Identification of Conserved ABC Importers Necessary for Intracellular Survival of Legionella pneumophila in Multiple Hosts

Legionella pneumophila, a Rosetta stone to understanding bacterial pathogenesis

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.

InSeq analysis of defined Legionella pneumophila libraries identifies a transporter-encoding gene cluster important for intracellular replication in mammalian hosts

Legionella pneumophila is an intracellular bacterial pathogen that replicates inside human alveolar macrophages to cause a severe pneumonia known as Legionnaires' disease. L. pneumophila requires the Dot/Icm Type IV secretion system to deliver hundreds of bacterial proteins to the host cytosol that manipulate cellular processes to establish a protected compartment for bacterial replication known as the Legionella-containing vacuole. To better understand mechanisms apart from the Dot/Icm system that support survival and replication in this vacuole, we used transposon insertion sequencing in combination with defined mutant sublibraries to identify L. pneumophila fitness determinants in primary mouse macrophages and the mouse lung. This approach validated that many previously identified genes important for intracellular replication were critical for infection of a mammalian host. Further, the screens uncovered additional genes contributing to L. pneumophila replication in mammalian infection models. This included a cluster of seven genes in which insertion mutations resulted in L. pneumophila fitness defects in mammalian hosts. Generation of isogenic deletion mutants and genetic complementation studies verified the importance of genes within this locus for infection of mammalian cells. Genes in this cluster are predicted to encode nucleotide-modifying enzymes, a protein of unknown function, and an atypical ATP-binding cassette (ABC) transporter with significant homology to multidrug efflux pumps that has been named Lit, for Legionella infectivity transporter. Overall, these data provide a comprehensive overview of the bacterial processes that support L. pneumophila replication in a mammalian host and offer insight into the unique challenges posed by the intravacuolar environment.IMPORTANCEIntracellular bacteria employ diverse mechanisms to survive and replicate inside the inhospitable environment of host cells. Legionella pneumophila is an opportunistic human pathogen and a model system for studying intracellular host-pathogen interactions. Transposon sequencing is an invaluable tool for identifying bacterial genes contributing to infection, but current animal models for L. pneumophila are suboptimal for conventional screens using saturated mutant libraries. This study employed a series of defined transposon mutant libraries to identify determinants of L. pneumophila fitness in mammalian hosts, which include a newly identified bacterial transporter called Lit. Understanding the requirements for survival and replication inside host cells informs us about the environment bacteria encounter during infection and the mechanisms they employ to make this environment habitable. Such knowledge will be key to addressing future challenges in treating infections caused by intracellular bacteria.

Protozoan-Priming and Magnesium Conditioning Enhance Legionella pneumophila Dissemination and Monochloramine Resistance

Opportunistic pathogens (OPs) are of concern in drinking water distribution systems because they persist despite disinfectant residuals. While many OPs garner protection from disinfectants via a biofilm lifestyle, Legionella pneumophila (Lp) also gains disinfection resistance by being harbored within free-living amoebae (FLA). It has been long established, but poorly understood, that Lp grown within FLA show increased infectivity toward subsequent FLA or human cells (i.e., macrophage), via a process we previously coined "protozoan-priming". The objectives of this study are (i) to identify in Lp a key genetic determinant of how protozoan-priming increases its infectivity, (ii) to determine the chemical stimulus within FLA to which Lp responds during protozoan-priming, and (iii) to determine if more infectious forms of Lp also exhibit enhanced disinfectant resistance. Using Acanthamoeba castellanii as a FLA host, the priming effect was isolated to Lp's sidGV locus, which is activated upon sensing elevated magnesium concentrations. Supplementing growth medium with 8 mM magnesium is sufficient to produce Lp grown in vitro with an infectivity equivalent to that of Lp grown via the protozoan-primed route. Both Lp forms with increased infectivity (FLA-grown and Mg2+-supplemented) exhibit greater monochloramine resistance than Lp grown in standard media, indicating that passage through FLA not only increases Lp's infectivity but also enhances its monochloramine resistance. Therefore, laboratory-based testing of disinfection strategies should employ conditions that simulate or replicate intracellular growth to accurately assess disinfectant resistance.

Legionnaires' Disease in Pediatric Patients, Control Measures and 5-Year Follow-up

BACKGROUND

Legionnaires' disease (LD) occurs predominantly in adults and elderly people. Its incidence in Europe has been increasing in recent years. It is rare in younger age groups and prone to be reported as healthcare-associated infection with a higher risk of fatal outcome. Hospital-acquired LD is mostly associated with a colonized hospital water system. We describe 5 LD cases in a children's hospital in Slovakia, subsequent environmental investigation, control measures, and 5-year monitoring of Legionella colonization in hospital's water system.

