Novel induction of broad-spectrum antibiotics by the human pathogen Legionella.

The majority of antibiotics are natural products, with microorganism-generated molecules and their derivatives being the most prevalent source of drugs to treat infections. Thus, identifying natural products remains the most valuable resource for novel therapeutics. Here, we report the discovery of a series of dormant bacteria in honey that have bactericidal activity toward Legionella, a bacterial pathogen that causes respiratory disease in humans. We show that, in response to bacterial products secreted by Legionella, the honey bacteria release diffusible antimicrobial molecules. Remarkably, the honey bacteria only produce these molecules in response to Legionella spp., when compared to a panel of 24 bacterial pathogens from different genera. However, the molecules induced by Legionella have broad activity against several clinically important pathogens, including many high-priority pathogens. Thus, Legionella spp. are potent drivers of antimicrobial molecule production by uncharacterized bacteria isolated from honey, providing access to new antimicrobial products and an unprecedented strategy for discovering novel antibiotics. IMPORTANCE: Natural products generated by microorganisms remain the most viable and abundant source of new antibiotics. However, their discovery depends on the ability to isolate and culture the producing organisms and to identify conditions that promote antibiotic production. Here, we identify a series of previously undescribed bacteria isolated from raw honey and specific culture conditions that induce the production of antimicrobial molecules that are active against a wide variety of pathogenic bacteria.

An efficient and cost-effective method for disrupting genes in RAW264.7 macrophages using CRISPR-Cas9.

The clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 (Cas9) are widely used for genome editing in cultured cell lines. However, the implementation of genome editing is still challenging due to the complex and often costly multi-step process associated with this technique. Moreover, the efficiency of genome editing varies across cell types, often limiting utility. Herein, we describe pCRISPR-EASY, a vector for simplified cloning of single guide RNAs (sgRNAs) and its simultaneous introduction with CRISPR-Cas9 into cultured cells using a non-viral delivery system. We outline a comprehensive, step-by-step protocol for genome editing in RAW264.7 macrophages, a mouse macrophage cell line widely used in biomedical research for which genome editing using CRISPR-Cas9 has been restricted to lentiviral or expensive commercial reagents. This provides an economical, highly efficient and reliable method for genome editing that can easily be adapted for use in other systems.

Legionella pneumophila usurps host cell lipids for vacuole expansion and bacterial growth.

Vacuolar pathogens reside in membrane-bound compartments within host cells. Maintaining the integrity of this compartment is paramount to bacterial survival and replication as it protects against certain host surveillance mechanisms that function to eradicate invading pathogens. Preserving this compartment during bacterial replication requires expansion of the vacuole membrane to accommodate the increasing number of bacteria, and yet, how this is accomplished remains largely unknown. Here, we show that the vacuolar pathogen Legionella pneumophila exploits multiple sources of host cell fatty acids, including inducing host cell fatty acid scavenging pathways, in order to promote expansion of the replication vacuole and bacteria growth. Conversely, when exogenous lipids are limited, the decrease in host lipid availability restricts expansion of the replication vacuole membrane, resulting in a higher density of bacteria within the vacuole. Modifying the architecture of the vacuole prioritizes bacterial growth by allowing the greatest number of bacteria to remain protected by the vacuole membrane despite limited resources for its expansion. However, this trade-off is not without risk, as it can lead to vacuole destabilization, which is detrimental to the pathogen. However, when host lipid resources become extremely scarce, for example by inhibiting host lipid scavenging, de novo biosynthetic pathways, and/or diverting host fatty acids to storage compartments, bacterial replication becomes severely impaired, indicating that host cell fatty acid availability also directly regulates L. pneumophila growth. Collectively, these data demonstrate dual roles for host cell fatty acids in replication vacuole expansion and bacterial proliferation, revealing the central functions for these molecules and their metabolic pathways in L. pneumophila pathogenesis.

Legionella pneumophila inhibits type I interferon signaling to avoid cell-intrinsic host cell defense.

The host type I interferon (IFN) response protects against Legionella pneumophila infections. Other bacterial pathogens inhibit type I IFN-mediated cell signaling; however, the interaction between this signaling pathway and L. pneumophila has not been well described. Here, we demonstrate that L. pneumophila inhibits the IFN-ß signaling pathway but does not inhibit IFN-γ-mediated cell signaling. The addition of IFN-ß to L. pneumophila-infected macrophages limited bacterial growth independently of NOS2 and reactive nitrogen species. The type IV secretion system of L. pneumophila is required to inhibit IFN-ß-mediated cell signaling. Finally, we show that the inhibition of the IFN-ß signaling pathway occurs downstream of STAT1 and STAT2 phosphorylation. In conclusion, our findings describe a novel host cell signaling pathway inhibited by L. pneumophila via its type IV secretion system.

