The Life Cycle of L pneumophila as Currently Understood

Life cycles tell us a great deal about the natural history of a living organism. They are a synthesis of years of evolution, adaptation, specialization, and survival in nature. From an infectious disease standpoint, life cycles may provide clues to understand the mechanisms of pathogenesis of a particular parasite. The origin of pathogens and the evolution of virulence lie within the dominion of microbial ecology, and the life cycle of L. pneumophila surely is no exception.

Amoebae are the primary natural hosts of L. pneumophila and constitute the central axis around which the life cycle of L. pneumophila revolves (Fig. 4.1). Rowbotham (1980) first recognized L. pneumophila to be an intracellular parasite of amoebae, shortly after the isolation and identification of the "Legionnaires' disease bacterium" by McDade et al. (1977). Currently, ~40 species of Legionella have been identified (Benson and Fields 1998), all of which are able to parasitize amoebae. In addition, a large number of Legionella-like amoebal pathogens (Adeleke et al. 1996) that cannot be cultured in vitro demonstrate that Legionella has developed unique obligate intracellular relationships with specific amoebae. Finally, Legionella-related endosymbionts (known as X-bacteria) have reached a very specific mutual dependence with Amoeba proteus (Ahn et al. 1994, Jeon

2004). Collectively, these observations suggest that the genus Legionella is ancient, and very well adapted to amoebal hosts. Most likely, members of the genus Legionella (L. pneumophila included) first "learned" how to become intracellular parasites through frequent encounters with amoebae (Molmeret et al.

2005), and have since evolved to establish a wide spectrum of relationships with these hosts, from facultative intracellular and pathogenic to endosymbiotic. The amoebae-L. pneumophila relationship seems to occupy a place on the pathogenic side of the spectrum, as L. pneumophila often kills its host cells.

The life cycle of L. pneumophila, first described by Rowbotham (1986) at the light microscope level, starts with the invasion of an amoebal host, and proceeds through a series of intracellular events that lead to the active replication of L. pneumophila and then the release of the bacterial progeny that infect new hosts (Fig. 4.1). As expected, the cycle begins and ends at the same point (i.e., infection of a new host). The following is a brief description of the specific events that constitute the life cycle of L. pneumophila and its progression through the amoebal host.

Legionella Pneumophila Life Cycle

Figure 4.1. The life cycle of L. pneumophila. The cycle begins with the attachment to and the invasion of an amoebal host. Post-internalization events include inhibition of phago-some-lysosome fusion and alteration of organelle traffic. L. pneumophila replicates in a ribo-some-decorated vacuole that associates with the endoplasmic reticulum (ER) and acquires ER markers. After replication is finished the progeny exits the wasted amoeba to reinitiate the cycle. The life cycle of L. pneumophila is associated with bacterial differentiation. The two main morphological forms are the mature intracellular form, or MIF, and the replicative form, or RF, which differentiate into each other via intermediates. MIFs play a central role in the infection of amoebae and are the potential agents that spread Legionnaires' disease to humans, either free or packaged into vesicles. MIF-ladden vesicles may be directly released by amoebae or indirectly by ciliates via an additional packaging step.

Figure 4.1. The life cycle of L. pneumophila. The cycle begins with the attachment to and the invasion of an amoebal host. Post-internalization events include inhibition of phago-some-lysosome fusion and alteration of organelle traffic. L. pneumophila replicates in a ribo-some-decorated vacuole that associates with the endoplasmic reticulum (ER) and acquires ER markers. After replication is finished the progeny exits the wasted amoeba to reinitiate the cycle. The life cycle of L. pneumophila is associated with bacterial differentiation. The two main morphological forms are the mature intracellular form, or MIF, and the replicative form, or RF, which differentiate into each other via intermediates. MIFs play a central role in the infection of amoebae and are the potential agents that spread Legionnaires' disease to humans, either free or packaged into vesicles. MIF-ladden vesicles may be directly released by amoebae or indirectly by ciliates via an additional packaging step.

Was this article helpful?

0 0

Responses

  • No comments

Post a comment