FIG. 7.

Simplified model illustrating the intracellular and extracellular growth cycles of L. pneumophila. Only the intracellular growth cycle produces the fully differentiated MIF, which is more infectious and environmentally fit than the in vitro-grown SP bacteria (Stat). Presumably, host-derived signals (inducers) are required for post-stationary-phase differentiation of MIFs. Intracellular RFs would be equivalent to exponentially growing bacteria (Exp) in vitro, as they display similar morphological traits, but it remains to be determined whether or not they are physiologically different. The central double arrow indicates that L. pneumophila can move from one cycle to the other when the proper conditions are met. Notice that, to initiate a new intracellular cycle, L. pneumophila has to exit into the aquatic environment, where it is possible for MIFs to continue their maturation in response to extracellular signals (inducers).