Reduced expression of the global regulator protein CsrA in Legionella pneumophila affects virulence-associated regulators and growth in Acanthamoeba castellanii

Molecular regulation of virulence in Legionella pneumophila

Legionella pneumophila is a gram-negative bacteria found in natural and anthropogenic aquatic environments such as evaporative cooling towers, where it reproduces as an intracellular parasite of cohabiting protozoa. If L. pneumophila is aerosolized and inhaled by a susceptible person, bacteria may colonize their alveolar macrophages causing the opportunistic pneumonia Legionnaires' disease. L. pneumophila utilizes an elaborate regulatory network to control virulence processes such as the Dot/Icm Type IV secretion system and effector repertoire, responding to changing nutritional cues as their host becomes depleted. The bacteria subsequently differentiate to a transmissive state that can survive in the environment until a replacement host is encountered and colonized. In this review, we discuss the lifecycle of L. pneumophila and the molecular regulatory network that senses nutritional depletion via the stringent response, a link to stationary phase-like metabolic changes via alternative sigma factors, and two-component systems that are homologous to stress sensors in other pathogens, to regulate differentiation between the intracellular replicative phase and more transmissible states. Together, we highlight how this prototypic intracellular pathogen offers enormous potential in understanding how molecular mechanisms enable intracellular parasitism and pathogenicity.

The LysR-type transcriptional regulator LelA co-regulates various effectors in different Legionella species

The intracellular pathogen Legionella pneumophila translocates more than 300 effector proteins into its host cells. The expression levels of the genes encoding these effectors are orchestrated by an intricate regulatory network. Here, we introduce LelA, the first L. pneumophila LysR-type transcriptional regulator of effectors. Through bioinformatic and experimental analyses, we identified the LelA target regulatory element and demonstrated that it directly activates the expression of three L. pneumophila effectors (legL7, legL6, and legU1). We further found that the gene encoding LelA is positively regulated by the RpoS sigma factor, thus linking it to the known effector regulatory network. Examination of other species throughout the Legionella genus revealed that this regulatory element is found upstream of 34 genes encoding validated effectors, putative effectors, and hypothetical proteins. Moreover, ten of these genes were examined and found to be activated by the L. pneumophila LelA as well as by their orthologs in the corresponding species. LelA represents a novel type of Legionella effector regulator, which coordinates the expression of both adjacently and distantly located effector-encoding genes, thus forming small groups of co-regulated effectors.

Developing New Tools to Fight Human Pathogens: A Journey through the Advances in RNA Technologies

A long scientific journey has led to prominent technological advances in the RNA field, and several new types of molecules have been discovered, from non-coding RNAs (ncRNAs) to riboswitches, small interfering RNAs (siRNAs) and CRISPR systems. Such findings, together with the recognition of the advantages of RNA in terms of its functional performance, have attracted the attention of synthetic biologists to create potent RNA-based tools for biotechnological and medical applications. In this review, we have gathered the knowledge on the connection between RNA metabolism and pathogenesis in Gram-positive and Gram-negative bacteria. We further discuss how RNA techniques have contributed to the building of this knowledge and the development of new tools in synthetic biology for the diagnosis and treatment of diseases caused by pathogenic microorganisms. Infectious diseases are still a world-leading cause of death and morbidity, and RNA-based therapeutics have arisen as an alternative way to achieve success. There are still obstacles to overcome in its application, but much progress has been made in a fast and effective manner, paving the way for the solid establishment of RNA-based therapies in the future.

Autorepressor PsrA is required for optimal Legionella pneumophila growth in Acanthamoeba castellanii protozoa

Legionella pneumophila possesses a unique intracellular lifecycle featuring distinct morphological stages that include replicative forms and transmissive cyst forms. Expression of genes associated with virulence traits and cyst morphogenesis is concomitant, and governed by a complex stringent response based-regulatory network and the stationary phase sigma factor RpoS. In Pseudomonas spp., rpoS expression is controlled by the autorepressor PsrA, and orthologs of PsrA and RpoS are required for cyst formation in Azotobacter. Here we report that the L. pneumophila psrA ortholog, expressed as a leaderless monocistronic transcript, is also an autorepressor, but is not a regulator of rpoS expression. Further, the binding site sequence recognized by L. pneumophila PsrA is different from that of Pseudomonas PsrA, suggesting a repertoire of target genes unique to L. pneumophila. While PsrA was dispensable for growth in human U937-derived macrophages, lack of PsrA affected bacterial intracellular growth in Acanthamoeba castellanii protozoa, but also increased the quantity of poly-3-hydroxybutyrate (PHB) inclusions in matured transmissive cysts. Interestingly, overexpression of PsrA increased the size and bacterial load of the replicative vacuole in both host cell types. Taken together, we report that PsrA is a host-specific requirement for optimal temporal progression of L. pneumophila intracellular lifecycle in A. castellanii.

