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

Efficient generation of unmarked deletions in Legionella pneumophila

Unmarked gene deletions facilitate studies of Legionella pneumophila multicomponent processes, such as motility and exonuclease activity. For this purpose, FRT-flanked alleles constructed in Escherichia coli using λ-Red recombinase were transferred to L. pneumophila by natural transformation. Resistance cassettes were then efficiently excised using the Flp site-specific recombinase encoded on a plasmid that is readily lost.

Comprehensive identification of protein substrates of the Dot/Icm type IV transporter of Legionella pneumophila

A large number of proteins transferred by the Legionella pneumophila Dot/Icm system have been identified by various strategies. With no exceptions, these strategies are based on one or more characteristics associated with the tested proteins. Given the high level of diversity exhibited by the identified proteins, it is possible that some substrates have been missed in these screenings. In this study, we took a systematic method to survey the L. pneumophila genome by testing hypothetical orfs larger than 300 base pairs for Dot/Icm-dependent translocation. 798 of the 832 analyzed orfs were successfully fused to the carboxyl end of β-lactamase. The transfer of the fusions into mammalian cells was determined using the β-lactamase reporter substrate CCF4-AM. These efforts led to the identification of 164 proteins positive in translocation. Among these, 70 proteins are novel substrates of the Dot/Icm system. These results brought the total number of experimentally confirmed Dot/Icm substrates to 275. Sequence analysis of the C-termini of these identified proteins revealed that Lpg2844, which contains few features known to be important for Dot/Icm-dependent protein transfer can be translocated at a high efficiency. Thus, our efforts have identified a large number of novel substrates of the Dot/Icm system and have revealed the diverse features recognizable by this protein transporter.

Legionella pneumophila type II secretion dampens the cytokine response of infected macrophages and epithelia

The type II secretion (T2S) system of Legionella pneumophila is required for the ability of the bacterium to grow within the lungs of A/J mice. By utilizing mutants lacking T2S (lsp), we now document that T2S promotes the intracellular infection of both multiple types of macrophages and lung epithelia. Following infection of macrophages, lsp mutants (but not a complemented mutant) elicited significantly higher levels of interleukin 6 (IL-6), tumor necrosis factor alpha (TNF-α), IL-10, IL-8, IL-1β, and MCP-1 within tissue culture supernatants. A similar result was obtained with infected lung epithelial cell lines and the lungs of infected A/J mice. Infection with a mutant specifically lacking the T2S-dependent ProA protease (but not a complemented proA mutant) resulted in partial elevation of cytokine levels. These data demonstrate that the T2S system of L. pneumophila dampens the cytokine/chemokine output of infected host cells. Upon quantitative reverse transcription (RT)-PCR analysis of infected host cells, an lspF mutant, but not the proA mutant, produced significantly higher levels of cytokine transcripts, implying that some T2S-dependent effectors dampen signal transduction and transcription but that others, such as ProA, act at a posttranscriptional step in cytokine expression. In summary, the impact of T2S on lung infection is a combination of at least three factors: the promotion of growth in macrophages, the facilitation of growth in epithelia, and the dampening of the chemokine and cytokine output from infected host cells. To our knowledge, these data are the first to identify a link between a T2S system and the modulation of immune factors following intracellular infection.

Oligonucleotides stimulate genomic alterations of Legionella pneumophila

Genetic variation generates diversity in all kingdoms of life. The corresponding mechanisms can also be harnessed for laboratory studies of fundamental cellular processes. Here we report that oligonucleotides (oligos) generate mutations on the Legionella pneumophila chromosome by a mechanism that requires homologous DNA, but not RecA, RadA or any known phage recombinase. Instead we propose that DNA replication contributes, as oligo-induced mutagenesis required ≥ 21 nucleotides of homology, was strand-dependent, and was most efficient in exponential phase. Mutagenesis did not require canonical 5' phosphate or 3' hydroxyl groups, but the primosomal protein PriA and DNA Pol I contributed. After electroporation, oligos stimulated excision of 2.1 kb of chromosomal DNA or insertion of 18 bp, and non-homologous flanking sequences were also processed. We exploited this endogenous activity to generate chromosomal deletions and to insert an epitope into a chromosomal coding sequence. Compared with Escherichia coli, L. pneumophila encodes fewer canonical single-stranded exonucleases, and the frequency of mutagenesis increased substantially when either its RecJ and ExoVII nucleases were inactivated or the oligos modified by nuclease-resistant bases. In addition to genetic engineering, oligo-induced mutagenesis may have evolutionary implications as a mechanism to incorporate divergent DNA sequences with only short regions of homology.

Striking a balance: modulation of host cell death pathways by legionella pneumophila

Programmed cell death is considered the ultimate solution for the host to eliminate infected cells, leading to the abolishment of the niche for microbial replication and the ablation of infection. Thus, it is not surprising that successful pathogens have evolved diverse strategies to reprogram the cell death pathways for their proliferation. Using effector proteins translocated by the Dot/Icm type IV secretion system, the facultative intracellular pathogen Legionella pneumophila manipulates multiple host cellular processes to create a niche within host cells to support its replication. Investigation in the past decade has established that in mammalian cells this bacterium actively modulates two host cell death pathways, namely the canonical apoptotic pathway controlled by the mitochondrion and the pyroptotic pathway controlled by the Nod-like receptor Naip5 and the Ipaf inflammasome. In this review, I will discuss the recent progress in understanding the mechanisms the bacterium employs to interfere with these host cell death pathways and how such modulation contribute to the intracellular life cycle of the pathogen.

Secreted bacterial effectors that inhibit host protein synthesis are critical for induction of the innate immune response to virulent Legionella pneumophila

The intracellular bacterial pathogen Legionella pneumophila causes an inflammatory pneumonia called Legionnaires' Disease. For virulence, L. pneumophila requires a Dot/Icm type IV secretion system that translocates bacterial effectors to the host cytosol. L. pneumophila lacking the Dot/Icm system is recognized by Toll-like receptors (TLRs), leading to a canonical NF-κB-dependent transcriptional response. In addition, L. pneumophila expressing a functional Dot/Icm system potently induces unique transcriptional targets, including proinflammatory genes such as Il23a and Csf2. Here we demonstrate that this Dot/Icm-dependent response, which we term the effector-triggered response (ETR), requires five translocated bacterial effectors that inhibit host protein synthesis. Upon infection of macrophages with virulent L. pneumophila, these five effectors caused a global decrease in host translation, thereby preventing synthesis of IκB, an inhibitor of the NF-κB transcription factor. Thus, macrophages infected with wildtype L. pneumophila exhibited prolonged activation of NF-κB, which was associated with transcription of ETR target genes such as Il23a and Csf2. L. pneumophila mutants lacking the five effectors still activated TLRs and NF-κB, but because the mutants permitted normal IκB synthesis, NF-κB activation was more transient and was not sufficient to fully induce the ETR. L. pneumophila mutants expressing enzymatically inactive effectors were also unable to fully induce the ETR, whereas multiple compounds or bacterial toxins that inhibit host protein synthesis via distinct mechanisms recapitulated the ETR when administered with TLR ligands. Previous studies have demonstrated that the host response to bacterial infection is induced primarily by specific microbial molecules that activate TLRs or cytosolic pattern recognition receptors. Our results add to this model by providing a striking illustration of how the host immune response to a virulent pathogen can also be shaped by pathogen-encoded activities, such as inhibition of host protein synthesis.

