This chapter takes up four groups of Gram-negative bacilli whose obligate or preferred growth is inside eukaryotic cells where they rely on the host cell for some essential nutrients. They are animal pathogens transmitted by arthropods to humans who are in the wrong place at the wrong time. The diseases vary depending on whether the target is endothelial cells, phagocytes, or erythrocytes. Most are prolonged fevers, often with vasculitis. These include classic ones like typhus, Rocky Mountain spotted fever, and cat-scratch disease, as well newly recognized infections like human ehrlichiosis and anaplasmosis.
Obligate intracellular parasites
Rickettsiae are small coccobacilli (Figure 40-1) which measure no more than 0.3 to 0.5 μm. Although the Gram reaction is negative, rickettsiae take the usual bacterial stains poorly and are better demonstrated by specific immunofluorescence. The ultrastructural morphology, which is similar to that of other Gram-negative bacteria, includes a Gram-negative type of cell envelope, ribosomes, and a nuclear body. Chemically, the cell wall contains lipopolysaccharide and at least two large proteins in the outer membrane, as well as peptidoglycan. The outer membrane proteins extend to the cell surface, where they are the most abundant protein present. They are discussed here as members of either the spotted fever group (SFG) or typhus group (TG). Due to differences in protein composition and its lack of lipopolysaccharide Orientia tsutsugamushi (formerly R tsutsugamushi) has been placed in a separate genus.
Rickettsia. Rickettsial morphology and reproduction. A. A human fibroblast filled with Rickettsia prowazekii. B. Rickettsiae attachment to endothelial cell and subsequent phagocytosis. Rickettsiae leave a disrupted phagosome (arrow) and enter the cytoplasmic matrix.
Small, Gram-negative coccobacilli stained best by immunofluorescence
Abundant outer membrane proteins at surface
Rickettsia grow freely in the cytoplasm of eukaryotic cells to which they are highly adapted, in contrast to Ehrlichia and Bartonella, that replicate in cytoplasmic vacuoles. Rickettsiae can be grown only in the living eukaryotic cells found in cell cultures, organ cultures, and embryonated eggs. Rickettsia are able to adhere to a wide variety of cell types through the binding of outer membrane proteins. They enter cells by induced phagocytosis and escape to the cytoplasms by elaboration of a phospholipase. In the cytoplasm the SFG rickettsiae move about utilizing an actin-based motility similar to that already described for Listeria and Shigella (Chapters 26, 33). Intracytoplasmic growth eventually produces lysis of the cell.
Grow in cytoplasm following induced endocytosis
Growth slow compared with most bacteria
The obligate intracellular parasitism of Rickettsiae has several interesting features. Failure to survive outside the ...