''Ehrlichia ewingii'' is a species of

rickettsiales The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria. They are obligate intracellular parasites, and some are notable pathogens, including ''Rickettsia'', which causes a variety of diseases in humans, and ' ...

bacteria. It has recently been associated with human infection, and can be detected via PCR serological testing. The name ''Ehrlichia ewingii'' was proposed in 1992.

Taxonomy and characterization

Taxonomy

The current classification is

Bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

, Pseudomonadota,

Alphaproteobacteria Alphaproteobacteria is a class of bacteria in the phylum Pseudomonadota (formerly Proteobacteria). The Magnetococcales and Mariprofundales are considered basal or sister to the Alphaproteobacteria. The Alphaproteobacteria are highly diverse and ...

Rickettsiales The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria. They are obligate intracellular parasites, and some are notable pathogens, including ''Rickettsia'', which causes a variety of diseases in humans, and ' ...

Anaplasmataceae The Ehrlichiaceae are a family of bacteria, included in the order Rickettsiales The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria. They are obligate intracellular parasites, and some are notable patho ...

, ''Ehrlichia ewingii''. Classification of different members of the genus ''

Ehrlichia ''Ehrlichia'' is a genus of Rickettsiales bacteria that are transmitted to vertebrates by ticks. These bacteria cause the disease ehrlichiosis, which is considered zoonotic, because the main reservoirs for the disease are animals. ''Ehrlichia ...

'' has been disputed, however, it is generally agreed that close relatives of ''Ehrlichia ewingii'' are ''

Ehrlichia chaffeensis ''Ehrlichia chaffeensis'' is an obligate intracellular, Gram-negative species of Rickettsiales bacteria. It is a zoonotic pathogen transmitted to humans by the lone star tick (''Amblyomma americanum''). It is the causative agent of human monoc ...

'' and ''

Ehrlichia canis ''Ehrlichia canis'' is an obligate intracellular bacterium that acts as the causative agent of ehrlichiosis, a disease most commonly affecting canine species. This pathogen is present throughout the United States (but is most prominent in the So ...

''. It is also closely related to ''

Wolbachia ''Wolbachia'' is a genus of intracellular bacteria that infects mainly arthropod species, including a high proportion of insects, and also some nematodes. It is one of the most common parasitic microbes, and is possibly the most common reproducti ...

'', ''

Anaplasma ''Anaplasma'' is a genus of bacteria of the alphaproteobacterial order Rickettsiales, family Anaplasmataceae. ''Anaplasma'' species reside in host blood cells and lead to the disease anaplasmosis. The disease most commonly occurs in areas where ...

'', and '' Neorickettsia'' bacteria, with ''

Rickettsia ''Rickettsia'' is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The term "ricke ...

'' as a more distant genus.

Characterization

Species in the family

have unique characteristics that can help differentiate them from other families including: sensitivity to mechanical stress, changes in

osmolarity Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/L ...

, and thawing. All ''Ehrlichia sp''. are

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 ...

and spherical with a rippled outer membrane that shows no

peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid cell wall (murein sacculus) characteristic of most ba ...

layer or any

lipopolysaccharide Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the outer ...

History

Discovery

In 1971 Ewing et al. detected a new ''Ehrlichia'' strain, which was thought of as another ''Ehrlichia canis'' strain. ''Ehrlichia canis'' is usually found in

lymphocyte A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. Lymphocytes include natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic ad ...

s and

monocyte Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and conventional dendritic cells. As a part of the vertebrate innate immune system monocytes also ...

s, but this new strain was found in granulocytes.Ewing, S. A., W. R. Roberson, R. G. Buckner, and C. S. Hayat. 1971. A new strain of Ehrlichia canis. J. Am. Vet. Med. Assoc. 159:1771-1774. Due to the variable location, this strain was called Canine Granulocytic ''Ehrlichia'' (CGE).

Method of identification

A field study was conducted by Anderson et al. on June 18, 1987 using a canine diagnosed with granulocytic

ehrlichiosis Ehrlichiosis is a tick-borne bacterial infection, caused by bacteria of the family Anaplasmataceae, genera '' Ehrlichia'' and ''Anaplasma''. These obligate intracellular bacteria infect and kill white blood cells. The average reported annual in ...