Peptidoglycan deacetylation controls type IV secretion and the intracellular survival of the bacterial pathogen Legionella pneumophila.

Peptidoglycan is a critical component of the bacteria cell envelope. Remodeling of the peptidoglycan is required for numerous essential cellular processes and has been linked to bacterial pathogenesis. Peptidoglycan deacetylases that remove the acetyl group of the N-acetylglucosamine (NAG) subunit protect bacterial pathogens from immune recognition and digestive enzymes secreted at the site of infection. However, the full extent of this modification on bacterial physiology and pathogenesis is not known. Here, we identify a polysaccharide deacetylase of the intracellular bacterial pathogen Legionella pneumophila and define a two-tiered role for this enzyme in Legionella pathogenesis. First, NAG deacetylation is important for the proper localization and function of the Type IVb secretion system, linking peptidoglycan editing to the modulation of host cellular processes through the action of secreted virulence factors. As a consequence, the Legionella vacuole mis-traffics along the endocytic pathway to the lysosome, preventing the formation of a replication permissive compartment. Second, within the lysosome, the inability to deacetylate the peptidoglycan renders the bacteria more sensitive to lysozyme-mediated degradation, resulting in increased bacterial death. Thus, the ability to deacetylate NAG is important for bacteria to persist within host cells and in turn, Legionella virulence. Collectively, these results expand the function of peptidoglycan deacetylases in bacteria, linking peptidoglycan editing, Type IV secretion, and the intracellular fate of a bacterial pathogen.

Complete Genome Sequence of Bacillus safensis Strain AHB2, Isolated from African Raw Honey from Kajo Keji, South Sudan.

We report the complete genome sequence of Bacillus safensis strain AHB2, isolated from African raw honey originating in Kajo Keji, South Sudan, and purchased from a third-party vendor. The genome consists of 3,785,324 bp encompassing 3,774 predicted protein-coding sequences and 183 RNA genes.

Complete Genome Sequence of an African Raw Honey Bacterial Isolate, Bacillus safensis Strain AHB11.

We present the complete genome sequence of Bacillus safensis strain AHB11, which was isolated from African raw honey from Kajo Keji, South Sudan, that had been purchased from a third-party vendor. The genome is composed of a 3,697,357-bp chromosome and a 7,105-bp plasmid, collectively encompassing 3,699 predicted protein-coding sequences and 110 RNA genes.

Combinatorial selection in amoebal hosts drives the evolution of the human pathogen Legionella pneumophila.

Virulence mechanisms typically evolve through the continual interaction of a pathogen with its host. In contrast, it is poorly understood how environmentally acquired pathogens are able to cause disease without prior interaction with humans. Here, we provide experimental evidence for the model that Legionella pathogenesis in humans results from the cumulative selective pressures of multiple amoebal hosts in the environment. Using transposon sequencing, we identify Legionella pneumophila genes required for growth in four diverse amoebae, defining universal virulence factors commonly required in all host cell types and amoeba-specific auxiliary genes that determine host range. By comparing genes that promote growth in amoebae and macrophages, we show that adaptation of L. pneumophila to each amoeba causes the accumulation of distinct virulence genes that collectively allow replication in macrophages and, in some cases, leads to redundancy in this host cell type. In contrast, some bacterial proteins that promote replication in amoebae restrict growth in macrophages. Thus, amoebae-imposed selection is a double-edged sword, having both positive and negative impacts on disease. Comparing the genome composition and host range of multiple Legionella species, we demonstrate that their distinct evolutionary trajectories in the environment have led to the convergent evolution of compensatory virulence mechanisms.

From Many Hosts, One Accidental Pathogen: The Diverse Protozoan Hosts of Legionella.

