Orientia tsutsugamushi
Encyclopedia
Orientia tsutsugamushi is the causative organism of scrub typhus
, and the natural vector and reservoir is probably trombiculid mites (genus Leptotrombidium
).
The organism is an obligate intracellular pathogen, which needs to infect eukaryotic cells in order to multiply. The envelope is similar to that of Gram negative bacteria
, but it is not easily stained with Gram stain and the Gimenez stain
is preferred. There are a large number of serotypes described: these include Karp (which accounts for approximately 50% of all infection), Gilliam (25%), Kato (less than 10%), and Kawasaki, but there exists enormous variability, with eight serotypes being reported in a single field in Malaysia, and many more serotypes continue to be reported. Genetic methods have revealed even greater complexity than had been previously described (for example, Gilliam is further divided into Gilliam and JG types). Infection with one serotype does not confer immunity to other serotypes (no cross immunity). Repeated infection in the same individual is therefore possible, and this complicates vaccine design.
, but is now classed in a separate genus, Orientia, in which it is the only species.
It is 0.5µm wide and 1.2 to 3.0µm long, and is an obligatory intracellular organism that can only be cultured in cell monolayers. The organism is highly virulent and should only be handled in a laboratory with biosafety level
3 facilities.
, rifampicin
and azithromycin
. It is innately resistant to all β-lactam
antibiotics (for example, penicillin
) because it lacks a peptidoglycan
cell wall. Aminoglycoside
s (for example, gentamicin
) are also ineffective in human infection because the organism is intracellular, and aminoglycosides do not penetrate intracellularly.
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...
, and the natural vector and reservoir is probably trombiculid mites (genus Leptotrombidium
Leptotrombidium
Leptotrombidium is a genus of mites in the family Trombiculidae, that are able to infect humans with scrub typhus through their bite. The larval form feeds on rodents, but also occasionally humans and other large mammals...
).
The organism is an obligate intracellular pathogen, which needs to infect eukaryotic cells in order to multiply. The envelope is similar to that of Gram negative bacteria
Bacteria
Bacteria 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...
, but it is not easily stained with Gram stain and the Gimenez stain
Gimenez stain
The Gimenez staining technique uses biological stains to detect and identify bacterial infections in tissue samples. Although largely superseded by techniques like Giemsa staining, the Gimenez technique may be valuable for detecting certain slow-growing or fastidious bacteria.Basic fuchsin stain ...
is preferred. There are a large number of serotypes described: these include Karp (which accounts for approximately 50% of all infection), Gilliam (25%), Kato (less than 10%), and Kawasaki, but there exists enormous variability, with eight serotypes being reported in a single field in Malaysia, and many more serotypes continue to be reported. Genetic methods have revealed even greater complexity than had been previously described (for example, Gilliam is further divided into Gilliam and JG types). Infection with one serotype does not confer immunity to other serotypes (no cross immunity). Repeated infection in the same individual is therefore possible, and this complicates vaccine design.
Microbiology
The bacterium was initially categorised in the genus RickettsiaRickettsia
Rickettsia is a genus of non-motile, Gram-negative, non-sporeforming, highly pleomorphic bacteria that can present as cocci , rods or thread-like . Being obligate intracellular parasites, the Rickettsia survival depends on entry, growth, and replication within the cytoplasm of eukaryotic host cells...
, but is now classed in a separate genus, Orientia, in which it is the only species.
It is 0.5µm wide and 1.2 to 3.0µm long, and is an obligatory intracellular organism that can only be cultured in cell monolayers. The organism is highly virulent and should only be handled in a laboratory with biosafety level
Biosafety level
A biosafety level is the level of the biocontainment precautions required to isolate dangerous biological agents in an enclosed facility. The levels of containment range from the lowest biosafety level 1 to the highest at level 4 . In the United States, the Centers for Disease Control and...
3 facilities.
Antimicrobial therapy
O. tsutsugamushi is sensitive in vitro to doxycyclineDoxycycline
Doxycycline INN is a member of the tetracycline antibiotics group, and is commonly used to treat a variety of infections. Doxycycline is a semisynthetic tetracycline invented and clinically developed in the early 1960s by Pfizer Inc. and marketed under the brand name Vibramycin. Vibramycin...
, rifampicin
Rifampicin
Rifampicin or rifampin is a bactericidal antibiotic drug of the rifamycin group. It is a semisynthetic compound derived from Amycolatopsis rifamycinica ...
and azithromycin
Azithromycin
Azithromycin is an azalide, a subclass of macrolide antibiotics. Azithromycin is one of the world's best-selling antibiotics...
. It is innately resistant to all β-lactam
Beta-lactam
A β-lactam ring, is a four-membered lactam. It is named as such, because the nitrogen atom is attached to the β-carbon relative to the carbonyl...
antibiotics (for example, penicillin
Penicillin
Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....
) because it lacks a peptidoglycan
Peptidoglycan
Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
cell wall. Aminoglycoside
Aminoglycoside
An aminoglycoside is a molecule or a portion of a molecule composed of amino-modifiedsugars.Several aminoglycosides function as antibiotics that are effective against certain types of bacteria...
s (for example, gentamicin
Gentamicin
Gentamicin is an aminoglycoside antibiotic, used to treat many types of bacterial infections, particularly those caused by Gram-negative organisms. However, gentamicin is not used for Neisseria gonorrhoeae, Neisseria meningitidis or Legionella pneumophila...
) are also ineffective in human infection because the organism is intracellular, and aminoglycosides do not penetrate intracellularly.