: an infection of the granulocytes by a member of the ''Ehrlichia'' sp. The blood from this canine was infused in another specimen to ensure the blood transmission of the intracellular parasite. This transmission was then completed using

tick Ticks (order Ixodida) are parasitic arachnids that are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, species, and "fullness". Ticks are external parasites, living by ...

s of the species ''

Amblyomma americanum ''Amblyomma americanum'', also known as the lone star tick, the northeastern water tick, or the turkey tick, or the "Cricker Tick", is a type of tick indigenous to much of the eastern United States and Mexico, that bites painlessly and commonly ...

'' by exposing them to infected dogs and then to susceptible ones. From blood samples,

16S rRNA 16S rRNA may refer to: * 16S ribosomal RNA 16 S ribosomal RNA (or 16 S rRNA) is the RNA component of the 30S subunit of a prokaryotic ribosome ( SSU rRNA). It binds to the Shine-Dalgarno sequence and provides most of the SSU structure. The g ...

genes were amplified using standard

polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete or partial) of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it (or a part of it) ...

(PCR), and the genes were analyzed using gel electrophoresis and a GAP (Genetic Analysis Program) system. This data was compared to all other ''Ehrlichia'' species, and Anderson et al. found that CGE is most closely related to ''E. chaffeensis'' and ''E. canis.'' These three—''E. ewingii, E. chaffeensis,'' and ''E. canis—''form a group in terms of relatedness within the genus ''Ehrlichia''. Despite being related, due to the level of divergence between CGE and the other species, it has been determined that CGE deserves to have

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

level recognition. The name was proposed to be ''Ehrlichia ewingii'', named after S.A. Ewing who initially identified the parasitic organism.

Metabolism and genomics

Genomics

This organism mostly uses

host cell In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...

machinery to replicate its

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...

. Although the genome itself has not been sequenced, the use of the host cell for replication is known based on comparisons to other ''Ehrlichia sp''. Since ''Ehrlichia ewingii'' is unable to synthesize all the

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...

s required for growth, also known as an

auxotroph Auxotrophy ( grc, αὐξάνω "to increase"; ''τροφή'' "nourishment") is the inability of an organism to synthesize a particular organic compound required for its growth (as defined by IUPAC). An auxotroph is an organism that displays this ...

, the bacterium requires a host for growth and survival.

Metabolism

No specific studies on ''E. ewingii''

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...

have been conducted so far. However, the organism is assumed to have a similar metabolism to members of the genus due to the high similarity between functional 16S rRNA gene sequencing and the highly related infections they cause within cells.

Ecology

All ''Ehrlichia sp.'' are obligate intracellular parasites that are transmitted by ticks to vertebrate animals; ''Ehrlichia ewingii'' is specifically transmitted by the North American Tick, ''Amblyomma americanum''. This bacterium then infects granulocytes in canines and humans. Currently, ''Ehrlichia ewingii'' cannot be cultivated in a number of

cell lines An immortalised cell line is a population of cells from a multicellular organism which would normally not proliferate indefinitely but, due to mutation, have evaded normal cellular senescence and instead can keep undergoing division. The cells ...

, meaning that study of this organism is dependent on extraction from

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

and

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

hosts.

Disease and treatment

''Ehrlichia ewingii'' is a human

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 germ ...

which results in a serious

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 ...

if not treated in a timely manner.

Ehrlichiosis Ehrlichiosis is a tick-borne bacterial infection, caused by bacteria of the family Anaplasmataceae, genera '' Ehrlichia'' and ''Anaplasma''. These obligate intracellular bacteria infect and kill white blood cells. The average reported annual in ...

, the disease caused by ''E. ewingii'' and ''E. chaffeensis'', presents with fever, headache, fatigue, and muscle aches. These symptoms are extremely unspecific which makes the infections difficult to diagnose. Doxycycline is effective when administered early in the infection, but despite this, the estimated

fatality rate In epidemiology, case fatality rate (CFR) – or sometimes more accurately case-fatality risk – is the proportion of people diagnosed with a certain disease, who end up dying of it. Unlike a disease's mortality rate, the CFR does not take int ...

is still 1.8%. This is partially due to the difficulty diagnosing the

causative agent 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 ge ...

of infection, however further ''Ehrlichia ewingii'' research could allow better

diagnostic tests A medical test is a medical procedure performed to detect, diagnose, or monitor diseases, disease processes, susceptibility, or to determine a course of treatment. Medical tests such as, physical and visual exams, diagnostic imaging, genetic te ...

to identify ehrlichial infections before they progress to severe cases.

References

External links

Ehrlichia infection in deer
{{Taxonbar, from=Q5348630 Rickettsiales