The 1976 outbreak of Legionnaires' disease led to the discovery of the intracellular bacterial pathogen Legionella pneumophila. Given their impact on human health, Legionella species and the mechanisms responsible for their replication within host cells are often studied in alveolar macrophages, the primary human cell type associated with disease. Despite the potential severity of individual cases of disease, Legionella are not spread from person-to-person. Thus, from the pathogen's perspective, interactions with human cells are accidents of time and space-evolutionary dead ends with no impact on Legionella's long-term survival or pathogenic trajectory. To understand Legionella as a pathogen is to understand its interaction with its natural hosts: the polyphyletic protozoa, a group of unicellular eukaryotes with a staggering amount of evolutionary diversity. While much remains to be understood about these enigmatic hosts, we summarize the current state of knowledge concerning Legionella's natural host range, the diversity of Legionella-protozoa interactions, the factors influencing these interactions, the importance of avoiding the generalization of protozoan-bacterial interactions based on a limited number of model hosts and the central role of protozoa to the biology, evolution, and persistence of Legionella in the environment.

Beyond Paralogs: The Multiple Layers of Redundancy in Bacterial Pathogenesis.

Redundancy has been referred to as a state of no longer being needed or useful. Microbiologists often theorize that the only case of true redundancy in a haploid organism would be a recent gene duplication event, prior to divergence through selective pressure. However, a growing number of examples exist where an organism encodes two genes that appear to perform the same function. For example, many pathogens translocate multiple effector proteins into hosts. While disruption of individual effector genes does not result in a discernable phenotype, deleting genes in combination impairs pathogenesis: this has been described as redundancy. In many cases, this apparent redundancy could be due to limitations of laboratory models of pathogenesis that do not fully recapitulate the disease process. Alternatively, it is possible that the selective advantage achieved by this perceived redundancy is too subtle to be measured in the laboratory. Moreover, there are numerous possibilities for different types of redundancy. The most common and recognized form of redundancy is functional redundancy whereby two proteins have similar biochemical activities and substrate specificities allowing each one to compensate in the absence of the other. However, redundancy can also exist between seemingly unrelated proteins that manipulate the same or complementary host cell pathways. In this article, we outline 5 types of redundancy in pathogenesis: molecular, target, pathway, cellular process, and system redundancy that incorporate the biochemical activities, the host target specificities and the impact of effector function on the pathways and cellular process they modulate. For each type of redundancy, we provide examples from Legionella pathogenesis as this organism employs over 300 secreted virulence proteins and loss of individual proteins rarely impacts intracellular growth. We also discuss selective pressures that drive the maintenance of redundant mechanisms, the current methods used to resolve redundancy and features that distinguish between redundant and non-redundant virulence mechanisms.

Iron Limitation Triggers Early Egress by the Intracellular Bacterial Pathogen Legionella pneumophila.

Legionella pneumophila is an intracellular bacterial pathogen that replicates in alveolar macrophages, causing a severe form of pneumonia. Intracellular growth of the bacterium depends on its ability to sequester iron from the host cell. In the L. pneumophila strain 130b, one mechanism used to acquire this essential nutrient is the siderophore legiobactin. Iron-bound legiobactin is imported by the transport protein LbtU. Here, we describe the role of LbtP, a paralog of LbtU, in iron acquisition in the L. pneumophila strain Philadelphia-1. Similar to LbtU, LbtP is a siderophore transport protein and is required for robust growth under iron-limiting conditions. Despite their similar functions, however, LbtU and LbtP do not contribute equally to iron acquisition. The Philadelphia-1 strain lacking LbtP is more sensitive to iron deprivation in vitro Moreover, LbtP is important for L. pneumophila growth within macrophages while LbtU is dispensable. These results demonstrate that LbtP plays a dominant role over LbtU in iron acquisition. In contrast, loss of both LbtP and LbtU does not impair L. pneumophila growth in the amoebal host Acanthamoeba castellanii, demonstrating a host-specific requirement for the activities of these two transporters in iron acquisition. The growth defect of the ΔlbtP mutant in macrophages is not due to alterations in growth kinetics. Instead, the absence of LbtP limits L. pneumophila replication and causes bacteria to prematurely exit the host cell. These results demonstrate the existence of a preprogrammed exit strategy in response to iron limitation that allows L. pneumophila to abandon the host cell when nutrients are exhausted.

iMAD, a genetic screening strategy for dissecting complex interactions between a pathogen and its host.