The inflammasomes: molecular effectors of host resistance against bacterial, viral, parasitic, and fungal infections

The inflammasomes are large multi-protein complexes scaffolded by cytosolic pattern recognition receptors (PRRs) that form an important part of the innate immune system. They are activated following the recognition of microbial-associated molecular patterns or host-derived danger signals (danger-associated molecular patterns) by PRRs. This recognition results in the recruitment and activation of the pro-inflammatory protease caspase-1, which cleaves its preferred substrates pro-interleukin-1β (IL-1β) and pro-IL-18 into their mature biologically active cytokine forms. Through processing of a number of other cellular substrates, caspase-1 is also required for the release of “alarmins” and the induction and execution of an inflammatory form of cell death termed pyroptosis. A growing spectrum of inflammasomes have been identified in the host defense against a variety of pathogens. Reciprocally, pathogens have evolved effector strategies to antagonize the inflammasome pathway. In this review we discuss recent developments in the understanding of inflammasome-mediated recognition of bacterial, viral, parasitic, and fungal infections and the beneficial or detrimental effects of inflammasome signaling in host resistance.

Molecular Characterization of Exploitation of the Polyubiquitination and Farnesylation Machineries of Dictyostelium Discoideum by the AnkB F-Box Effector of Legionella Pneumophila

The Dot/Icm-translocated Ankyrin B (AnkB) F-box effector of Legionella pneumophila is essential for intra-vacuolar proliferation and functions as a platform for the docking of polyubiquitinated proteins to the Legionella-containing vacuole (LCV) within macrophages and ameba. Here we show that ectopically expressed AnkB in Dictyostelium discoideum is targeted to the plasma membrane where it recruits polyubiquitinated proteins and it trans-rescues the intracellular growth defect of the ankB null mutant, which has never been demonstrated for any effector in ameba. Using co-immunoprecipitation and bimolecular fluorescence complementation we show specific interaction of Skp1 of D. discoideum with the F-box domain of AnkB, which has never been demonstrated in ameba. We show that anchoring of AnkB to the cytosolic face of the LCV membrane in D. discoideum is mediated by the host farnesylation of the C-terminal eukaryotic CaaX motif of AnkB and is independent of the F-box and the two ANK domains, which has never been demonstrated in ameba. Importantly, the three host farnesylation enzymes farnesyl transferase, RCE-1, and isoprenyl cysteine carboxyl methyl transferase of D. discoideum are recruited to the LCV in a Dot/Icm-dependent manner, which has never been demonstrated in ameba. We conclude that the polyubiquitination and farnesylation enzymatic machineries of D. discoideum are recruited to the LCV in a Dot/Icm-dependent manner and the AnkB effector exploits the two evolutionarily conserved eukaryotic machineries to proliferate within ameba, similar to mammalian cells. We propose that L. pneumophila has acquired ankB through inter-kingdom horizontal gene transfer from primitive eukaryotes, which facilitated proliferation of L. pneumophila within human cells and the emergence of Legionnaires’ disease.

Do free-living amoebae in treated drinking water systems present an emerging health risk?

There is an expanding body of evidence that free-living amoebae (FLA) increase both the numbers and virulence of water-based, human-pathogenic, amoeba-resisting microorganisms (ARM). Legionella spp., Mycobacterium spp., and other opportunistic human pathogens are known to be both ARM and also the etiologic agents of potentially fatal human lung infections. However, comparatively little is known about the FLA that may facilitate ARM growth in drinking water. This review examines the available literature on FLA in treated drinking water systems; in total 26 studies from 18 different countries. FLA were reported to breakthrough the water treatment barrier and enter distribution systems, in addition to the expected post-treatment system ingress. Once in the distribution system there is evidence of FLA colonization and regrowth especially in reservoirs and in-premise plumbing storage tanks. At the point of use the average FLA detection rate was 45% but highly variable (n = 16, σ = 31) due to both differences in both assay methods and the type of water systems examined. This review reveals that FLA are consistently detected in treated drinking water systems around the world and present a yet unquantified emerging health risk. However, more research is urgently required before accurate risks assessments can be undertaken to assess the impacts on human health, in households and institutions, due to exposure to FLA facilitated pathogenic ARM.

Legionella pneumophila LbtU acts as a novel, TonB-independent receptor for the legiobactin siderophore

Gram-negative Legionella pneumophila produces a siderophore (legiobactin) that promotes lung infection. We previously determined that lbtA and lbtB are required for the synthesis and secretion of legiobactin. DNA sequence and reverse transcription-PCR (RT-PCR) analyses now reveal the presence of an iron-repressed gene (lbtU) directly upstream of the lbtAB-containing operon. In silico analysis predicted that LbtU is an outer membrane protein consisting of a 16-stranded transmembrane β-barrel, multiple extracellular domains, and short periplasmic tails. Immunoblot analysis of cell fractions confirmed an outer membrane location for LbtU. Although replicating normally in standard media, lbtU mutants, like lbtA mutants, were impaired for growth on iron-depleted agar media. While producing typical levels of legiobactin, lbtU mutants were unable to use supplied legiobactin to stimulate growth on iron-depleted media and displayed an inability to take up iron. Complemented lbtU mutants behaved as the wild type did. The lbtU mutants were also impaired for infection in a legiobactin-dependent manner. Together, these data indicate that LbtU is involved in the uptake of legiobactin and, based upon its location, is most likely the Legionella siderophore receptor. The sequence and predicted two-dimensional (2D) and 3D structures of LbtU were distinct from those of all known siderophore receptors, which generally contain a 22-stranded β-barrel and an extended N terminus that binds TonB in order to transduce energy from the inner membrane. This observation coupled with the fact that L. pneumophila does not encode TonB suggests that LbtU is a new type of receptor that participates in a form of iron uptake that is mechanistically distinct from the existing paradigm.

Nonhematopoietic cells are key players in innate control of bacterial airway infection

Airborne pathogens encounter several hurdles during host invasion, including alveolar macrophages (AMs) and airway epithelial cells (AECs) and their products. Although growing evidence indicates pathogen-sensing capacities of epithelial cells, the relative contribution of hematopoietic versus nonhematopoietic cells in the induction of an inflammatory response and their possible interplay is still poorly defined in vivo in the context of infections with pathogenic microorganisms. In this study, we show that nonhematopoietic cells, including AECs, are critical players in the inflammatory process induced upon airway infection with Legionella pneumophila, and that they are essential for control of bacterial infections. Lung parenchymal cells, including AECs, are not infected themselves by L. pneumophila in vivo but rather act as sensors and amplifiers of inflammatory cues delivered by L. pneumophila-infected AM. We identified AM-derived IL-1β as the critical mediator to induce chemokine production in nonhematopoietic cells in the lung, resulting in swift and robust recruitment of infection-controlling neutrophils into the airways. These data add a new level of complexity to the coordination of the innate immune response to L. pneumophila and illustrate how the cross talk between leukocytes and nonhematopoietic cells contributes to efficient host protection.

Cyclic diguanylate signaling proteins control intracellular growth of Legionella pneumophila

Proteins that metabolize or bind the nucleotide second messenger cyclic diguanylate regulate a wide variety of important processes in bacteria. These processes include motility, biofilm formation, cell division, differentiation, and virulence. The role of cyclic diguanylate signaling in the lifestyle of Legionella pneumophila, the causative agent of Legionnaires' disease, has not previously been examined. The L. pneumophila genome encodes 22 predicted proteins containing domains related to cyclic diguanylate synthesis, hydrolysis, and recognition. We refer to these genes as cdgS (cyclic diguanylate signaling) genes. Strains of L. pneumophila containing deletions of all individual cdgS genes were created and did not exhibit any observable growth defect in growth medium or inside host cells. However, when overexpressed, several cdgS genes strongly decreased the ability of L. pneumophila to grow inside host cells. Expression of these cdgS genes did not affect the Dot/Icm type IVB secretion system, the major determinant of intracellular growth in L. pneumophila. L. pneumophila strains overexpressing these cdgS genes were less cytotoxic to THP-1 macrophages than wild-type L. pneumophila but retained the ability to resist grazing by amoebae. In many cases, the intracellular-growth inhibition caused by cdgS gene overexpression was independent of diguanylate cyclase or phosphodiesterase activities. Expression of the cdgS genes in a Salmonella enterica serovar Enteritidis strain that lacks all diguanylate cyclase activity indicated that several cdgS genes encode potential cyclases. These results indicate that components of the cyclic diguanylate signaling pathway play an important role in regulating the ability of L. pneumophila to grow in host cells.

