Legionella pneumophila, a Rosetta stone to understanding bacterial pathogenesis

Abstract

Legionella pneumophila is an environmentally acquired pathogen that causes respiratory disease in humans. While the discovery of L. pneumophila is relatively recent compared to other bacterial pathogens, over the past 50 years, L. pneumophila has emerged as a powerhouse for studying host-pathogen interactions. In its natural habitat of fresh water, L. pneumophila interacts with a diverse array of protozoan hosts and readily evolve to expand their host range. This has led to the accumulation of the most extensive arsenal of secreted virulence factors described for a bacterial pathogen and their ability to infect humans. Within amoebae and human alveolar macrophages, the bacteria replicate within specialized membrane-bound compartments, establishing L. pneumophila as a model for studying intracellular vacuolar pathogens. In contrast, the virulence factors required for intracellular replication are specifically tailored to individual host cells types, allowing the pathogen to adapt to variation between disparate niches. The broad host range of this pathogen, combined with the extensive diversity and genome plasticity across the Legionella genus, has thus established this bacterium as an archetype to interrogate pathogen evolution, functional genomics, and ecology. In this review, we highlight the features of Legionella that establish them as a versatile model organism, new paradigms in bacteriology and bacterial pathogenesis resulting from the study of Legionella, as well as current and future questions that will undoubtedly expand our understanding of the complex and intricate biology of the microbial world.

Keywords: Legionella; Type IV secretion systems; amoeba; bacterial pathogenesis; effector; genome plasticity; horizontal gene transfer; host-pathogen interactions; inflammasome; macrophage; pathogen evolution; protozoa; redundancy; respiratory disease; vacuolar pathogen; virulence mechanisms.

Fig 1

Legionella is an intracellular vacuolar pathogen of protozoa and humans. Legionella replicates within protozoa, particularly free-living amoeba in the environment. Intracellular replication requires a Type IVB secretion system that translocates effector proteins into the host cell where they function to modulate host cellular processes. Legionella evolution, often through the acquisition of foreign genetic material, has enabled expansion of its host range, including its ability to transition from environmental reservoirs to humans. Legionella is able to replicate within alveolar macrophages causing respiratory illness in humans; however, disease severity varies depending on immune system competency. LCV, Legionella-containing vacuole.

Fig 2

Legionella remodels the nascent phagosome, through interactions with ER vesicles and tubules (upper panel; reproduced from reference [108] with permission from J Cell Science), into a replication-permissive compartment (middle panel; reproduced from reference [21] with permission from the American Society of Microbiology), replicating to high numbers (lower panel; reproduced from reference [22] with permission from the American Society of Microbiology), often reaching over a hundred bacteria, before exiting from the host cell.

Fig 3

The Legionella Dot/Icm secretion system. (A) The Dot/Icm translocon is a macromolecular complex spanning the inner membrane (IM), periplasm and outer membrane (OM) (reproduced from reference [140] under the creative commons attribution 4.0 international license). (B) The Dot/Icm machinery localizes to the bacterial poles and, upon contact with the host cell, engages with the host membrane (reproduced from reference [141] under the creative commons attribution 4.0 international license). LCV, Legionella-containing vacuole; Lcyt, Legionella cytoplasm; Hcyt, host cytoplasm, LCVM, LCV membrane. (C) Polar deliver of effectors (SdeC) to the host cell (reproduced from reference [144] with permission from Proc Natl Acad Sci USA). b, bacteria; n, nucleus.

Fig 4

L. pneumophila modulate a diverse array of host cellular processes through the activities of secreted effector proteins. L. pneumophila translocates over 300 effectors (red) into host cells that target a wide range of host proteins (blue) and their associated pathways and cellular processes. Despite the extensive body of research characterizing effectors, depicted here, remarkably, these represent less than 25% of the full arsenal of L. pneumophila effectors. ADPR, adenosine diphosphate ribosyl; DAG, diacylglycerol; Glc, glucose; GTP, guanosine triphosphate; Me, methyl; P, phosphate; PA, phosphatidic acid; PC, phosphatidyl choline; PI, phosphatidylinositol; PI(3)P, phosphatidylinositol 3-phosphate; PI(4)P, phosphatidylinositol 4-phosphate; PI(4,5)P₃, phosphatidylinositol (4,5) trisphosphate; PI(3–5)P₂, phosphatidylinositol (3–5) bisphosphate; PI(4,5)P₂, phosphatidylinositol (4,5) bisphosphate; PI(3,4)P, phosphatidylinositol (3,4) phosphate; PR-Ub, phosphoribosyl-ubiquitin; S1P, sphingosine-1-phosphate; SCF, Skp-Cullin-F-box complex; SUMO, small ubiquitin-like modifier; Ub, ubiquitin.

Fig 5

Emerging features and functional relationships among L. pneumophila effectors. The study of L. pneumophila effectors (red) has revealed several themes regarding the function and interplay of effectors with their target proteins (blue) and each other. Novel protein biochemistry: RavZ deconjugates Atg8 to inhibit autophagy-mediated deliver of the L. pneumophila to lysosomes, but in a manner that removes the terminal glycine (Gly) such that Atg8 cannot be recycled and reconjugated by the host cell machinery. Molecular on/off switches: Paired effectors can reversibly activate host target proteins. For example, the effector AnkX modifies the host GTPase Rab1 through covalent attachment of a phosphatidylcholine (PC) moeity, regulating its interactions with endogenous guanosine nucleotide exchange factors (GEFs), guanosine dissociation inhibitor (GDIs), and GTPase-activating proteins (GAPs), which collectively stabilizes Rab1 at the membrane in a GDP bound form. Conversely, Lem3 de-phosphocholinates Rab1, allowing for Rab1 activation and recycling. Metaeffectors: Some effectors regulate the activity of other effectors, often to prevent the toxic effects of their constitutive activities in the host cell. SdeA ubiquitinates host targets proteins such as Rtn4, while SidJ controls the activity of SdeA by glutamylating (Glu) an active site residue that renders the SdeA enzymatically inactive. Redundancy: L. pneumophila often employs more than one effector to modulate a specific host cell target, pathway or process, which can be achieved using paralogs or unrelated proteins that act through different mechanisms. Host protein translation is modulated through the activity of at least seven effectors. Lgt1, Lgt2, and Lgt3 are paralogs, each capable of covalently modifying the ribosome subunit eEF1a through glycosylation with glucose (Glc). SidI similarly binds to eEF1a but exhibits tRNA mimicry and glycosylates ribosomes with mannose (Man), inhibiting translation via an alternative mechanism that involves ribosome stalling. LegK4 impairs translation through phosphorylation of ribosome-associated heat-shock protein, Hsp70, while the mechanism of SidL- and RavX-mediated translation inhibition is unknown.