METHODS

In 2014-2019, we tested clinical specimens from 75 hospitalized patients. Respiratory samples were cultured for Legionella, patient's urine was tested for Legionella urinary antigens, and the microagglutination test was used for serologic testing. Samples of water were collected in 2015-2019 and processed according ISO11731.

RESULTS

We identified 5 Legionella infections in 2014-2015. Median age of patients was 15 years. All were high-risk patients hospitalized for their underlying diseases. All patients required admission to intensive care unit, and artificial ventilation due to general deterioration and respiratory failure. Legionella pneumophila was isolated from 72% of water samples. Chlorine dioxide dosing into water system above 0.3 ppm caused significant decrease of Legionella concentration in water samples. Samples taken from outlets with antimicrobial filter installed were legionellae-negative.

CONCLUSIONS

Control measures led to decreased risk of infection, but not to eradication of Legionellae. It is necessary to extend the diagnostics for Legionella infection in hospitalized children with pneumonia, especially in hospitals with colonized water system.

Evasion of phagotrophic predation by protist hosts and innate immunity of metazoan hosts by Legionella pneumophila

Legionella pneumophila is a ubiquitous environmental bacterium that has evolved to infect and proliferate within amoebae and other protists. It is thought that accidental inhalation of contaminated water particles by humans is what has enabled this pathogen to proliferate within alveolar macrophages and cause pneumonia. However, the highly evolved macrophages are equipped with more sophisticated innate defence mechanisms than are protists, such as the evolution of phagotrophic feeding into phagocytosis with more evolved innate defence processes. Not surprisingly, the majority of proteins involved in phagosome biogenesis (~80%) have origins in the phagotrophy stage of evolution. There are a plethora of highly evolved cellular and innate metazoan processes, not represented in protist biology, that are modulated by L. pneumophila, including TLR2 signalling, NF-κB, apoptotic and inflammatory processes, histone modification, caspases, and the NLRC-Naip5 inflammasomes. Importantly, L. pneumophila infects haemocytes of the invertebrate Galleria mellonella, kill G. mellonella larvae, and proliferate in and kill Drosophila adult flies and Caenorhabditis elegans. Although coevolution with protist hosts has provided a substantial blueprint for L. pneumophila to infect macrophages, we discuss the further evolutionary aspects of coevolution of L. pneumophila and its adaptation to modulate various highly evolved innate metazoan processes prior to becoming a human pathogen.

Mammalian Solute Carrier (SLC)-like transporters of Legionella pneumophila

Acquisition of nutrients during intra-vacuolar growth of L. pneumophila within macrophages or amoebae is poorly understood. Since many genes of L. pneumophila are acquired by inter-kingdom horizontal gene transfer from eukaryotic hosts, we examined the presence of human solute carrier (SLC)-like transporters in the L. pneumophila genome using I-TASSER to assess structural alignments. We identified 11 SLC-like putative transporters in L. pneumophila that are structurally similar to SLCs, eight of which are amino acid transporters, and one is a tricarboxylate transporter. The two other transporters, LstA and LstB, are structurally similar to the human glucose transporter, SLC2a1/Glut1. Single mutants of lstA or lstB have decreased ability to import, while the lstA/lstB double mutant is severely defective for uptake of glucose. While lstA or lstB single mutants are not defective in intracellular proliferation within Acanthamoeba polyphaga and human monocyte-derived macrophages, the lstA/lstB double mutant is severely defective in both host cells. The two phenotypic defects of the lstA/lstB double mutant in uptake of glucose and intracellular replication are both restored upon complementation of either lstA or lstB. Our data show that the two glucose transporters, LstA and LstB, are redundant and are required for intracellular replication within human macrophages and amoebae.

A Legionella pneumophila amylase is essential for intracellular replication in human macrophages and amoebae

Legionella pneumophila invades protozoa with an "accidental" ability to cause pneumonia upon transmission to humans. To support its nutrition during intracellular residence, L. pneumophila relies on host amino acids as the main source of carbon and energy to feed the TCA cycle. Despite the apparent lack of a requirement for glucose for L. pneumophila growth in vitro and intracellularly, the organism contains multiple amylases, which hydrolyze polysaccharides into glucose monomers. Here we describe one predicted putative amylase, LamB, which is uniquely present only in L. pneumophila and L. steigerwaltii among the ~60 species of Legionella. Our data show that LamB has a strong amylase activity, which is abolished upon substitutions of amino acids that are conserved in the catalytic pocket of amylases. Loss of LamB or expression of catalytically-inactive variants of LamB results in a severe growth defect of L. pneumophila in Acanthamoeba polyphaga and human monocytes-derived macrophages. Importantly, the lamB null mutant is severely attenuated in intra-pulmonary proliferation in the mouse model and is defective in dissemination to the liver and spleen. Our data show an essential role for LamB in intracellular replication of L. pneumophila in amoeba and human macrophages and in virulence in vivo.