The Coxiella burnetii cryptic plasmid is enriched in genes encoding type IV secretion system substrates

The intracellular bacterial pathogen Coxiella burnetii directs biogenesis of a phagolysosome-like parasitophorous vacuole (PV), in which it replicates. The organism encodes a Dot/Icm type IV secretion system (T4SS) predicted to deliver to the host cytosol effector proteins that mediate PV formation and other cellular events. All C. burnetii isolates carry a large, autonomously replicating plasmid or have chromosomally integrated plasmid-like sequences (IPS), suggesting that plasmid and IPS genes are critical for infection. Bioinformatic analyses revealed two candidate Dot/Icm substrates with eukaryotic-like motifs uniquely encoded by the QpH1 plasmid from the Nine Mile reference isolate. CpeC, containing an F-box domain, and CpeD, possessing kinesin-related and coiled-coil regions, were secreted by the closely related Legionella pneumophila Dot/Icm T4SS. An additional QpH1-specific gene, cpeE, situated in a predicted operon with cpeD, also encoded a secreted effector. Further screening revealed that three hypothetical proteins (CpeA, CpeB, and CpeF) encoded by all C. burnetii plasmids and IPS are Dot/Icm substrates. By use of new genetic tools, secretion of plasmid effectors by C. burnetii during host cell infection was confirmed using β-lactamase and adenylate cyclase translocation assays, and a C-terminal secretion signal was identified. When ectopically expressed in HeLa cells, plasmid effectors trafficked to different subcellular sites, including autophagosomes (CpeB), ubiquitin-rich compartments (CpeC), and the endoplasmic reticulum (CpeD). Collectively, these results suggest that C. burnetii plasmid-encoded T4SS substrates play important roles in subversion of host cell functions, providing a plausible explanation for the absolute maintenance of plasmid genes by this pathogen.

Replication of Legionella Pneumophila in Human Cells: Why are We Susceptible?

Legionella pneumophila is the causative agent of Legionnaires’ disease, a serious and often fatal form of pneumonia. The susceptibility to L. pneumophila arises from the ability of this intracellular pathogen to multiply in human alveolar macrophages and monocytes. L. pneumophila also replicates in several professional and non-professional phagocytic human-derived cell lines. With the exception of the A/J mouse strain, most mice strains are restrictive, thus they do not support L. pneumophila replication. Mice lacking the NOD-like receptor Nlrc4 or caspase-1 are also susceptible to L. pneumophila. On the other hand, in the susceptible human hosts, L. pneumophila utilizes several strategies to ensure intracellular replication and protect itself against the host immune system. Most of these strategies converge to prevent the fusion of the L. pneumophila phagosome with the lysosome, inhibiting host cell apoptosis, activating survival pathways, and sequestering essential nutrients for replication and pathogenesis. In this review, we summarize survival mechanisms employed by L. pneumophila to maintain its replication in human cells. In addition, we highlight different human-derived cell lines that support the multiplication of this intracellular bacterium. Therefore, these in vitro models can be applicable and are reproducible when investigating L. pneumophila/phagocyte interactions at the molecular and cellular levels in the human host.

Evidence that intracellular stages of Leishmania major utilize amino sugars as a major carbon source

Intracellular parasites, such as Leishmania spp, must acquire suitable carbon sources from the host cell in order to replicate. Here we present evidence that intracellular amastigote stages of Leishmania exploit amino sugars in the phagolysosome of mammalian macrophages as a source of carbon and energy. L. major parasites are capable of using N-acetylglucosamine and glucosamine as primarily carbon sources and contain key enzymes required for conversion of these sugars to fructose-6-phosphate. The last step in this pathway is catalyzed by glucosamine-6-phosphate deaminase (GND), which was targeted to glycosomes via a canonical C-terminal targeting signal when expressed as a GFP fusion protein. Mutant parasites lacking GND were unable to grow in medium containing amino sugars as sole carbohydrate source and rapidly lost viability, concomitant with the hyper-accumulation of hexosamine-phosphates. Expression of native GND, but not a cytosolic form of GND, in Δgnd parasites restored hexosamine-dependent growth, indicating that toxicity is due to depletion of glycosomal pools of ATP. Non-lethal increases in hexosamine phosphate levels in both Δgnd and wild type parasites was associated with a defect in promastigote metacyclogenesis, suggesting that hexosamine phosphate levels may influence parasite differentiation. Promastigote and amastigote stages of the Δgnd mutant were unable to replicate within macrophages and were either completely cleared or exhibited reduced lesion development in highly susceptible Balb/c mice. Our results suggest that hexosamines are a major class of sugars in the macrophage phagolysosome and that catabolism of scavenged amino sugars is required to sustain essential metabolic pathways and prevent hexosamine toxicity.

Protozoan parasites belonging to the genus Leishmania are transmitted by sandfly vectors and cause a number of important diseases in humans. These parasites proliferate within mature lysosome compartments in macrophages and other phagocytic cells in the mammalian host. How intracellular stages of Leishmania survive within this hydrolytic compartment and the extent to which they utilize different carbon sources is poorly defined. Previous studies have suggested that sugar uptake is important for growth, although the nature of these sugars is unknown. In this study we show that Leishmania express all of the enzymes needed to degrade the amino sugars, glucosamine and N-acetylglucosamine. We show that a key enzyme in this pathway is sequestered within modified peroxisomes, or glycosomes, and that this localization is essential for growth on amino sugars and avoidance of amino sugar toxicity. This pathway is also required for parasite proliferation within cultured macrophages and for normal infection of highly susceptible mice. Mutant parasites are either completely eradicated or induce small lesions in Balb/c mice after an extended lag period. These findings suggest that amino sugars generated by the lysosomal breakdown of host glycoconjugates are an important carbon source for intracellular stages of Leishmania, particularly during the early stages of infection.

Host Kinase Activity is Required for Coxiella burnetii Parasitophorous Vacuole Formation

Coxiella burnetii is the etiologic agent of human Q fever and targets alveolar phagocytic cells in vivo wherein the pathogen generates a phagolysosome-like parasitophorous vacuole (PV) for replication. C. burnetii displays a prolonged growth cycle, making PV maintenance critical for bacterial survival. Previous studies showed that C. burnetii mediates activation of eukaryotic kinases to inhibit cell death, indicating the importance of host signaling during infection. In the current study, we examined the role of eukaryotic kinase signaling in PV establishment. A panel of 113 inhibitors was analyzed for their impact on C. burnetii infection of human THP-1 macrophage-like cells and HeLa cells. Inhibition of 11 kinases or two phosphatases altered PV formation and prevented pathogen growth, with most inhibitor-treated cells harboring organisms in tight-fitting phagosomes, indicating kinase/phosphatase activation is required for PV maturation. Five inhibitors targeted protein kinase C (PKC), suggesting a critical role for this protein during intracellular growth. The PKC-specific substrate MARCKS was phosphorylated at 24 h post-infection and remained phosphorylated through 5 days post-infection, indicating prolonged regulation of the PKC pathway by C. burnetii. Infection also altered the activation status of p38, myosin light chain kinase, and cAMP-dependent protein kinase, suggesting C. burnetii subverts numerous phosphorylation cascades. These results underscore the importance of intracellular host signaling for C. burnetii PV biogenesis.