Insertional mutagenesis and depletion (iMAD) is a genetic screening strategy for dissecting complex interactions between two organisms. The simultaneous genetic manipulation of both organisms allows the identification of aggravating and alleviating genetic interactions between pairs of gene disruptions, one from each organism. Hierarchial clustering and genetic interaction networks are then used to identify common behavioral patterns among subsets of genes, which allow functional relationships between proteins and their component pathways to be elucidated. Here we present a protocol for dissecting the interaction between a pathogen (Legionella pneumophila) and its host (cultured Drosophila melanogaster cells) using bacterial mutagenesis and host RNAi. The key stages covered in the PROCEDURE include the design, execution and data analysis of an iMAD screen; details for determining the abundance of individual mutants by microarray analysis and next-generation sequencing are not included because detailed protocols have been published elsewhere. Adapting and optimizing iMAD to a specific experimental system can require 6-18 months. Once a bacterial mutant library, host cell factor depletion strategies and conditions to monitor the interaction are established, an iMAD screen can be completed in 4-8 weeks, depending on the organisms' growth rates, the duration of the interaction and the types of data analysis performed.

Analysis of Legionella infection using RNAi in Drosophila cells.

RNA interference (RNAi) is the process of specific gene silencing by the use of double-stranded RNA (dsRNA). In cultured Drosophila cells, RNAi methodologies are well established and easily executed: dsRNA, when added to the cell culture medium, is efficiently internalized by the cells and, through the activity of endogenous processing machinery, targets the specified mRNA for degradation resulting in reduced levels of its encoded protein. This technique has proven very useful in studying the role of host genes during Legionella pneumophila infections, as it allows the effect of host factor depletion on intracellular growth of the bacterium to be examined. In this chapter we present the methods commonly used in our laboratory to study intracellular growth of L. pneumophila using dsRNA in Drosophila cells.

Aggravating genetic interactions allow a solution to redundancy in a bacterial pathogen.

Interactions between hosts and pathogens are complex, so understanding the events that govern these interactions requires the analysis of molecular mechanisms operating in both organisms. Many pathogens use multiple strategies to target a single event in the disease process, confounding the identification of the important determinants of virulence. We developed a genetic screening strategy called insertional mutagenesis and depletion (iMAD) that combines bacterial mutagenesis and RNA interference, to systematically dissect the interplay between a pathogen and its host. We used this technique to resolve the network of proteins secreted by the bacterium Legionella pneumophila to promote intracellular growth, a critical determinant of pathogenicity of this organism. This strategy is broadly applicable, allowing the dissection of any interface between two organisms involving numerous interactions.

The Legionella effector RavZ inhibits host autophagy through irreversible Atg8 deconjugation.

Eukaryotic cells can use the autophagy pathway to defend against microbes that gain access to the cytosol or reside in pathogen-modified vacuoles. It remains unclear if pathogens have evolved specific mechanisms to manipulate autophagy. Here, we found that the intracellular pathogen Legionella pneumophila could interfere with autophagy by using the bacterial effector protein RavZ to directly uncouple Atg8 proteins attached to phosphatidylethanolamine on autophagosome membranes. RavZ hydrolyzed the amide bond between the carboxyl-terminal glycine residue and an adjacent aromatic residue in Atg8 proteins, producing an Atg8 protein that could not be reconjugated by Atg7 and Atg3. Thus, intracellular pathogens can inhibit autophagy by irreversibly inactivating Atg8 proteins during infection.

Minimization of the Legionella pneumophila genome reveals chromosomal regions involved in host range expansion.

Legionella pneumophila is a bacterial pathogen of amoebae and humans. Intracellular growth requires a type IVB secretion system that translocates at least 200 different proteins into host cells. To distinguish between proteins necessary for growth in culture and those specifically required for intracellular replication, a screen was performed to identify genes necessary for optimal growth in nutrient-rich medium. Mapping of these genes revealed that the L. pneumophila chromosome has a modular architecture consisting of several large genomic islands that are dispensable for growth in bacteriological culture. Strains lacking six of these regions, and thus 18.5% of the genome, were viable but required secondary point mutations for optimal growth. The simultaneous deletion of five of these genomic loci had no adverse effect on growth of the bacterium in nutrient-rich media. Remarkably, this minimal genome strain, which lacked 31% of the known substrates of the type IVB system, caused only marginal defects in intracellular growth within mouse macrophages. In contrast, deletion of single regions reduced growth within amoebae. The importance of individual islands, however, differed among amoebal species. The host-specific requirements of these genomic islands support a model in which the acquisition of foreign DNA has broadened the L. pneumophila host range.