The small regulatory RNA Lpr10 regulates the expression of RpoS in Legionella pneumophila

Legionella pneumophila (Lp) is a waterborne bacterium able to infect human alveolar macrophages, causing Legionnaires' disease. Lp can survive for several months in water, while searching for host cells to grow in, such as ciliates and amoeba. In Lp, the sigma factor RpoS is essential for survival in water. A previous transcriptomic study showed that RpoS positively regulates the small regulatory RNA Lpr10. In the present study, deletion of lpr10 results in an increased survival of Lp in water. Microarray analysis and RT-qPCR revealed that Lpr10 negatively regulates the expression of RpoS in the postexponential phase. Electrophoretic mobility shift assay and in-line probing showed that Lpr10 binds to a region upstream of the previously identified transcription start sites (TSS) of rpoS. A third putative transcription start site was identified by primer extension analysis, upstream of the Lpr10 binding site. In addition, nlpD TSS produces a polycistronic mRNA including the downstream gene rpoS, indicating a fourth TSS for rpoS. Our results suggest that the transcripts from the third and fourth TSS are negatively regulated by the Lpr10 sRNA. Therefore, we propose that Lpr10 is involved in a negative regulatory feedback loop to maintain expression of RpoS to an optimal level.

A Novel Legionella Genomic Island Encodes a Copper-Responsive Regulatory System and a Single Icm/Dot Effector Protein Transcriptionally Activated by Copper

Legionella pneumophila is an intracellular human pathogen that utilizes amoebae as its environmental host. The adaptation of L. pneumophila to the intracellular environment requires coordination of expression of its multicomponent pathogenesis system, which is composed of a secretion system and effector proteins. However, the regulatory factors controlling the expression of this pathogenesis system are only partially uncovered. Here, we discovered a novel regulatory system that is activated by copper and controls the expression of a single effector protein. The genes encoding both the regulatory system and the effector protein are located on a genomic island that undergoes horizontal gene transfer within the Legionella genus. This regulator-effector genomic island represents the first reported case of local regulation of effectors in Legionella. The discovery of this regulatory mechanism is an important step forward in the understanding of how the regulatory network of effectors functions and evolves in the Legionella genus.

The intracellular pathogen Legionella pneumophila utilizes the Icm/Dot type IV secretion system to translocate >300 effector proteins into host cells during infection. The regulation of some of these effector-encoding genes was previously shown to be coordinated by several global regulators, including three two-component systems (TCSs) found in all the Legionella species examined. Here, we describe the first Legionella genomic island encoding a single Icm/Dot effector and a dedicated TCS, which regulates its expression. This genomic island, which we named Lci, undergoes horizontal gene transfer in the Legionella genus, and the TCS encoded from this island (LciRS) is homologous to TCSs that control the expression of various metal resistance systems found in other bacteria. We found that the L. pneumophila sensor histidine kinase LciS is specifically activated by copper via a unique, small periplasmic sensing domain. Upon activation by LciS, the response regulator LciR directly binds to a conserved regulatory element and activates the expression of the adjacently located lciE effector-encoding gene. Thus, LciR represents the first local regulator of effectors identified in L. pneumophila. Moreover, we found that the expression of the lciRS operon is repressed by the Fis1 and Fis3 regulators, leading to Fis-mediated effects on copper induction of LciE and silencing of the expression of this genomic island in the absence of copper. This island represents a novel type of effector regulation in Legionella, shedding new light on the ways by which the Legionella pathogenesis system evolves its effector repertoire and expands its activating signals.

The Temporal Expression of Global Regulator Protein CsrA Is Dually Regulated by ClpP During the Biphasic Life Cycle of Legionella pneumophila

Legionella pneumophila, an environmental bacterium that parasitizes protozoa, is the causative pathogen of Legionnaires' disease. L. pneumophila adopts a distinct biphasic life cycle that allows it to adapt to environmental conditions for survival, replication, and transmission. This cycle consists of a non-virulent replicative phase (RP) and a virulent transmissive phase (TP). Timely and fine-tuned expression of growth and virulence factors in a life cycle-dependent manner is crucial. Herein, we report evidence that CsrA, a key regulator of the switch between the RP and the TP, is dually regulated in a ClpP-dependent manner during the biphasic life cycle of L. pneumophila. First, we show that the protein level of CsrA is temporal during the life cycle and is degraded by ClpP during the TP. The ectopic expression of CsrA in a ΔclpP mutant, but not in the wild type, inhibits both the initiation of the RP in vitro and the invasiveness to Acanthamoeba castellanii, indicating that the ClpP-mediated proteolytic pathway regulates the CsrA protein level. We further show that the temporally expressed IHFB is the transcriptional inhibitor of csrA and is degraded via a ClpP-dependent manner during the RP. During the RP, the level of CsrA is increased by promoting the degradation of IHFB and reducing the degradation of the accumulated CsrA via a ClpP-dependent manner. During the TP, the level of CsrA is decreased by inhibiting the degradation of IHFB and promoting the degradation of the accumulated CsrA via a ClpP-dependent manner as well. In conclusion, our results show that the growth-stage-specific expression level of CsrA is dually regulated by ClpP-dependent proteolysis at both the transcription and protein levels during the biphasic life cycle of L. pneumophila.

