Electron microscopic observations of Coxiella burnetii in the guinea pig

Animal models of Q fever (Coxiella burnetii)

Q fever, caused by the pathogen Coxiella burnetii, is an acute disease that can progress to become a serious chronic illness. The organism leads an obligate, intracellular lifecycle, during which it multiplies in the phagolytic compartments of the phagocytic cells of the immune system of its hosts. This characteristic makes study of the organism particularly difficult and is perhaps one of the reasons why, more than 70 y after its discovery, much remains unknown about the organism and its pathogenesis. A variety of animal species have been used to study both the acute and chronic forms of the disease. Although none of the models perfectly mimics the disease process in humans, each opens a window onto an important aspect of the pathology of the disease. We have learned that immunosuppression, overexpression of IL10, or physical damage to the heart muscle in mice and guinea pigs can induce disease that is similar to the chronic disease seen in humans, suggesting that this aspect of disease may eventually be fully understood. Models using species from mice to nonhuman primates have been used to evaluate and characterize vaccines to protect against the disease and may ultimately yield safer, less expensive vaccines.

The Coxiella burnetii parasitophorous vacuole

Coxiella burnetii is a bacterial intracellular parasite of eucaryotic cells that replicates within a membrane-bound compartment, or "parasitophorous vacuole" (PV). With the exception of human macrophages/monocytes, the consensus model of PV trafficking in host cells invokes endolysosomal maturation culminating in lysosome fusion. C. burnetii resists the degradative functions of the vacuole while at the same time exploiting the acidic pH for metabolic activation. While at first glance the mature PV resembles a large phagolysosome, an increasing body of evidence indicates the vacuole is in fact a specialized compartment that is actively modified by the pathogen. Adding to the complexity of PV biogenesis is new data showing vacuole engagement with autophagic and early secretory pathways. In this chapter, we review current knowledge of PV nature and development, and discuss disparate data related to the ultimate maturation state of PV harboring virulent or avirulent C. burnetii lipopolysaccharide phase variants in human mononuclear phagocytes.

Some contributions of electron microscopy to the study of the rickettsiae

Electron microscopy has provided valuable insights into the study of rickettsiae as intracellular parasites from several important perspectives. This tool has allowed researchers to delineate the fine structural features of these organisms and to show that they truly resemble free-living bacteria. Furthermore, it has been shown that there are subtle, but distinct differences in the outer envelope structure of some members of the genus Rickettsia that may explain reported differences in tinctorial properties and in their sensitivity to certain antibiotics. With Coxiella burnetii, electron microscopy has helped significantly in the characterization of the pleomorphic nature of the organism including formation of terminal bodies that resemble endospores of gram-positive bacteria. Electron microsxopy has also helped to define the relationship of the rickettsiae to their host cells. For example, ultrastructural analysis can reveal whether organisms exist free within the cytoplasm or nucleus (members of the genus Rickettsia), or whether they are bound by a phagosomal or phagolysosomal membrane (Ehrlichia and Coxiella). Finally, although all rickettsiae eventually destroy their host cell, it has been shown through transmission electron microscopy that this destruction might be mediated by different mechanisms that are specific for different rickettsial species.

Interferon-gamma inhibits growth of Coxiella burnetii in mouse fibroblasts

We studied the effects of various mouse interferon preparations on the growth of Coxiella burnetii in mouse fibroblasts. The addition of both recombinant interferon-gamma and a crude lymphokine preparation that contained interferon-gamma to infected L929 cells inhibited the growth of C. burnetii, whereas the addition of a crude preparation of type I interferons did not. Cycloheximide suppressed the inhibitory effects of recombinant interferon-gamma and crude lymphokines.

