Selected observations on rickettsiae and their host cells.

The study of the interaction between rickettsiae and their host cells is in its infancy. Members of the genera Rickettsia, Coxiella and Rochalimaea show considerable diversity in host cell range (in vivo vs. in vitro), kind of association with host cell (pericellular, intracellular), mode of entry, interactions with various host cell membranes, intracellular localization (intraphagosomal, free in cytoplasm, intranuclear), adaptation to preferred microhabitat (e.g., optimal pH for enzymes), details of growth cycle, mechanisms of host cell damage. Quantitative in vitro methods exist for the study of infection cycles. Knowledge of nutritional requirements is almost non-existent. Host factors, (e.g., antibody, lymphokines, immune interferon) influence intracellular rickettsiae. Rickettsia-host cell interactions remain a fertile field for discovery.