Legionella pneumophila CsrA regulates a metabolic switch from amino acid to glycerolipid metabolism

Legionella pneumophila CsrA plays a crucial role in the life-stage-specific expression of virulence phenotypes and metabolic activity. However, its exact role is only partly known. To elucidate how CsrA impacts L. pneumophila metabolism we analysed the CsrA depended regulation of metabolic functions by comparative ¹³C-isotopologue profiling and oxygen consumption experiments of a L. pneumophila wild-type (wt) strain and its isogenic csrA⁻ mutant. We show that a csrA⁻ mutant has significantly lower respiration rates when serine, alanine, pyruvate, α-ketoglutarate or palmitate is the sole carbon source. By contrast, when grown in glucose or glycerol, no differences in respiration were detected. Isotopologue profiling uncovered that the transfer of label from [U-¹³C₃]serine via pyruvate into the citrate cycle and gluconeogenesis was lower in the mutant as judged from the labelling patterns of protein-derived amino acids, cell-wall-derived diaminopimelate, sugars and amino sugars and 3-hydroxybutyrate derived from polyhydroxybutyrate (PHB). Similarly, the incorporation of [U-¹³C₆]glucose via the glycolysis/Entner–Doudoroff (ED) pathway but not via the pentose phosphate pathway was repressed in the csrA⁻ mutant. On the other hand, fluxes due to [U-¹³C₃]glycerol utilization were increased in the csrA⁻ mutant. In addition, we showed that exogenous [1,2,3,4-¹³C₄]palmitic acid is efficiently used for PHB synthesis via ¹³C₂-acetyl-CoA. Taken together, CsrA induces serine catabolism via the tricarboxylic acid cycle and glucose degradation via the ED pathway, but represses glycerol metabolism, fatty acid degradation and PHB biosynthesis, in particular during exponential growth. Thus, CsrA has a determining role in substrate usage and carbon partitioning during the L. pneumophila life cycle and regulates a switch from amino acid usage in replicative phase to glycerolipid usage during transmissive growth.

The LetA/S two-component system regulates transcriptomic changes that are essential for the culturability of Legionella pneumophila in water

Surviving the nutrient-poor aquatic environment for extended periods of time is important for the transmission of various water-borne pathogens, including Legionella pneumophila (Lp). Previous work concluded that the stringent response and the sigma factor RpoS are essential for the survival of Lp in water. In the present study, we investigated the role of the LetA/S two-component signal transduction system in the successful survival of Lp in water. In addition to cell size reduction in the post-exponential phase, LetS also contributes to cell size reduction when Lp is exposed to water. Importantly, absence of the sensor kinase results in a significantly lower survival as measured by CFUs in water at various temperatures and an increased sensitivity to heat shock. According to the transcriptomic analysis, LetA/S orchestrates a general transcriptomic downshift of major metabolic pathways upon exposure to water leading to better culturability, and likely survival, suggesting a potential link with the stringent response. However, the expression of the LetA/S regulated small regulatory RNAs, RsmY and RsmZ, is not changed in a relAspoT mutant, which indicates that the stringent response and the LetA/S response are two distinct regulatory systems contributing to the survival of Lp in water.

Global effect of CsrA on gene expression in enterohemorrhagic Escherichia coli O157:H7

The post-transcriptional regulator CsrA regulates multiple unrelated processes such as central carbon metabolism, motility, biofilm formation and bacterial virulence in different bacteria. However, regulation by CsrA in enterohemorrhagic Escherichia coli (EHEC) O157:H7 is still largely unknown. In this study, we performed a detailed analysis of gene expression differences between the EHEC O157:H7 wild-type strain and a corresponding csrA::kan mutant using RNA-seq technology. Genes whose expression was affected by CsrA were identified and grouped into different clusters of orthologous group categories. Genes located in the locus of enterocyte effacement (LEE) pathogenicity island were significantly upregulated, whereas expression of flagella-related genes was significantly reduced in the csrA::kan mutant. Subsequent bacterial adherence and motility assays showed that inactivation of CsrA in EHEC O157:H7 resulted in a significant increase in bacterial adherence to host epithelial cells, with a concomitant loss of swimming motility on semi-solid agar plates. Furthermore, we also found that CsrA regulates genes not previously identified in other bacterial species, including genes encoding cytochrome oxidases and those required for nitrogen metabolism. Our results provide essential insight into the regulatory function of CsrA.