Q fever and Coxiella burnetii: a model for host-parasite interactions

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Developmental cycle of Coxiella burnetii: structure and morphogenesis of vegetative and sporogenic differentiations

Coxiella burnetii is a gram-variable obligate intracellular bacterium which carries out its development cycle in the phagolysosome of eucaryotic cells. Ultrastructural analysis of C. burnetii, in situ and after Renografin purification, by transmission electron microscopy of lead-stained thin sections has revealed extreme pleomorphism as demonstrated by two morphological cell types, a large cell variant (LCV) and a small cell variant (SCV). Potassium permanganate staining of purified rickettsiae revealed a number of differences in the internal structures of the cell variants. (i) The outer membrane of the sCV and LCV were comparable; however, the underlying dense layer of the SCV was much wider and more prominent than that of the LCV. The periplasmic space of the SCV was not readily visualized, whereas the periplasmic space of the LCV was apparent and resembled that of other gram-negative bacteria. (ii) Complex internal membranous intrusions which appeared to originate from the cytoplasmic membrane were observed in the SCV. The LCV did not harbor an extensive membranous system. (iii) Some LCVs contained a dense body in the periplasmic space. This endogenous structure appeared to arise in one pole of the LCV as an electrondense "cap" formation with the progressive development of a dense body approximately 130 to 170 nm in diameter which was eventually surrounded by a coat of at least four layers. Our observations suggest that the morphogenesis of C. burnetii is comparable, although not identical, to cellular differentiation of endospore formation. A developmental cycle consisting of vegetative and sporogenic differentiation is proposed.

Host-parasite interaction and development of infraforms in chicken embryos infected with Coxiella burnetii via the yolk sac

Two phase I strains of Coxiella burnetii of different virulence were injected into the yolk sacs of chicken embryos, and the yolk sacs and livers were examined at intervals by light, fluorescent, and electron microscopy. The high absorptive and digestive capacities of the yolk endoderm contributed to he entrance of the organisms into endodermal epithelial cells where C. burnetii multiplied. Organisms multiplied not only inside specific vacuoles originating from phagolysosomes but also in the cytoplasm itself. Lysis of the limiting membrane of some phagolysosomes, a normal function of endodermal cells, as well as rupture of vacuoles, provided the release of C. burnetii into the cytoplasm. The C. burnetii strain of greater virulence infected 100% of the endodermal cells, whereas the strain of lesser virulence infected only 60%. Budding of very small particles from the C. burnetii bodies was demonstrated. The particles were regarded as filterable forms of the organism. Despite the enormous multiplication of C. burnetii in the endodermal cells, organisms were only rarely detected in the vitelline blood vessels and liver sinusoids of the embryos. Peculiarities of the infectious process of C. burnetii in chicken embryos and possible mechanisms of limitation of spread of the infection are discussed.

Synthesis of ribonucleotides and their participation in ribonucleic acid synthesis by Coxiella burnetii

Synthesis of ribonucleic acid (RNA) by the deoxyribonucleic acid-dependent RNA polymerase of Coxiella burnetii required adenosine, uridine, guanosine, and cytidine 5'-triphosphates. Cell-free preparations of this obligate intracellular procaryotic parasite had competence to phosphorylate ribonucleoside mono- and diphosphates in the presence of exogenous adenosine and guanosine 5'-triphosphates to the corresponding di- and triphosphates. C. burnetii contained about 2 nmol of adenosine 5'-triphosphate per mg of protein, which could serve as a approximately P donor for in vivo synthesis of nucleoside triphosphates. The latter were then used as substrates in the synthesis of RNA in a coordinated metabolic system with C. burnetii RNA polymerase. It is suggested that during infection the rickettsiae might obtain the nucleotides necessary for RNA synthesis from the vacuoles in which C. burnetii proliferates.

Electron microscopic study on the interaction between normal guinea pig peritoneal macrophages and Coxiella burnetii

An electron microscopic study was conducted to explore the interaction between normal guinea pig peritoneal macrophages and phase I and II Coxeilla burnetii previously treated with either normal or immune serum. A comparison was made on the efficiency of phagocytosis and subsequent killing of rickettsiae by macrophages. Both phases of rickettsiae previously treated with normal serum multiplied within phagosomes after phagocytosis with resultant destruction of macrophages. In contrast, suspending rickettsiae in immune serum rendered them more susceptible to phagocytosis and potentiated their destruction within macrophages.

Ribonucleic acid and protein synthesis in guinea pig liver during Q fever

Protein and ribonucleic acid (RNA) syntheses increase in the liver during Q fever, coincident with progressive increments in cortisol levels. Cell-free protein-synthesizing systems indicate that the pH 5 enzyme fraction was chiefly responsible for enhanced activity. Although the polysome profiles from normal and infected livers were similar, the number of liver ribosomes doubled during infection. There was a concomitant increase in orotate incorporation in the 28s, 18s, and 4s RNA species of infected liver.