Rickettsia is a
genusIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of
non-motileMotility is a biological term which refers to the ability to move spontaneously and actively, consuming energy in the process. Most animals are motile but the term applies to single-celled and simple multicellular organisms, as well as to some mechanisms of fluid flow in multicellular organs, in...
,
Gram-negativeGram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
,
non-sporeformingAn endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...
, highly pleomorphic
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that can present as cocci (0.1 μm in diameter), rods (1–4 μm long) or thread-like (10 μm long). Being obligate intracellular parasites, the
Rickettsia survival depends on entry, growth, and replication within the
cytoplasmThe cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
of
eukaryoticA eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
host cells (typically endothelial cells). Because of this,
Rickettsia cannot live in artificial nutrient environments and are grown either in
tissueTissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
or
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
cultures (typically, chicken embryos are used). In the past they were positioned somewhere between viruses and true
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
. The majority of
Rickettsia bacteria are susceptible to
antibioticAn antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...
s of the
tetracycline group.
Despite the similar name,
Rickettsia bacteria do not cause
ricketsRickets is a softening of bones in children due to deficiency or impaired metabolism of vitamin D, magnesium , phosphorus or calcium, potentially leading to fractures and deformity. Rickets is among the most frequent childhood diseases in many developing countries...
, which is a result of vitamin D
deficiencyAvitaminosis is any disease caused by chronic or long-term vitamin deficiency or caused by a defect in metabolic conversion, such as tryptophan to niacin...
.
Rickettsia species are carried by many
tickTicks are small arachnids in the order Ixodida, along with mites, constitute the subclass Acarina. Ticks are ectoparasites , living by hematophagy on the blood of mammals, birds, and sometimes reptiles and amphibians...
s,
fleaFlea is the common name for insects of the order Siphonaptera which are wingless insects with mouthparts adapted for piercing skin and sucking blood...
s, and lice, and cause
diseaseA disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
s in
humanHumans are the only living species in the Homo genus...
s such as
typhusEpidemic typhus is a form of typhus so named because the disease often causes epidemics following wars and natural disasters...
,
rickettsialpoxRickettsialpox is an illness caused by bacteria of the Rickettsia genus . Physician Robert Huebner and self-trained entomologist Charles Pomerantz played major roles in identifying the etiology of the disease after an outbreak in 1946 in a New York City apartment complex, documented in medical...
,
Boutonneuse feverBoutonneuse fever is a fever as a result of a Rickettsia infection caused by the bacterium Rickettsia conorii and transmitted by the dog tick Rhipicephalus sanguineus...
,
African tick bite feverAfrican tick bite fever is a condition that is often associated with a rash characterized by scant lesions, often macular and/or vesicular.It is caused by Rickettsia africae.- See also :* Flea-borne spotted fever* Rocky Mountain spotted fever...
,
Rocky Mountain spotted feverRocky Mountain spotted fever is the most lethal and most frequently reported rickettsial illness in the United States. It has been diagnosed throughout the Americas. Some synonyms for Rocky Mountain spotted fever in other countries include “tick typhus,” “Tobia fever” , “São Paulo fever” or “febre...
,
Flinders Island spotted feverFlinders Island spotted fever is a condition characterized by a rash in approximately 85% of cases.It is associated with Rickettsia honei.- See also :* Japanese spotted fever* North Asian tick typhus* List of cutaneous conditions* Flinders Island...
and
Queensland tick typhusQueensland tick typhus is a condition caused by a bacterium Rickettsia australis.It is transmitted by Ixodes holocyclus and Ixodes tasmani.- See also :* Typhus...
(Australian Tick Typhus). They have also been associated with a range of plant diseases. The name rickettsia is often used for any member of the
RickettsialesThe Rickettsiales, also called rickettsias, are an order of small proteobacteria. Most of those described survive only as endosymbionts of other cells. Some are notable pathogens, including Rickettsia, which causes a variety of diseases in humans...
. They are one of closest living relatives to bacteria that were the origin of the mitochondria organelle that exists inside most eukaryotic cells.
The method of growing Rickettsia in chicken embryos was invented by
Ernest William GoodpastureDr. Ernest William Goodpasture was an American pathologist and physician. Goodpasture advanced the scientific understanding of the pathogenesis of infectious diseases, parasitism, and a variety of rickettsial and viral infections...
and his colleagues at
Vanderbilt UniversityVanderbilt University is a private research university located in Nashville, Tennessee, United States. Founded in 1873, the university is named for shipping and rail magnate "Commodore" Cornelius Vanderbilt, who provided Vanderbilt its initial $1 million endowment despite having never been to the...
in the early 1930s.
Classification
The classification of
Rickettsia into three groups (spotted fever, typhus and scrub typhus) was based on serology. This grouping has since been confirmed by DNA sequencing. All three of these contain human pathogens. The scrub typhus group has been reclassified as a new genus –
OrientiaOrientia is a genus of bacteria in family Rickettsiaceae.They are obligate intracellular gram negative bacteria found in insects and mammals. They are spread through the bites or feces of infected insects.One species is Orientia tsutsugamushi....