Phosphoinositides differentially regulate bacterial uptake and Nramp1-induced resistance to Legionella infection in Dictyostelium

Membrane phosphatidylinositides recruit cytosolic proteins to regulate phagocytosis, macropinocytosis and endolysosomal vesicle maturation. Here, we describe effects of inactivation of PI3K, PTEN or PLC on Escherichia coli and Legionella pneumophila uptake by the professional phagocyte Dictyostelium discoideum. We show that L. pneumophila is engulfed by macropinocytosis, a process that is partially sensitive to PI3K inactivation, unlike phagocytosis of E. coli. Both processes are blocked by PLC inhibition. Whereas E. coli is rapidly digested, Legionella proliferates intracellularly. Proliferation is blocked by constitutively expressing Nramp1, an endolysosomal iron transporter that confers resistance against invasive bacteria. Inactivation of PI3K, but not PTEN or PLC, enhances Legionella infection and suppresses the protective effect of Nramp1 overexpression. PI3K activity is restricted to early infection and is not mediated by effects on the actin cytoskeleton; rather L. pneumophila, in contrast to E. coli, subverts phosphoinositide-sensitive fusion of Legionella-containing macropinosomes with acidic vesicles, without affecting Nramp1 recruitment. A model is presented to explain how Legionella escapes fusion with acidic vesicles and Nramp1-induced resistance to pathogens.

Manipulation of the host by pathogens to survive the lysosome

Lysosomes form part of our innate immunity and are an important line of defence against microbes, viruses and parasites. Although it is more than 50 years since de Duve discovered lysosomes, it is only in more recent years that we are slowly unravelling the molecular mechanisms involved in the delivery of material to the lysosome. However, successful intracellular pathogens often have a better grip on the mechanisms involved in delivery to the lysosome and can manipulate membrane trafficking pathways to create an intracellular environment that is favourable for replication. By studying pathogen effector proteins that are secreted into the host's cytosol, we can learn about both pathogen-survival mechanisms and further regulatory elements involved in trafficking to the lysosome.

Host-mediated post-translational prenylation of novel dot/icm-translocated effectors of legionella pneumophila

The Dot/Icm type IV translocated Ankyrin B (AnkB) effector of Legionella pneumophila is modified by the host prenylation machinery that anchors it into the outer leaflet of the Legionella-containing vacuole (LCV), which is essential for biological function of the effector in vitro and in vivo. Prenylation involves the covalent linkage of an isoprenoid lipid moiety to a C-terminal CaaX motif in eukaryotic proteins enabling their anchoring into membranes. We show here that the LCV harboring an ankB null mutant is decorated with prenylated proteins in a Dot/Icm-dependent manner, indicating that other LCV membrane-anchored proteins are prenylated. In silico analyses of four sequenced L. pneumophila genomes revealed the presence of eleven other genes that encode proteins with a C-terminal eukaryotic CaaX prenylation motif. Of these eleven designated Prenylated effectors of Legionella (Pel), seven are also found in L. pneumophila AA100. We show that six L. pneumophila AA100 Pel proteins exhibit distinct cellular localization when ectopically expressed in mammalian cells and this is dependent on action of the host prenylation machinery and the conserved cysteine residue of the CaaX motif. Although inhibition of the host prenylation machinery completely blocks intra-vacuolar proliferation of L. pneumophila, it only had a modest effect on intracellular trafficking of the LCV. Five of the Pel proteins are injected into human macrophages by the Dot/Icm type IV translocation system of L. pneumophila. Taken together, the Pel proteins are novel Dot/Icm-translocated effectors of L. pneumophila that are post-translationally modified by the host prenylation machinery, which enables their anchoring into cellular membranes, and the prenylated effectors contribute to evasion of lysosomal fusion by the LCV.

Apoptosis-associated speck-like protein (ASC) controls Legionella pneumophila infection in human monocytes

The ability of Legionella pneumophila to cause pneumonia is determined by its capability to evade the immune system and grow within human monocytes and their derived macrophages. Human monocytes efficiently activate caspase-1 in response to Salmonella but not to L. pneumophila. The molecular mechanism for the lack of inflammasome activation during L. pneumophila infection is unknown. Evaluation of the expression of several inflammasome components in human monocytes during L. pneumophila infection revealed that the expression of the apoptosis-associated speck-like protein (ASC) and the NOD-like receptor NLRC4 are significantly down-regulated in human monocytes. Exogenous expression of ASC maintained the protein level constant during L. pneumophila infection and conveyed caspase-1 activation and restricted the growth of the pathogen. Further depletion of ASC with siRNA was accompanied with improved NF-κB activation and enhanced L. pneumophila growth. Therefore, our data demonstrate that L. pneumophila manipulates ASC levels to evade inflammasome activation and grow in human monocytes. By targeting ASC, L. pneumophila modulates the inflammasome, the apoptosome, and NF-κB pathway simultaneously.

Exploitation of Host Polyubiquitination Machinery through Molecular Mimicry by Eukaryotic-Like Bacterial F-Box Effectors

Microbial pathogens have evolved exquisite mechanisms to interfere and intercept host biological processes, often through molecular mimicry of specific host proteins. Ubiquitination is a highly conserved eukaryotic post-translational modification essential in determining protein fate, and is often hijacked by pathogenic bacteria. The conserved SKP1/CUL1/F-box (SCF) E3 ubiquitin ligase complex plays a key role in ubiquitination of proteins in eukaryotic cells. The F-box protein component of the SCF complex provides specificity to ubiquitination by binding to specific cellular proteins, targeting them to be ubiquitinated by the SCF complex. The bacterial pathogens. Legionella pneumophila, Agrobacterium tumefaciens, and Ralstonia solanacearum utilize type III or IV translocation systems to inject into the host cell eukaryotic-like F-box effectors that interact with the host SKP1 component of the SCF complex to trigger ubiquitination of specific host cells targets, which is essential to promote proliferation of these pathogens. Our bioinformatic analyses have identified at least 74 genes encoding putative F-box proteins belonging to 22 other bacterial species, including human pathogens, plant pathogens, and amebal endosymbionts. Therefore, subversion of the host ubiquitination machinery by bacterial F-box proteins may be a widespread strategy amongst pathogenic bacteria. The findings that bacterial F-box proteins harbor Ankyrin repeats as protein–protein interaction domains, which are present in F-box proteins of primitive but not higher eukaryotes, suggest acquisition of many bacterial F-box proteins from primitive eukaryotic hosts rather than the mammalian host.

Molecular Detection of Legionella: Moving on From mip

The detection of Legionella pneumophila in environmental and clinical samples is frequently performed by PCR amplification of the mip and/or 16S rRNA genes. Combined with DNA sequencing, these two genetic loci can be used to distinguish different species of Legionella and identify L. pneumophila. However, the recent Legionella genome sequences have opened up hundreds of possibilities for the development of new molecular targets for detection and diagnosis. Ongoing comparative genomics has the potential to fine tune the identification of Legionella species and serogroups by combining specific and general genetic targets. For example, the coincident detection of LPS biosynthesis genes and virulence genes may allow the differentiation of both pathogen and serogroup without the need for nucleotide sequencing. We tested this idea using data derived from a previous genomic subtractive hybridization we performed between L. pneumophila serogroup 1 and L. micdadei. Although not yet formally tested, these targets serve as an example of how comparative genomics has the potential to improve the scope and accuracy of Legionella molecular detection if embraced by laboratories undertaking Legionella surveillance.

Antibodies protect against intracellular bacteria by Fc receptor-mediated lysosomal targeting

The protective effect of antibodies (Abs) is generally attributed to neutralization or complement activation. Using Legionella pneumophila and Mycobacterium bovis bacillus Calmette-Guérin as a model, we discovered an additional mechanism of Ab-mediated protection effective against intracellular pathogens that normally evade lysosomal fusion. We show that Fc receptor (FcR) engagement by Abs, which can be temporally and spatially separated from bacterial infection, renders the host cell nonpermissive for bacterial replication and targets the pathogens to lysosomes. This process is strictly dependent on kinases involved in FcR signaling but not on host cell protein synthesis or protease activation. Based on these findings, we propose a mechanism whereby Abs and FcR engagement subverts the strategies by which intracellular bacterial pathogens evade lysosomal degradation.

