Genetic Manipulation of Rickettsiae: a Preview

Abstract

The genus Rickettsia includes the causative agents of some of the most historically significant and severe bacterial diseases of humans. These include Rickettsia prowazekii, the agent responsible for epidemic typhus, and R. rickettsii, the agent of Rocky Mountain spotted fever. Additional species, including R. typhi, responsible for murine or endemic typhus, and others, responsible for a variety of spotted fevers occurring worldwide, have also been identified (8, 12). Increasingly, as new rickettsioses are identified and the disease potential of well-known rickettsial diseases are assessed, the rickettsiae are being recognized as emerging or reemerging pathogens (18). For example, the most notorious rickettsial disease, epidemic typhus, considered by many to be a disease of historical importance only, remains capable of producing severe morbidity and mortality in those populations where the classical precipitating factors of war, poverty, and poor vector control exist. Recent outbreaks testify to the continued threat that R. prowazekii poses (17, 20). Even the successes resulting from public hygiene in the United States have failed to eliminate R. prowazekii. In addition to individuals that carry rickettsiae for many years after the initial infection and then experience a recrudescence of the disease (Brill-Zinsser disease), a zoonotic reservoir, the flying squirrel, that maintains R. prowazekii has been identified in the eastern United States (5). In addition, all of the rickettsial diseases pose a diagnostic challenge for the physician, with failure to properly diagnose and treat these diseases resulting in high mortality. As described in a number of excellent reviews and books dealing with the biology of rickettsiae (8, 13, 22, 23, 25, 26, 28), rickettsiae are gram-negative, obligate, intracellular parasitic bacteria that are able to grow only within the cytoplasm or, occasionally, the nucleus of a variety of eukaryotic host cells. Thus, they differ from the obligate intracellular bacteria of the genera Chlamydia, Coxiella, and Ehrlichia that grow within phagosomes or phagolysosomes. Another distinguishing characteristic of these novel bacteria is their association with arthropod vectors (4). Lice, fleas, ticks, and mites serve as vectors for one or more rickettsial species. Rickettsiae enter host cells by induced phagocytosis, a process that requires active participation by both the rickettsia and the host cell and the appearance of a phospholipase A activity (24). The rickettsiae rapidly escape from the phagosome and enter the host cell cytoplasm, where they are able to exploit high-energy compounds present there, such as ATP, using specialized transport systems (27, 29). However, they are not strict energy parasites and retain the ability to generate ATP via an intact tricarboxylic acid cycle and oxidative phosphorylation. Some members of the genus, most notably R. rickettsii, are capable of polymerizing actin for movement within and between cells while others, such as R. prowazekii, do not exhibit this property (7, 9, 10). The end result of rickettsial growth, and the basis of their pathogenicity, is lysis of the host cell. Delineating the mechanisms involved in this unique, obligate, intracytoplasmic parasitism is the goal of current studies.