User:Alyssa.Vickers/Rickettsia typhi

Murine typhus is a disease that is known to be caused by the bacterial species Rickettsia typhi.

Cell characteristics
Rickettsia typhi is a small, aerobic, obligate intracellular, gram negative bacteria. There are four species groups for the genus Rickettsia, in which Rickettsia typhi belongs to the typhus group, it resides in this group with Rickettsia prowazekii. R. typhi is a zoonotic bacteria that perpetuate using an arthropod vector such as a flea for transmission, aerosols of infected material is another method of transmitting the disease. This species is recognized as a biocontainment level 3 organism.

Once infected, R. typhi invades host endothelial cells and induces phagocytosis. Once phagocytosed it multiplies rapidly within the cells cytoplasm, when ready to exit and further infiltrate other cells, R. typhi induces lysis, bursting and killing the host cell. R. typhi’s method of movement in its host is through actin based motility, with little control over directionality and speed, R. typhi is often seen moving in a circular pattern, this characteristic is another means of differentiation from the Rickettsia spotted fever group which has a much more uniform pattern of movement.

Rickettsia species have a relatively small genome, R. typhi consists of a single circular chromosome that has 1,111,496 base pairs and 838 genes, 24 of which are unique to its species. R. typhi and its typhus group member R. prowazekii are composed of very similar genomes, R. typhi can be differentiated from R. prowazekii by 12000 base pairs inserted into R. prowazekii.

Identification
R. typhi is widely identified using serological methods, the main method being immunofluorescent analysis, but a variety of other methods such as Weil-Felix test, dot-blot assay, and enzyme-linked immunosorbent assay, among others, have been used. PCR based methods are also used and membrane protein ompB and prsA  gene fragments have been shown to be identifiable markers in qPCR for R. typhi.

R. typhi are obligate intracellular pathogens which means they can be difficult to isolate. They are not able to grow in axenic or sterile conditions, and must be grown in tissue, or embryo samples. Even when stringent physiologic conditions are met, when grown in media that mimics the environment of host cytoplasm, pathogen activity cannot survive very long. A common method for growing R. typhi is through the yolk sacs of embryonic eggs. Shell vial assay is another promising method for Rickettsia culture, in which the pathogen is inoculated onto a thin cell layer, centrifuged and incubated.

History
Murine typhus may have gone unidentified as the true source of some milder epidemic typhus cases in the early 20th century. During this time, epidemic typhus ran rampant throughout many parts of the world. It was associated with high mortality, high virulence, and thought to be transmitted via louse. During this period, less severe and untraceable cases were also appearing. Epidemiologist Kenneth F Maxy was among the first to begin questioning and isolating the presence of another typhus within the United States aside from the R. prowazekii, which was the source of the epidemic, he detailed this in an article released in 1926. Maxy speculated the presence of an arthropod vector transmitting this new form of typhus, which would later be discovered as R. typhi.