6 rickettsiia chlamydia,,,.doc

. .................................................................. Lecture 4 …………………………………………….Rickettsia &Chlamydiae Rickettsia & Ehrlichia The human pathogens in the family Rickettsiaceae are small bacteria of the genera Rickettsia, Orientia, Coxiella, and Ehrlichia. They are obligate intracellular parasites and, except for Q fever, are transmitted to humans by arthropods. Many rickettsiae are transmitted transovarially in the arthropod, which serves as both vector and reservoir. Rickettsial infections—except Q fever and the ehrlichioses—typically are manifested by fever, rashes, and vasculitis. They are grouped on the basis of their clinical features, epidemiologic aspects, and immunologic characteristics. Properties of Rickettsiae Rickettsiae are pleomorphic coccobacilli, appearing either as short rods (0.3 × 1–2 μm) or as cocci (0.3 μm in diameter). They do not stain well with Gram's stain but are readily visible under the light microscope when stained with Giemsa, Gimenez, acridine orange, or other stains. Rickettsiae grow readily in yolk sacs of embryonated eggs. Pure preparations of rickettsiae for use in laboratory testing can be obtained by differential centrifugation of yolk sac suspensions. Many strains of rickettsiae also grow in cell culture. In cell culture, the generation time is 8–10 hours at 34 °C. For reasons of biosafety, isolation of rickettsiae should be done only in reference laboratories. Rickettsiae have gram-negative cell wall structures that include peptidoglycan-containing muramic acid and diaminopimelic acid. The typhus and spotted fever groups contain lipopolysaccharide. The cell wall proteins include surface protein (OmpA and OmpB) which are important in the humoral immune response and provide the basis for serotyping. Rickettsiae grow in different parts of the cell. Those of the typhus group are usually found in the cytoplasm; those of the spotted fever group, in the nucleus. Coxiellae grow only in cytoplasmic vacuoles. Rickettsial growth is enhanced in the presence of sulfonamides, and rickettsial diseases are made more severe by these drugs. Tetracyclines and chloramphenicol inhibit the growth o f rickettsiae and can be therapeutically effective. Most rickettsiae survive only for short times outside of the vector or host. Rickettsiae are quickly destroyed by heat, drying, and bactericidal chemicals. Dried feces of infected lice may contain infectious Rickettsia prowazekii for months at room temperature. The organism of Q fever is the rickettsial agent most resistant to drying. This organism may survive pasteurization at 60 °C for 30 minutes and can survive for months in dried feces or milk. This may be due to the formation of endospore-like structures by Coxiella burnetii. Rickettsial Antigens & Serology The direct immunofluorescent antibody test can be used to detect rickettsiae in ticks and sections of tissues. The test has been most useful to detect R rickettsii in skin biopsy specimens to aid in the diagnosis of Rocky Mountain spotted fever; however, the test is performed in only a few reference laboratories. 1

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