Inhibition of pathogen-induced apoptosis by a Coxiella burnetii type IV effector protein

Coxiella burnetii and Legionella pneumophila are evolutionarily related pathogens with different intracellular infection strategies. C. burnetii persists within and is transmitted by mammalian hosts, whereas, L. pneumophila is found primarily in the environment associated with protozoan hosts. Although a type IV secretion system encoded by the defect in organelle trafficking (dot) and intracellular multiplication (icm) genes is a virulence determinant that remains highly conserved in both bacteria, the two pathogens encode a different array of effector proteins that are delivered into host cells by the Dot/Icm machinery. This difference suggests that adaptations to evolutionarily distinct hosts may be reflected in the effector protein repertoires displayed by these two pathogens. Here we provide evidence in support of this hypothesis. We show that a unique C. burnetii effector from the ankyrin repeat (Ank) family called AnkG interferes with the mammalian apoptosis pathway. AnkG was found to interact with the host protein gC1qR (p32). Either the addition of AnkG to the repertoire of L. pneumophila effector proteins or the silencing of p32 in mouse dendritic cells resulted in a gain of function that allowed intracellular replication of L. pneumophila in these normally restrictive mammalian host cells by preventing rapid pathogen-induced apoptosis. These data indicate that p32 regulates pathogen-induced apoptosis and that AnkG functions to block this pathway. Thus, emergence of an effector protein that interferes with a proapoptotic signaling pathway directed against intracellular bacteria correlates with adaptation of a pathogen to mammalian hosts.

Chemical probing reveals insights into the signaling mechanism of inflammasome activation

Caspase-1-mediated IL-1β production is generally controlled by two pathways. Toll-like receptors (TLRs) recognize pathogen-derived products and induce NF-κB-dependent pro-IL-1β transcription; NOD-like receptors (NLRs) assemble caspase-1-activating inflammasome complexes that sense bacterial products/danger signals. Through a targeted chemical screen, we identify bromoxone, a marine natural product, as a specific and potent inhibitor of the caspase-1 pathway. Bromoxone is effective over diverse inflammatory stimuli including TLR ligands plus ATP/nigericin, cytosolic DNA, flagellin and Bacillus anthracis lethal toxin. Bromoxone also efficiently suppresses caspase-1 activation triggered by several types of bacterial infection. Bromoxone acts upstream or at the level of the inflammasome in a transcription-independent manner. Bromoxone also inhibits pro-IL-1β expression by targeting components upstream of IKK in the TLR-NF-κB pathway. The unique dual activities of bromoxone are shared by the known TAK1 inhibitor that specifically blocks Nalp3 inflammasome activation. Hinted from the mechanistic and pharmacological properties of bromoxone, we further discover that several known NF-κB inhibitors that act upstream of IKK, but not those targeting IKK or IKK downstream, are potent blockers of different NLRs-mediated caspase-1 activation. Our study uncovers a possible non-transcriptional molecular link between the NLR (Nalp3)-mediated inflammasome pathway and TLR-NF-κB signaling, and suggests a potential strategy to develop new anti-inflammatory drugs.

Legionella pneumophila strain 130b possesses a unique combination of type IV secretion systems and novel Dot/Icm secretion system effector proteins

Legionella pneumophila is a ubiquitous inhabitant of environmental water reservoirs. The bacteria infect a wide variety of protozoa and, after accidental inhalation, human alveolar macrophages, which can lead to severe pneumonia. The capability to thrive in phagocytic hosts is dependent on the Dot/Icm type IV secretion system (T4SS), which translocates multiple effector proteins into the host cell. In this study, we determined the draft genome sequence of L. pneumophila strain 130b (Wadsworth). We found that the 130b genome encodes a unique set of T4SSs, namely, the Dot/Icm T4SS, a Trb-1-like T4SS, and two Lvh T4SS gene clusters. Sequence analysis substantiated that a core set of 107 Dot/Icm T4SS effectors was conserved among the sequenced L. pneumophila strains Philadelphia-1, Lens, Paris, Corby, Alcoy, and 130b. We also identified new effector candidates and validated the translocation of 10 novel Dot/Icm T4SS effectors that are not present in L. pneumophila strain Philadelphia-1. We examined the prevalence of the new effector genes among 87 environmental and clinical L. pneumophila isolates. Five of the new effectors were identified in 34 to 62% of the isolates, while less than 15% of the strains tested positive for the other five genes. Collectively, our data show that the core set of conserved Dot/Icm T4SS effector proteins is supplemented by a variable repertoire of accessory effectors that may partly account for differences in the virulences and prevalences of particular L. pneumophila strains.

Exploitation of conserved eukaryotic host cell farnesylation machinery by an F-box effector of Legionella pneumophila

Farnesylation involves covalent linkage of eukaryotic proteins to a lipid moiety to anchor them into membranes, which is essential for the biological function of Ras and other proteins. A large cadre of bacterial effectors is injected into host cells by intravacuolar pathogens through elaborate type III–VII translocation machineries, and many of these effectors are incorporated into the pathogen-containing vacuolar membrane by unknown mechanisms. The Dot/Icm type IV secretion system of Legionella pneumophila injects into host cells the F-box effector Ankyrin B (AnkB), which functions as platforms for the docking of polyubiquitinated proteins to the Legionella-containing vacuole (LCV) to enable intravacuolar proliferation in macrophages and amoeba. We show that farnesylation of AnkB is indispensable for its anchoring to the cytosolic face of the LCV membrane, for its biological function within macrophages and Dictyostelium discoideum, and for intrapulmonary proliferation in mice. Remarkably, the protein farnesyltransferase, RCE-1 (Ras-converting enzyme-1), and isoprenyl cysteine carboxyl methyltransferase host farnesylation enzymes are recruited to the LCV in a Dot/Icm-dependent manner and are essential for the biological function of AnkB. In conclusion, this study shows novel localized recruitment of the host farnesylation machinery and its anchoring of an F-box effector to the LCV membrane, and this is essential for biological function in vitro and in vivo.

The Legionella effector protein DrrA AMPylates the membrane traffic regulator Rab1b

In the course of Legionnaires' disease, the bacterium Legionella pneumophila affects the intracellular vesicular trafficking of infected eukaryotic cells by recruiting the small guanosine triphosphatase (GTPase) Rab1 to the cytosolic face of the Legionella-containing vacuole. In order to accomplish this, the Legionella protein DrrA contains a specific guanine nucleotide exchange activity for Rab1 activation that exchanges guanosine triphosphate (GTP) for guanosine diphosphate on Rab1. We found that the amino-terminal domain of DrrA possesses adenosine monophosphorylation (AMPylation) activity toward the switch II region of Rab1b, leading to posttranslational covalent modification of tyrosine 77. AMPylation of switch II by DrrA restricts the access of GTPase activating proteins, thereby rendering Rab1b constitutively active.

High-affinity binding of phosphatidylinositol 4-phosphate by Legionella pneumophila DrrA

The DrrA protein of Legionella pneumophila is involved in mistargeting of endoplasmic reticulum-derived vesicles to Legionella-containing vacuoles through recruitment of the small GTPase Rab1. To this effect, DrrA binds specifically to phosphatidylinositol 4-phosphate (PtdIns(4)P) lipids on the cytosolic surface of the phagosomal membrane shortly after infection. In this study, we present the atomic structure of the PtdIns(4)P-binding domain of a protein (DrrA) from a human pathogen. A detailed kinetic investigation of its interaction with PtdIns(4)P reveals that DrrA binds to this phospholipid with, as yet unprecedented, high affinity, suggesting that DrrA can sense a very low abundance of the lipid.

Legionella pneumophila induces human beta defensin-3 in pulmonary cells

Background

Legionella pneumophila is an important causative agent of severe pneumonia in humans. Human alveolar epithelium and macrophages are effective barriers for inhaled microorganisms and actively participate in the initiation of innate host defense. The beta defensin-3 (hBD-3), an antimicrobial peptide is an important component of the innate immune response of the human lung. Therefore we hypothesize that hBD-3 might be important for immune defense towards L. pneumophila.

