Protein and DNA Biosynthesis Demonstrated in Host Cell-Free Phagosomes Containing Anaplasma phagocytophilum or Ehrlichia chaffeensis in Axenic Media

Rickettsiae belong to the Anaplasmataceae family, which includes mostly tick-transmitted pathogens causing human, canine, and ruminant diseases. Biochemical characterization of the pathogens remains a major challenge because of their obligate parasitism. We investigated the use of an axenic medium for growth of two important pathogens—Anaplasma phagocytophilum and Ehrlichia chaffeensis—in host cell-free phagosomes. We recently reported that the axenic medium promotes protein and DNA biosynthesis in host cell-free replicating form of E. chaffeensis, although the bacterial replication is limited. We now tested the hypothesis that growth on axenic medium can be improved if host cell-free rickettsia-containing phagosomes are used. Purification of phagosomes from A. phagocytophilum- and E. chaffeensis-infected host cells was accomplished by density gradient centrifugation combined with magnet-assisted cell sorting. Protein and DNA synthesis was observed for both organisms in cell-free phagosomes with glucose-6-phosphate and/or ATP. The levels of protein and DNA synthesis were the highest for a medium pH of 7. The data demonstrate bacterial DNA and protein synthesis for the first time in host cell-free phagosomes for two rickettsial pathogens. The host cell support-free axenic growth of obligate pathogenic rickettsiae will be critical in advancing research goals in many important tick-borne diseases impacting human and animal health.