However, the functionality of these genes or how the proteins encoded within PAI subvert the macrophage has not yet been established.


Taxonomic debate

While this organism is generally known as ''Rhodococcus equi'', there has been taxonomic debate since the 1980s about whether this name is the valid name, with ''Rhodococcus hoagii'' and ''Prescottella equi'' both proposed as official alternative names. Other names used include ''Nocardia restricta'', ''Corynebacterium equi'', ''Bacillus hoagii'', ''Corynebacterium purulentus'', ''Mycobacterium equi'', ''Mycobacterium restrictum'', and ''Proactinomyces restrictus''.


References


Further reading

* {{Taxonbar, from=Q293148 Mycobacteriales Gram-positive bacteria Bacteria described in 1923