The E Block motif is associated with Legionella pneumophila translocated substrates.

Legionella pneumophila promotes intracellular growth by moving bacterial proteins across membranes via the Icm/Dot system. A strategy was devised to identify large numbers of Icm/Dot translocated proteins, and the resulting pool was used to identify common motifs that operate as recognition signals. The 3' end of the sidC gene, which encodes a known translocated substrate, was replaced with DNA encoding 200 codons from the 3' end of 442 potential substrate-encoding genes. The resulting hybrid proteins were then tested in a high throughput assay, in which translocated SidC antigen was detected by indirect immunofluorescence. Among translocated substrates, regions of 6-8 residues called E Blocks were identified that were rich in glutamates. Analysis of SidM/DrrA revealed that loss of three Glu residues, arrayed in a triangle on an α-helical surface, totally eliminated translocation of a reporter protein. Based on this result, a second strategy was employed to identify Icm/Dot substrates having carboxyl terminal glutamates. From the fusion assay and the bioinformatic queries, carboxyl terminal sequences from 49 previously unidentified proteins were shown to promote translocation into target cells. These studies indicate that by analysing subsets of translocated substrates, patterns can be found that allow predictions of important motifs recognized by Icm/Dot.

The Legionella pneumophila replication vacuole: making a cosy niche inside host cells.

The pathogenesis of Legionella pneumophila is derived from its growth within lung macrophages after aerosols are inhaled from contaminated water sources. Interest in this bacterium stems from its ability to manipulate host cell vesicular-trafficking pathways and establish a membrane-bound replication vacuole, making it a model for intravacuolar pathogens. Establishment of the replication compartment requires a specialized translocation system that transports a large cadre of protein substrates across the vacuolar membrane. These substrates regulate vesicle traffic and survival pathways in the host cell. This Review focuses on the strategies that L. pneumophila uses to establish intracellular growth and evaluates why this microorganism has accumulated an unprecedented number of translocated substrates that are targeted at host cells.

Pivotal roles for the receiver domain in the mechanism of action of the response regulator RamR of Streptomyces coelicolor.

The response regulator RamR activates expression of the ramCSAB operon, the source of the morphogenetic peptide SapB, and is therefore important for morphogenesis of the bacterium Streptomyces coelicolor. Like most response regulators, RamR consists of an amino-terminal receiver domain and a carboxy-terminal DNA binding domain. Four of five highly conserved active site residues known to be important in other response regulators are present in RamR: D12, D56 (the predicted site of phosphorylation), T84 and K105. Here, we show that in spite of this, RamR did not demonstrate an ability to autophosphorylate in vitro in the presence of small molecule phosphodonors. The unphosphorylated protein behaved as a dimer and bound cooperatively to three sites in the ramC promoter, one with very high affinity and two with lower affinity. On its own, the RamR DNA binding domain could not bind DNA but was able to interfere with the action of full length RamR in a manner suggesting direct protein-protein contact. Surprisingly, substitution of residues D12 or T84 had no effect on RamR function in vivo. In contrast, D56A and K105A substitutions caused defects in both dimer formation and DNA binding while the more conservative substitution, D56N permitted dimer formation but not DNA binding. L102 in RamR corresponds to a well-conserved tyrosine (or aromatic) residue that is important for function in the other response regulators. While a L102Y variant, which introduced the aromatic side-chain usually found at this position, functioned normally, L102A and L102W substitutions blocked RamR function in vivo. We show that these substitutions specifically impaired cooperative DNA binding by RamR at the lower affinity recognition sequences. The biochemical properties of RamR therefore differ markedly from those of other well-characterized response regulators.

The ramC gene is required for morphogenesis in Streptomyces coelicolor and expressed in a cell type-specific manner under the direct control of RamR.

The bacterium Streptomyces coelicolor produces two cell types during the course of its life cycle: the aerial hyphae, which metamorphose into spores, and the substrate hyphae, which synthesize antibiotics. We show that the genes ramC and ramR are required for the production of the aerial hyphae but are dispensable for vegetative growth and antibiotic synthesis. We find that ramC is expressed in the substrate hyphae and shut off in the aerial hyphae by the time visible signs of sporulation-associated septation are evident. Production of RamC requires the developmental regulators bldD, cprA and ramR, but not bldM or bldN, and we show that the RamR protein interacts directly with DNA in the ramC promoter region suggesting that it is, at least in part, responsible for regulating ramC expression.