Methods

We investigated the effects of L. pneumophila and different TLR agonists on pulmonary cells in regard to hBD-3 expression by ELISA. Furthermore, siRNA-mediated inhibition of TLRs as well as chemical inhibition of potential downstream signaling molecules was used for functional analysis.

Results

L. pneumophila induced release of hBD-3 in pulmonary epithelium and alveolar macrophages. A similar response was observed when epithelial cells were treated with different TLR agonists. Inhibition of TLR2, TLR5, and TLR9 expression led to a decreased hBD-3 expression. Furthermore expression of hBD-3 was mediated through a JNK dependent activation of AP-1 (c-Jun) but appeared to be independent of NF-κB. Additionally, we demonstrate that hBD-3 elicited a strong antimicrobial effect on L. pneumophila replication.

Conclusions

Taken together, human pulmonary cells produce hBD-3 upon L. pneumophila infection via a TLR-JNK-AP-1-dependent pathway which may contribute to an efficient innate immune defense.

Bactericidal and anti-inflammatory properties of a standardized Echinacea extract (Echinaforce): dual actions against respiratory bacteria

Common symptoms of upper respiratory infections, such as sore throat, cough, and inflammation, are often caused by bacteria, sometimes as a complication of virus infection. Extracts of Echinacea purpurea (Asteraceae) have been advocated traditionally for use by individuals suffering from these symptoms, although the underlying basis for the beneficial effects of Echinacea is not known. We hypothesized that Echinacea could inactivate certain respiratory bacteria and could also reverse inflammatory effects caused by these bacteria in epithelial cells. In order to test this we used a commercial standardized extract of Echinacea purpurea (Echinaforce), and a novel cytokine array system designed to measure simultaneously the levels of 20 different cytokines secreted by bronchial epithelial cell cultures in response to infection. Streptococcus pyogenes (Group A Strep), which is often associated with sore throat and more severe pulmonary infections, was readily inactivated by Echinacea, which also completely reversed the cellular pro-inflammatory response. Hemophilus influenzae and Legionella pneumophila were also readily inactivated, and their pro-inflammatory responses reversed. Staphylococcus aureus (methicillin-resistant and sensitive strains) and Mycobacterium smegmatis were less sensitive to the bactericidal effects of Echinacea however, but their pro-inflammatory responses were still completely reversed. In contrast some other pathogens tested, including Candida albicans, were relatively resistant. Thus Echinaforce) exerts a dual action against several important respiratory bacteria, a killing effect and an anti-inflammatory effect. These results support the concept of using a standardized Echinacea preparation to control symptoms associated with bacterial respiratory infections.

Molecular pathogenesis of infections caused by Legionella pneumophila

The genus Legionella contains more than 50 species, of which at least 24 have been associated with human infection. The best-characterized member of the genus, Legionella pneumophila, is the major causative agent of Legionnaires' disease, a severe form of acute pneumonia. L. pneumophila is an intracellular pathogen, and as part of its pathogenesis, the bacteria avoid phagolysosome fusion and replicate within alveolar macrophages and epithelial cells in a vacuole that exhibits many characteristics of the endoplasmic reticulum (ER). The formation of the unusual L. pneumophila vacuole is a feature of its interaction with the host, yet the mechanisms by which the bacteria avoid classical endosome fusion and recruit markers of the ER are incompletely understood. Here we review the factors that contribute to the ability of L. pneumophila to infect and replicate in human cells and amoebae with an emphasis on proteins that are secreted by the bacteria into the Legionella vacuole and/or the host cell. Many of these factors undermine eukaryotic trafficking and signaling pathways by acting as functional and, in some cases, structural mimics of eukaryotic proteins. We discuss the consequences of this mimicry for the biology of the infected cell and also for immune responses to L. pneumophila infection.

E3 ubiquitin ligase activity and targeting of BAT3 by multiple Legionella pneumophila translocated substrates

The intracellular bacterial pathogen Legionella pneumophila modulates a number of host processes during intracellular growth, including the eukaryotic ubiquitination machinery, which dictates the stability, activity, and/or localization of a large number of proteins. A number of L. pneumophila proteins contain eukaryotic-like motifs typically associated with ubiquitination. Central among these is a family of five F-box-domain-containing proteins of Legionella pneumophila. Each of these five proteins is translocated to the host cytosol by the Dot/Icm type IV protein translocation system during infection. We show that three of these proteins, LegU1, LegAU13, and LicA, interact with components of the host ubiquitination machinery in vivo. In addition, LegU1 and LegAU13 are integrated into functional Skp-Cullin-F-box (SCF) complexes that confer E3 ubiquitin ligase activity. LegU1 specifically interacts with and can direct the ubiquitination of the host chaperone protein BAT3. In a screen for additional L. pneumophila proteins that associate with LegU1 in mammalian cells, we identified the bacterial protein Lpg2160. We demonstrate that Lpg2160 also associates with BAT3 independently of LegU1. We show that Lpg2160 is a translocated substrate of the Dot/Icm system and contains a C-terminal translocation signal. We propose a model in which LegU1 and Lpg2160 may function redundantly or in concert to modulate BAT3 activity during the course of infection.

Manipulation of host membrane machinery by bacterial pathogens

Subversion of host membrane machinery is important for the uptake, survival, and replication of bacterial pathogens. Understanding how pathogens manipulate host membrane transport pathways provides mechanistic insight into how infection occurs and is also revealing new information on biochemical processes involved in the functioning of eukaryotic cells. In this review we discuss several of the canonical host pathways targeted by bacterial pathogens and emerging areas of investigation in this exciting field.

Isolation of Legionella-containing vacuoles by immuno-magnetic separation

The environmental bacterium Legionella pneumophila naturally parasitizes free-living amoebae. L. pneumophila is an opportunistic human pathogen that grows in macrophages, thus causing a life-threatening pneumonia termed Legionnaires' disease. The bacteria replicate intracellularly in environmental and immune phagocytes within a unique compartment, the Legionella-containing vacuole (LCV). Formation of LCVs is a complex and robust process involving >150 secreted bacterial effector proteins, which are believed to subvert host cell signaling and vesicle trafficking pathways. This unit describes a simple approach to purify intact LCVs from Dictyostelium discoideum amoebae. The method comprises a two-step purification protocol that includes immuno-magnetic separation by means of an antibody against an effector protein specifically binding to LCVs, followed by density gradient centrifugation. The use of D. discoideum producing a fluorescent LCV marker and fluorescently labeled L. pneumophila allow tracking the enrichment of LCVs by light microscopy.

Type IV secretion machinery promotes ton-independent intracellular survival of Neisseria gonorrhoeae within cervical epithelial cells

Survival of Neisseria gonorrhoeae within host epithelial cells is expected to be important in the pathogenesis of gonococcal disease. We previously demonstrated that strain FA1090 derives iron from a host cell in a process that requires the Ton complex and a putative TonB-dependent transporter, TdfF. FA1090, however, lacks the gonococcal genetic island (GGI) that is present in the majority of strains. The GGI in strain MS11 has been partially characterized, and it encodes a type IV secretion system (T4SS) involved in DNA release. In this study we investigated the role of iron acquisition and GGI-encoded gene products in gonococcal survival within cervical epithelial cells. We demonstrated that intracellular survival of MS11 was dependent on acquisition of iron from the host cell, but unlike the findings for FA1090, expression of the Ton complex was not required. Survival was not dependent on a putative TonB-like protein encoded in the GGI but instead was directly linked to T4SS structural components in a manner independent of the ability to release or internalize DNA. These data suggest that expression of selected GGI-encoded open reading frames confers an advantage during cervical cell infection. This study provides the first link between expression of the T4SS apparatus and intracellular survival of gonococci.

ppGpp conjures bacterial virulence

Like for all microbes, the goal of every pathogen is to survive and replicate. However, to overcome the formidable defenses of their hosts, pathogens are also endowed with traits commonly associated with virulence, such as surface attachment, cell or tissue invasion, and transmission. Numerous pathogens couple their specific virulence pathways with more general adaptations, like stress resistance, by integrating dedicated regulators with global signaling networks. In particular, many of nature's most dreaded bacteria rely on nucleotide alarmones to cue metabolic disturbances and coordinate survival and virulence programs. Here we discuss how components of the stringent response contribute to the virulence of a wide variety of pathogenic bacteria.

