Diverse protist grazers select for virulence-related traits in Legionella

Abstract

It is generally accepted that selection for resistance to grazing by protists has contributed to the evolution of Legionella pneumophila as a pathogen. Grazing resistance is becoming more generally recognized as having an important role in the ecology and evolution of bacterial pathogenesis. However, selection for grazing resistance presupposes the existence of protist grazers that provide the selective pressure. To determine whether there are protists that graze on pathogenic Legionella species, we investigated the existence of such organisms in a variety of environmental samples. We isolated and characterized diverse protists that graze on L. pneumophila and determined the effects of adding L. pneumophila on the protist community structures in microcosms made from these environmental samples. Several unrelated organisms were able to graze efficiently on L. pneumophila. The community structures of all samples were markedly altered by the addition of L. pneumophila. Surprisingly, some of the Legionella grazers were closely related to species that are known hosts for L. pneumophila, indicating the presence of unknown specificity determinants for this interaction. These results provide the first direct support for the hypothesis that protist grazers exert selective pressure on Legionella to acquire and retain adaptations that contribute to survival, and that these properties are relevant to the ability of the bacteria to cause disease in people. We also report a novel mechanism of killing of amoebae by one Legionella species that requires an intact Type IV secretion system but does not involve intracellular replication. We refer to this phenomenon as 'food poisoning'.

Creating a customized intracellular niche: subversion of host cell signaling byLegionellatype IV secretion system effectors

“…However, the everchanging diversity of protozoa, which Legionella encounters in the environment, seems to be a driving force for extensively expanding its T4SS effector repertoire. Importantly, even closely related protozoan species could either serve as a host for Legionella or consume it as a food source (Amaro et al 2015). As main cellular pathways and their mediators are conserved between protozoa and human macrophages, this effector arsenal also enables Legionella to thrive in human alveolar macrophages and cause disease.…”

“…are ubiquitously found in the environment, where they occur as planktonic bacteria or reside in biofilm communities (Fliermans et al 1981;Abdel-Nour et al 2013). However, the most important step of their environmental life cycle and main driving force for their evolution seems to be their interaction with protozoa (Rowbotham 1980;Amaro et al 2015). Protozoa are a large diverse group of unicellular eukaryotes.…”

Surveillance of parasiticLegionellain surface waters by using immunomagnetic separation and amoebae enrichment

“…According to the description of Amaro et al, 18 some of the Legionella grazers were closely related to species that are known hosts for L. pneumophila, indicating the presence of unknown specificity determinants for this interaction. Several unrelated organisms were able to graze efficiently on L. pneumophila.…”

“…Some Legionella grazers are closely related to species that are known hosts for L. pneumophila, indicating the presence of unknown specificity determinants for this interaction to occur. 18 Immunomagnetic separation (IMS) technology uses antibody-coated magnetic microbeads to target specific biological substances. It is widely applied for separating pathogens in foods, clinical specimens and environmental samples.…”

Selective Requirement of the Shikimate Pathway of Legionella pneumophila for Intravacuolar Growth within Human Macrophages but Not within Acanthamoeba

“…egionella pneumophila is an environmental organism of aquatic reservoirs, where the bacteria replicate within selected species of amoebae and other unicellular protists (1)(2)(3)(4). Upon transmission to the human accidental host, L. pneumophila causes Legionnaires' disease, an often fatal pneumonia.…”