Phagolysosomes of Coxiella burnetii-infected cell lines maintain an acidic pH during persistent infection.

Coxiella burnetii, the agent of Q fever, is an obligate intracellular bacterium that multiples within vacuoles of phagolysosomal origin. Persistently infected cell lines were maintained in continuous culture for months. We studied the pH of the phagolysosomes by using two murine cell lines during early propagation of the bacteria and after establishment of persistent infection. Three strains of C. burnetii were studied because of the purported propensity of each strain to cause acute or chronic disease and to be resistant or susceptible to antibiotics. The pHs were calculated from fluorescence experiments with fluoresceinated dextran as a lysosomal probe. Phagolysosomal vacuoles maintained an acidic pH during a 36-day infection. Minimal variation of the pH occurred over the duration of the experiment with strains that caused either acute or chronic disease. Phagolysosomal pH remained stable for as long as 153 days with the Nine Mile phase II isolate. Thus, neither the course of C. burnetti infection nor the diversity of antibiotic susceptibility of the strains is related to variations in the phagolysosomal pH.