Induction of type I interferons by bacteria

Type I interferons (IFNs) are secreted cytokines that orchestrate diverse immune responses to infection. Although typically considered to be most important in the response to viruses, type I IFNs are also induced by most, if not all, bacterial pathogens. Although diverse mechanisms have been described, bacterial induction of type I IFNs occurs upon stimulation of two main pathways: (i) Toll-like receptor (TLR) recognition of bacterial molecules such as lipopolysaccharide (LPS); (ii) TLR-independent recognition of molecules delivered to the host cell cytosol. Cytosolic responses can be activated by two general mechanisms. First, viable bacteria can secrete stimulatory ligands into the cytosol via specialized bacterial secretion systems. Second, ligands can be released from bacteria that lyse or are degraded. The bacterial ligands that induce the cytosolic pathways remain uncertain in many cases, but appear to include various nucleic acids. In this review, we discuss recent advances in our understanding of how bacteria induce type I interferons and the roles type I IFNs play in host immunity.

Cellular pharmacodynamics of the novel biaryloxazolidinone radezolid: studies with infected phagocytic and nonphagocytic cells, using Staphylococcus aureus, Staphylococcus epidermidis, Listeria monocytogenes, and Legionella pneumophila

Radezolid is a novel biaryloxazolidinone in clinical development which shows improved activity, including against linezolid-resistant strains. In a companion paper (29), we showed that radezolid accumulates about 11-fold in phagocytic cells, with approximately 60% of the drug localized in the cytosol and approximately 40% in the lysosomes of the cells. The present study examines its activity against (i) bacteria infecting human THP-1 macrophages and located in different subcellular compartments (Listeria monocytogenes, cytosol; Legionella pneumophila, vacuoles; Staphylococcus aureus and Staphylococcus epidermidis, mainly phagolysosomal), (ii) strains of S. aureus with clinically relevant mechanisms of resistance, and (iii) isogenic linezolid-susceptible and -resistant S. aureus strains infecting a series of phagocytic and nonphagocytic cells. Radezolid accumulated to similar levels ( approximately 10-fold) in all cell types (human keratinocytes, endothelial cells, bronchial epithelial cells, osteoblasts, macrophages, and rat embryo fibroblasts). At equivalent weight concentrations, radezolid proved consistently 10-fold more potent than linezolid in all these models, irrespective of the bacterial species and resistance phenotype or of the cell type infected. This results from its higher intrinsic activity and higher cellular accumulation. Time kill curves showed that radezolid's activity was more rapid than that of linezolid both in broth and in infected macrophages. These data suggest the potential interest of radezolid for recurrent or persistent infections where intracellular foci play a determinant role.

Bacterial gene regulation by alpha-hydroxyketone signaling

Bacteria produce diffusible, small signaling molecules termed autoinducers to promote cell-cell communication. Recently, a novel class of signaling molecules, the alpha-hydroxyketones (AHKs), was discovered in the facultative human pathogens Legionella pneumophila and Vibrio cholerae. In this review, we summarize and compare findings on AHK signaling in these bacteria. The L. pneumophila lqs (Legionella quorum sensing) and V. cholerae cqs (cholera quorum sensing) gene clusters synthesize and detect Legionella autoinducer 1 (3-hydroxypentadecan-4-one) or cholera autoinducer-1 (3-hydroxytridecan-4-one), respectively. In addition to the autoinducer synthase and cognate sensor kinase encoded in the cqs locus, the lqs cluster also harbors a prototypic response regulator. AHK signaling regulates pathogen-host cell interactions, bacterial virulence, formation of biofilms or extracellular filaments, and expression of a genomic island. The lqs/cqs gene cluster is present in several environmental bacteria, suggesting that AHKs are widely used for cell-cell signaling.

Immunology taught by bacteria

Introduction

It has been proposed that the innate immune system might discriminate living and virulent pathogens from dead or harmless microbes, but the molecular mechanisms by which this discrimination could occur remain unclear. Although studies of model antigens and adjuvants have illuminated important principles underlying immune responses, the specific immune responses made to living, virulent pathogens can only be discovered by studies of the living, virulent pathogens themselves.

Methods and Findings

Here, I review what one particular bacterium, Legionella pneumophila, has taught us about the innate immune response. Pathogens differ greatly in the mechanisms they use to invade, replicate within, and transmit among their hosts. However, a theme that emerges is that the pathogenic activities sensed by host cells are conserved among multiple pathogenic bacteria.

Conclusion

Thus, immunology taught by L. pneumophila may lead to a more general understanding of the host response to infection.

Burkholderia cenocepacia creates an intramacrophage replication niche in zebrafish embryos, followed by bacterial dissemination and establishment of systemic infection

Bacteria belonging to the "Burkholderia cepacia complex" (Bcc) often cause fatal pulmonary infections in cystic fibrosis patients, yet little is know about the underlying molecular mechanisms. These Gram-negative bacteria can adopt an intracellular lifestyle, although their ability to replicate intracellularly has been difficult to demonstrate. Here we show that Bcc bacteria survive and multiply in macrophages of zebrafish embryos. Local dissemination by nonlytic release from infected cells was followed by bacteremia and extracellular replication. Burkholderia cenocepacia isolates belonging to the epidemic electrophoretic type 12 (ET12) lineage were highly virulent for the embryos; intravenous injection of <10 bacteria of strain K56-2 killed embryos within 3 days. However, small but significant differences between the clonal ET12 isolates K56-2, J2315, and BC7 were evident. In addition, the innate immune response in young embryos was sufficiently developed to control infection with other less virulent Bcc strains, such as Burkholderia vietnamiensis FC441 and Burkholderia stabilis LMG14294. A K56-2 cepR quorum-sensing regulator mutant was highly attenuated, and its ability to replicate and spread to neighboring cells was greatly reduced. Our data indicate that the zebrafish embryo is an excellent vertebrate model to dissect the molecular basis of intracellular replication and the early innate immune responses in this intricate host-pathogen interaction.

Inhibition of host vacuolar H+-ATPase activity by a Legionella pneumophila effector

Legionella pneumophila is an intracellular pathogen responsible for Legionnaires' disease. This bacterium uses the Dot/Icm type IV secretion system to inject a large number of bacterial proteins into host cells to facilitate the biogenesis of a phagosome permissive for its intracellular growth. Like many highly adapted intravacuolar pathogens, L. pneumophila is able to maintain a neutral pH in the lumen of its phagosome, particularly in the early phase of infection. However, in all cases, the molecular mechanisms underlying this observation remain unknown. In this report, we describe the identification and characterization of a Legionella protein termed SidK that specifically targets host v-ATPase, the multi-subunit machinery primarily responsible for organelle acidification in eukaryotic cells. Our results indicate that after being injected into infected cells by the Dot/Icm secretion system, SidK interacts with VatA, a key component of the proton pump. Such binding leads to the inhibition of ATP hydrolysis and proton translocation. When delivered into macrophages, SidK inhibits vacuole acidification and impairs the ability of the cells to digest non-pathogenic E. coli. We also show that a domain located in the N-terminal portion of SidK is responsible for its interactions with VatA. Furthermore, expression of sidK is highly induced when bacteria begin to enter new growth cycle, correlating well with the potential temporal requirement of its activity during infection. Our results indicate that direct targeting of v-ATPase by secreted proteins constitutes a virulence strategy for L. pneumophila, a vacuolar pathogen of macrophages and amoebae.