– but many medical textbooks still list this group under the rickettsial diseases.
However more recently it has become apparent that rickettsia are more widespread than previously believed and are known to be associated with
arthropodAn arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s,
leechLeeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like other oligochaetes such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from other oligochaetes in significant ways...
es and
protistProtists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s. Divisions have also been identified in the spotted fever group and it has been suggested that this should be divided into two clades. Arthropod-inhabiting rickettsiae are generally associated with reproductive manipulation (such as
parthenogenesisParthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
) to persist in host lineage
In March 2010 Swedish researchers reported a case of
bacterial meningitisBacterial meningitis refers to meningitis that is caused by bacterial infection.-Signs and Symptoms:*Fever*Seizures*Meningismus*Headache*Vomiting*Photophobia*Altered mental status and coma*Anorexia...
in woman caused by
Rickettsia helveticaRickettsia helvetica is a bacterium found in Dermacentor reticulatus and other ticks that was previously thought harmless. In 1997, a man living in eastern France had seroconverted to Rickettsia 4 weeks after the onset of a then unexplained febrile illness...
previously thought to be harmless.
Spotted fever group
- Rickettsia rickettsii
Rickettsia rickettsii is a unicellular, gram-negative coccobacillus that is native to the New World. It belongs to the spotted fever group of Rickettsia and is most commonly known as the causative agent of Rocky Mountain spotted fever . By nature, R...
(Western hemisphere)
-
- Rocky Mountain spotted fever
Rocky Mountain spotted fever is the most lethal and most frequently reported rickettsial illness in the United States. It has been diagnosed throughout the Americas. Some synonyms for Rocky Mountain spotted fever in other countries include “tick typhus,” “Tobia fever” , “São Paulo fever” or “febre...
- Rickettsia akari
Rickettsia akari is a species of Rickettsia which causes rickettsialpox.After a 1946 outbreak of a rickettsial-type disease at an apartment complex in Kew Gardens, Queens, an investigation was performed to identify the source of the infections...
(USA, former Soviet Union)
-
- Rickettsialpox
Rickettsialpox is an illness caused by bacteria of the Rickettsia genus . Physician Robert Huebner and self-trained entomologist Charles Pomerantz played major roles in identifying the etiology of the disease after an outbreak in 1946 in a New York City apartment complex, documented in medical...
- Rickettsia conorii
Rickettsia conorii is a unicellular, gram-negative, obligate intracellular bacterium of the genus Rickettsia that causes human disease called Boutonneuse fever, Mediterranean spotted fever, Israeli tick typhus, Astrakhan spotted fever, Kenya tick typhus, Indian tick typhus, or other names that...
(Mediterranean countries, Africa, Southwest Asia, India)
-
- Boutonneuse fever
Boutonneuse fever is a fever as a result of a Rickettsia infection caused by the bacterium Rickettsia conorii and transmitted by the dog tick Rhipicephalus sanguineus...
- Rickettsia sibirica
Rickettsia sibirica is a species of Rickettsia. This bacterium is the etiologic agent of North Asian tick typhus, which is also known as Siberian tick typhus. The ticks that transmit it are primarily various species of Dermacentor and Haemaphysalis....
(Siberia, Mongolia, northern China)
-
- Siberian tick typhus or North Asian tick typhus
North Asian tick typhus , also known as Siberian tick typhus, is a condition characterized by a maculopapular rash.It is associated with Rickettsia sibirica.- See also :* Flinders Island spotted fever...
- Rickettsia australis
Rickettsia australis is a bacterium that causes a medical condition called Queensland tick typhus. The probable vectors are the tick species, Ixodes holocyclus and Ixodes tasmani. Small marsupials are suspected reservoirs of this bacterium....
(Australia)
-
- Australian tick typhus
- Rickettsia felis
Rickettsia felis or cat-flea typhus is a species of Rickettsia that causes flea-borne spotted fever in cats. The bacteria can also infect humans and cause illness. Fleas are the vector carriers of the bacteria and it has been found in cat flea populations of North and South America, Southern...
(North and South America, Southern Europe, Australia)
-
- Flea-borne spotted fever
Flea-borne spotted fever is a condition characterized by a rash of maculopapules or furuncles.It is caused by Rickettsia felis.- See also :* American tick bite fever* Japanese spotted fever* List of cutaneous conditions...
- Rickettsia japonica
Rickettsia japonica is a species of Rickettsia.It can cause Japanese spotted fever....
(Japan)
-
- Oriental spotted fever
- Rickettsia africae
Rickettsia africae is a species of Rickettsia.It can cause African tick-bite fever....
(South Africa)
-
- African tick bite fever
African tick bite fever is a condition that is often associated with a rash characterized by scant lesions, often macular and/or vesicular.It is caused by Rickettsia africae.- See also :* Flea-borne spotted fever* Rocky Mountain spotted fever...
