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Coxiella burnetti, the etiologic agent of Q fever, is an oligate intracellular parasite of eukaryotes. Unlike the majority of successful bacterial parasites, which escape the bactericidal environment of the phagolysosome by various means, C. burnetii multiplies only in the phagolysosome. In view of the relatively harsh environment inhabited by C. burnetii, we have examined (i) the in vitro metabolism of glucose and glutamate by whole cells of C. burnetii under conditions designed to approximate the pH within the phagolysosome and (ii) the effect of manipulation of the phagolysosomal pH by lysosomotropic amines on the replication of C. burnetii in chicken embryo fibroblasts. The transport, catabolism, and incorporation of both glucose and glutamate were found to be highly stimulated by acidic conditions, whereas at pH 7.0 metabolism of these substrates was minimal. The transport processes were shown to be energy dependent and highly sensitive to inhibition by uncouplers of oxidative phosphorylation. Increasing the phagolysosomal pH ofinfected chicken embryo fibroblasts by use of the lysosomotropic agents chloroquine, methylamine, or ammonium chloride inhibited the multiplication of C. burnetii, thus demonstrating the in vivo requirement for the acidic conditions ofthe phagolysosome. This apparent dependence upon phagosome-lysosome fusion to generate pH conditions favorable to C. burnetii replication suggests a unique biochemical mechanism of parasite activation. A pathogenic mechanism based on regulation of microbial metabolism by H+-dependent stimulation of cell function is proposed.

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After repeated passages through embyronated eggs, the Nine Mile strain of Coxiella burnetii exhibits antigenic variation, a loss of virulence characteristics, and transition to a truncated lipopolysaccharide (LPS) structure. In two independently derived strains, Nine Mile phase II and RSA 514, these phenotypic changes were accompanied by a large chromosomal deletion (M. H. Vodkin and J. C. Williams, J. Gen. Microbiol. 132:2587-2594, 1986). In the work reported here, additional screening of a cosmid bank prepared from the wild-type strain was used to map the deletion termini of both mutant strains and to accumulate all the segments of DNA that comprise the two deletions. The corresponding DNAs were then sequenced and annotated. The Nine Mile phase II deletion was completely nested within the deletion of the RSA 514 strain. Basic alignment and homology studies indicated that a large group of LPS biosynthetic genes, arranged in an apparent O-antigen cluster, was deleted in both variants. Database homologies identified, in particular, mannose pathway genes and genes encoding sugar methylases and nucleotide sugar epimerase-dehydratase proteins. Candidate genes for addition of sugar units to the core oligosaccharide for synthesis of the rare sugar 6-deoxy-3-C-methylgulose (virenose) were identified in the deleted region. Repeats, redundancies, paralogous genes, and two regions with reduced G؉C contents were found within the deletions.Coxiella burnetii is an obligately parasitic bacterium that replicates within phagolysosomes of eukaryotic hosts (1,16,27). This organism is the causative agent of Q fever in humans. It is endemic in many species of domestic animals, especially sheep, goats, and cattle. C. burnetii is extremely infectious; inhalation of one organism is enough to initiate an acute illness in a guinea pig (32). In humans, the disease usually is a moderate to severe flu-like illness with headache, fever and chills, malaise, myalgia, and anorexia or an atypical pneumonia with a dry cough (26). Recrudescence and chronic illness, usually in the form of hepatitis or intractable endocarditis, are the most serious manifestations of the disease (24,25,28,55). Humans usually contract Q fever by inhaling contaminated dust, often in barnyard, stockyard, or abattoir settings (34, 45).Some strains of C. burnetii have been observed to undergo variation in surface antigens (18,40). Continual passage of the Nine Mile strain (the tick-derived United States prototype strain) through immunocompetent hosts (306 successive passages in guinea pigs) apparently maintained the organism in its original, native (wild-type) antigenic form. However, eight subsequent passages through embryonated eggs resulted in the emergence of organisms that were easily distinguished serologically from the original isolate (40). The new phenotype reacted strongly with complement-fixing antibody found in acute-phase serum, compared with the lack of a reaction demonstrated by the guinea pig-passaged antigenic form (40). The two antigenic types differed ...