Molecular characterization of the Dot/Icm-translocated AnkH and AnkJ eukaryotic-like effectors of Legionella pneumophila

Although most Dot/Icm-translocated effectors of Legionella pneumophila are not required for intracellular proliferation, the eukaryotic-like ankyrin effectors, AnkH and AnkJ are required for intracellular proliferation. In this report, we show that the IcmSW chaperones are essential for translocation of AnkJ but not AnkH. The 10 C-terminal residues and the ANK domains of AnkH and AnkJ are required for translocation. Our data indicate that the two ANK domains of AnkH are critical domains required for the function of the effector in intracellular replication of L. pneumophila. The ankH and ankJ mutants are severely defective in intrapulmonary proliferation in mice. Expression of AnkH and AnkJ fusions within HEK293 cells show a punctuate distribution in the cytosol but no association with endocytic vesicles, the Golgi apparatus or the endoplasmic reticulum. Interestingly, the defect in intracellular proliferation of the ankH or ankJ mutants is rescued in HEK293 cells expressing the respective protein. We conclude that AnkH and AnkJ are effectors translocated by the Dot/Icm system by distinct mechanisms and modulate distinct cytosolic processes in the host cell.

Indispensable role for the eukaryotic-like ankyrin domains of the ankyrin B effector of Legionella pneumophila within macrophages and amoebae

The Dot/Icm-translocated ankyrin B (AnkB) effector of Legionella pneumophila exhibits molecular mimicry of eukaryotic F-box proteins and is essential for intracellular replication in macrophages and protozoa. In addition to two eukaryotic-like ankyrin (ANK) domains, AnkB harbors a conserved eukaryotic F-box domain, which is involved in polyubiquitination of proteins throughout the eukaryotic kingdom. We have recently shown that the F-box domain of the AnkB effector is essential for decoration of the Legionella-containing vacuole (LCV) with polyubiquitinated proteins within macrophages and protozoan hosts. To decipher the role of the two ANK domains in the function of AnkB, we have constructed in-frame deletion of either or both of the ANK domain-encoding regions (ankB Delta A1, ankB Delta A2, and ankB Delta A1A2) to trans-complement the ankB null mutant. Deletion of the ANK domains results in defects in intracellular proliferation and decoration of the LCV with polyubiquitinated proteins. Export of the truncated variants of AnkB was reduced, and this may account for the observed defects. However, while full-length AnkB ectopically expressed in mammalian cells trans-rescues the ankB null mutant for intracellular proliferation, ectopic expression of AnkB Delta A1, AnkB Delta A2, and AnkB Delta A1A2 fails to trans-rescue the ankB null mutant. Importantly, ectopically expressed full-length AnkB is targeted to the host cell plasma membrane, where it recruits polyubiquitinated proteins. In contrast, AnkB Delta A1, AnkB Delta A2, and AnkB Delta A1A2 are diffusely distributed throughout the cytosol and fail to recruit polyubiquitinated proteins. We conclude that the two eukaryotic-like ANK domains of AnkB are essential for intracellular proliferation, for targeting AnkB to the host membranes, and for decoration of the LCV with polyubiquitinated proteins.

Distinct roles of ppGpp and DksA in Legionella pneumophila differentiation

To transit between hosts, intracellular Legionella pneumophila transform into a motile, infectious, transmissive state. Here we exploit the pathogen's life cycle to examine how guanosine tetraphosphate (ppGpp) and DksA cooperate to govern bacterial differentiation. Transcriptional profiling revealed that during transmission alarmone accumulation increases the mRNA for flagellar and Type IV-secretion components, secreted host effectors and regulators, and decreases transcripts for translation, membrane modification and ATP synthesis machinery. DksA is critical for differentiation, since mutants are defective for stationary phase survival, flagellar gene activation, lysosome avoidance and macrophage cytotoxicity. The roles of ppGpp and DksA depend on the context. For macrophage transmission, ppGpp is essential, whereas DksA is dispensable, indicating that ppGpp can act autonomously. In broth, DksA promotes differentiation when ppGpp levels increase, or during fatty acid stress, as judged by flaA expression and evasion of degradation by macrophages. For flagella morphogenesis, DksA is required for basal fliA (sigma(28)) promoter activity. When alarmone levels increase, DksA cooperates with ppGpp to generate a pulse of Class II rod RNA or to amplify the Class III sigma factor and Class IV flagellin RNAs. Thus, DksA responds to the level of ppGpp and other stress signals to co-ordinate L. pneumophila differentiation.

Structural mechanism of host Rab1 activation by the bifunctional Legionella type IV effector SidM/DrrA

Bacterial pathogens deliver effector proteins with diverse biochemical activities into host cells, thereby modulating various host functions. Legionella pneumophila hijacks host vesicle trafficking to avoid phagosome-lysosome fusion, a mechanism that is dependent on the Legionella Dot/Icm type IV secretion system. SidM/DrrA, a Legionella type IV effector, is important for the interactions of Legionella-containing vacuoles with host endoplasmic reticulum-derived vesicles. SidM is the only known protein that catalyzes both the exchange of GDP for GTP and GDI displacement from small GTPase Rab1. We determined the crystal structures of SidM alone (residues 317-647) and SidM (residues 193-550) in complex with nucleotide-free WT Rab1. The SidM structure contains an N-terminal helical domain with a potential new function, a Rab1-activation domain, and a C-terminal phosphatidylinositol 4-phosphate-binding P4M domain. The Rab1-activation domain has extensive strong interactions mainly with Rab1 switch I and II regions that undergo substantial conformational changes on SidM binding. Mutations of switch-contacting residues in SidM attenuate both the nucleotide exchange and GDI displacement activities. Structural comparisons of Rab1 in the SidM complex with Rab1-GDP and Ypt1-GDP in the GDI complex identify key conformational changes that disrupt the nucleotide and GDI binding of Rab1. Further biochemical and structural analyses reveal a unique mechanism of coupled GDP release and GDI displacement likely triggered by the SidM-induced drastic displacement of switch I of Rab1.

Analysis of the Legionella longbeachae genome and transcriptome uncovers unique strategies to cause Legionnaires' disease

Legionella pneumophila and L. longbeachae are two species of a large genus of bacteria that are ubiquitous in nature. L. pneumophila is mainly found in natural and artificial water circuits while L. longbeachae is mainly present in soil. Under the appropriate conditions both species are human pathogens, capable of causing a severe form of pneumonia termed Legionnaires' disease. Here we report the sequencing and analysis of four L. longbeachae genomes, one complete genome sequence of L. longbeachae strain NSW150 serogroup (Sg) 1, and three draft genome sequences another belonging to Sg1 and two to Sg2. The genome organization and gene content of the four L. longbeachae genomes are highly conserved, indicating strong pressure for niche adaptation. Analysis and comparison of L. longbeachae strain NSW150 with L. pneumophila revealed common but also unexpected features specific to this pathogen. The interaction with host cells shows distinct features from L. pneumophila, as L. longbeachae possesses a unique repertoire of putative Dot/Icm type IV secretion system substrates, eukaryotic-like and eukaryotic domain proteins, and encodes additional secretion systems. However, analysis of the ability of a dotA mutant of L. longbeachae NSW150 to replicate in the Acanthamoeba castellanii and in a mouse lung infection model showed that the Dot/Icm type IV secretion system is also essential for the virulence of L. longbeachae. In contrast to L. pneumophila, L. longbeachae does not encode flagella, thereby providing a possible explanation for differences in mouse susceptibility to infection between the two pathogens. Furthermore, transcriptome analysis revealed that L. longbeachae has a less pronounced biphasic life cycle as compared to L. pneumophila, and genome analysis and electron microscopy suggested that L. longbeachae is encapsulated. These species-specific differences may account for the different environmental niches and disease epidemiology of these two Legionella species.