- Rickettsia hoogstraalii (Croatia, Spain and Georgia USA)
-
- Unknown pathogenicity
Typhus group
- Rickettsia prowazekii
Rickettsia prowazekii is a species of gram negative, Alpha Proteobacteria, obligate intracellular parasitic, aerobic bacteria that is the etiologic agent of epidemic typhus, transmitted in the feces of lice. In North America, the main reservoir for R. prowazekii is the flying squirrel. R...
(Worldwide)
-
- Epidemic typhus, recrudescent typhus and sporadic typhus
- Rickettsia typhi
Rickettsia typhi is a species of the genus Rickettsia.It is the causative agent of Murine typhus.The genome is similar to that of Rickettsia prowazekii.-References:...
(Worldwide)
-
- Murine typhus
Murine typhus is a form of typhus transmitted by fleas , usually on rats. Murine typhus is an under-recognized entity, as it is often confused with viral illnesses...
(endemic typhus)
Scrub typhus group
- The causative agent of scrub typhus
Scrub typhus or Bush typhus is a form of typhus caused by Orientia tsutsugamushi first isolated and identified in 1930 in Japan., accessdate: 16 October 2011...
formerly known as R. tsutsugamushi has been reclassified into the genus OrientiaOrientia is a genus of bacteria in family Rickettsiaceae.They are obligate intracellular gram negative bacteria found in insects and mammals. They are spread through the bites or feces of infected insects.One species is Orientia tsutsugamushi....
.
Flora and fauna pathogenesis
The following plant diseases have been associated with Rickettsia-like organisms.
- Beet latent Rosette RLO
- Citrus Greening bacterium
- Clover leaf RLO
- Grapevine infectious necrosis RLO
- Grapevine Pierce's RLO
- Grapevine yellos RLO
- Larch witch's broom disease
- Peach phony RLO
Infection occurs in non-human mammals; for example, species of
Rickettsia have been found to afflict the South American Guanaco,
Lama guanacoe.
Genomics
Certain segments of Rickettsial
genomeIn modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
s resemble that of mitochondria. The deciphered genome of
R. prowazekii is 1,111,523
bpIn molecular biology and genetics, the linking between two nitrogenous bases on opposite complementary DNA or certain types of RNA strands that are connected via hydrogen bonds is called a base pair...
long and contains 834 protein-coding
genesGênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
. Unlike free-living bacteria, it contains no genes for
anaerobicAnaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen...
glycolysisGlycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...
or genes involved in the biosynthesis and regulation of
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s and
nucleosideNucleosides are glycosylamines consisting of a nucleobase bound to a ribose or deoxyribose sugar via a beta-glycosidic linkage...
s. In this regard it is similar to mitochondrial genomes; in both cases, nuclear (host) resources are used.
ATPAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
production in
Rickettsia is the same as that in mitochondria. In fact, of all the microbes known, the
Rickettsia is probably the closest relative (in a phylogenetic sense) to the mitochondria. Unlike the latter, the genome of
R. prowazekii, however, contains a complete set of genes encoding for the tricarboxylic acid cycle and the respiratory chain complex. Still, the genomes of the Rickettsia as well as the mitochondria are frequently said to be "small, highly derived products of several types of reductive evolution".
The recent discovery of another parallel between
Rickettsia and viruses may become a basis for fighting
HIVHuman immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
infection. Human immune response to the
scrub typhusScrub typhus or Bush typhus is a form of typhus caused by Orientia tsutsugamushi first isolated and identified in 1930 in Japan., accessdate: 16 October 2011...
pathogen,
Orientia tsutsugamushiOrientia tsutsugamushi is the causative organism of scrub typhus, and the natural vector and reservoir is probably trombiculid mites ....
rickettsia, appears to provide a beneficial effect against HIV infection progress, negatively influencing the virus replication process. A probable reason for this actively studied phenomenon is a certain degree of
homologyHomology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
between the rickettsia and the virus – namely, common
epitopeAn epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The part of an antibody that recognizes the epitope is called a paratope...
(s) due to common genome fragment(s) in both pathogens. Surprisingly, the other infection reported to be likely to provide the same effect (decrease in viral load) is the virus-caused illness
dengue feverDengue fever , also known as breakbone fever, is an infectious tropical disease caused by the dengue virus. Symptoms include fever, headache, muscle and joint pains, and a characteristic skin rash that is similar to measles...
.
Naming
The genus
Rickettsia is named after Howard Taylor Ricketts (1871–1910), who studied Rocky Mountain spotted fever in the
Bitterroot ValleyThe Bitterroot Valley is located in southwestern Montana in the northwestern United States. It extends over 100 miles from remote Horse Creek Pass north to a point near the city of Missoula...
of Montana, and eventually died of typhus after studying that disease in Mexico City. Despite the similar name,
Rickettsia bacteria do not cause
ricketsRickets is a softening of bones in children due to deficiency or impaired metabolism of vitamin D, magnesium , phosphorus or calcium, potentially leading to fractures and deformity. Rickets is among the most frequent childhood diseases in many developing countries...
, which is a result of vitamin D
deficiencyAvitaminosis is any disease caused by chronic or long-term vitamin deficiency or caused by a defect in metabolic conversion, such as tryptophan to niacin...
.
External links