Rickettsiae are small coccobacilli that often have a transverse septum between two bacilli, reflecting division by binary fission. They commonly measure no more than 0.3 to 0.5μm. Although the Gram reaction is negative, they take the usual bacterial stains poorly and are better demonstrated by the Giemsa stain, particularly in infected cells. 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.
Rickettsia grow freely in the cytoplasm of eukaryotic cells to which they are highlyadapted, in contrast to Ehrlichia and Coxiella, which replicate in cytoplasmic vacuoles. Rickettsiae can be grown only in living host cells such as cell cultures and embryonated eggs. Infection of the host cell begins by induction of an endocytic process, which is analogous to phagocytosis, but requires expenditure of energy by the rickettsiae. Penetration of infected cells appears to be facilitated by production of a rickettsial phospholipase. The organisms then escape the phagosome or endocytic vacuole to enter the cytoplasm, possi-bly aided by elaboration of the phospholipase. Recent studies indicate that intracellular and intercellular spread involves directional actin polymerization and use of the host cell cytoskeleton in a manner similar to Listeria andShigella . Intracytoplasmic growth eventually produces lysis of the cell. The estimated genera-tion time of rickettsiae is much longer than that of bacteria such asEscherichia coli but more rapid than that of Mycobacterium tuberculosis.
The obligate intracellular parasitism of rickettsiae has several interesting features. Failure to survive outside the cell is apparently related to requirements for nucleotide cofactors (coenzyme A, nicotinamide adenine dinucleotide) and adenosine triphosphate (ATP). Outside the host cell, rickettsiae not only cease metabolic activity, but leak protein, nucleic acids, and essential small molecules. This instability leads to rapid loss of infectiv-ity, because the penetration of another cell requires energy. In summary, rickettsiae have the metabolic capabilities of other bacteria, but must borrow some essential elements from host cells for adequate growth and, thus, do not survive well in the environment.
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