Life Cycle, Growth Cycles and Developmental Cycle of Legionella pneumophila

Life Cycle, Growth Cycles and Developmental Cycle of Legionella pneumophila,10.1007/978-0-387-70896-6_4,Rafael A. Garduño

Life Cycle, Growth Cycles and Developmental Cycle of Legionella pneumophila   (Citations: 2)
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The Gram-negative bacterium L. pneumophila is an inhabitant of freshwater. Although free legionellae may be readily detected in freshwater, L. pneumophila is not a free-living organism but an intracellular parasite of amoebae (Fields 1996) that recently became an opportunistic human pathogen. Only accidentally, when water aerosol containing L. pneumophila is inhaled or contaminated water is aspirated, does L. pneumophila enter the human lung to infect alveolar macrophages. In susceptible individuals (mainly immuno- compromised patients or the elderly), this initial infection may lead to an atypical pneumonia known as Legionnaires' disease (Winn 1988, Weeratna et al. 1993). Legionnaires' disease is always transmitted from the environment to humans, and thus constitutes an environmental disease that cannot be transmitted from person to person. A careful evaluation of the ecology of L. pneumophila would unequivocally lead to the conclusion that the accidental infection of humans is not advantageous to L. pneumophila, but rather a "lateral shuffle" (Spriggs 1987) leading to a dead end. Infection of the human host is a diversion from the natural life cycle of L. pneumophila in amoebae. In other words, it would be very difficult to argue that humans have played a defining role in the evolution of L. pneumophila. Despite years of accumulated clinical experience and intense epidemiological research, a satisfactory explanation for the lack of person-to-person transmission of Legionnaires' disease does not exist. However, some recent studies that will be discussed below are beginning to provide explanations. Central to these studies has been the understanding of the life cycle and the developmental cycle of L. pneumophila. Given the recent advances in our understanding of L. pneumophila's developmental cycle, I believe that the systematic study of the ecology and natural history of L. pneumophila will shed new light on the pathogenesis of L. pneumophila and the mechanism(s) by which Legionnaires' disease is transmitted. The purpose of this chapter is dual. It intends to integrate and summarize what we know about the natural history of L. pneumophila. An ever-increasing body of literature continues to describe both the dramatic changes that L. pneumophila experiences as it grows in different environments, as well as its complex interactions with host cells. Regrettably, the terminology used in the literature to describe these changes and complex
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