Temperature-Dependent Growth Modeling of Environmental and Clinical Legionella pneumophila Multilocus Variable-Number Tandem-Repeat Analysis (MLVA) Genotypes

Legionella pneumophila causes waterborne infections resulting in severe pneumonia. High-resolution genotyping of L. pneumophila isolates can be achieved by multiple-locus variable-number tandem-repeat analysis (MLVA). Recently, we found that different MLVA genotypes of L. pneumophila dominated different sites in a small drinking-water network, with a genotype-related temperature and abundance regime. The present study focuses on understanding the temperature-dependent growth kinetics of the genotypes that dominated the water network. Our aim was to model mathematically the influence of temperature on the growth kinetics of different environmental and clinical L. pneumophila genotypes and to compare it with the influence of their ecological niches. Environmental strains showed a distinct temperature preference, with significant differences among the growth kinetics of the three studied genotypes (Gt4, Gt6, and Gt15). Gt4 strains exhibited superior growth at lower temperatures (25 and 30°C), while Gt15 strains appeared to be best adapted to relatively higher temperatures (42 and 45°C). The temperature-dependent growth traits of the environmental genotypes were consistent with their distribution and temperature preferences in the water network. Clinical isolates exhibited significantly higher growth rates and reached higher maximal cell densities at 37°C and 42°C than the environmental strains. Further research on the growth preferences of L. pneumophila clinical and environmental genotypes will result in a better understanding of their ecological niches in drinking-water systems as well as in the human body.IMPORTANCELegionella pneumophila is a waterborne pathogen that threatens humans in developed countries. The bacteria inhabit natural and man-made freshwater environments. Here we demonstrate that different environmental L. pneumophila genotypes have different temperature-dependent growth kinetics. Moreover, Legionella strains that belong to the same species but were isolated from environmental and clinical sources possess adaptations for growth at different temperatures. These growth preferences may influence the bacterial colonization at specific ecological niches within the drinking-water network. Adaptations for growth at human body temperatures may facilitate the abilities of some L. pneumophila strains to infect and cause illness in humans. Our findings may be used as a tool to improve Legionella monitoring in drinking-water networks. Risk assessment models for predicting the risk of legionellosis should take into account not only Legionella concentrations but also the temperature-dependent growth kinetics of the isolates.

The Legionella pneumophila genome evolved to accommodate multiple regulatory mechanisms controlled by the CsrA-system

The carbon storage regulator protein CsrA regulates cellular processes post-transcriptionally by binding to target-RNAs altering translation efficiency and/or their stability. Here we identified and analyzed the direct targets of CsrA in the human pathogen Legionella pneumophila. Genome wide transcriptome, proteome and RNA co-immunoprecipitation followed by deep sequencing of a wild type and a csrA mutant strain identified 479 RNAs with potential CsrA interaction sites located in the untranslated and/or coding regions of mRNAs or of known non-coding sRNAs. Further analyses revealed that CsrA exhibits a dual regulatory role in virulence as it affects the expression of the regulators FleQ, LqsR, LetE and RpoS but it also directly regulates the timely expression of over 40 Dot/Icm substrates. CsrA controls its own expression and the stringent response through a regulatory feedback loop as evidenced by its binding to RelA-mRNA and links it to quorum sensing and motility. CsrA is a central player in the carbon, amino acid, fatty acid metabolism and energy transfer and directly affects the biosynthesis of cofactors, vitamins and secondary metabolites. We describe the first L. pneumophila riboswitch, a thiamine pyrophosphate riboswitch whose regulatory impact is fine-tuned by CsrA, and identified a unique regulatory mode of CsrA, the active stabilization of RNA anti-terminator conformations inside a coding sequence preventing Rho-dependent termination of the gap operon through transcriptional polarity effects. This allows L. pneumophila to regulate the pentose phosphate pathway and the glycolysis combined or individually although they share genes in a single operon. Thus the L. pneumophila genome has evolved to acclimate at least five different modes of regulation by CsrA giving it a truly unique position in its life cycle.

The RNA binding protein CsrA is the master regulator of the bi-phasic life cycle of Legionella pneumophila governing virulence expression in this intracellular pathogen. Here, we have used deep sequencing of RNA enriched by co-immunoprecipitation with epitope-tagged CsrA to identify CsrA-associated transcripts at the genome level. We found 479 mRNAs or non-coding RNAs to be targets of CsrA. Among those major regulators including FleQ, the regulator of flagella expression, LqsR, the regulator of quorum sensing and RpoS implicated in stress response were identified. The expression of over 40 type IV secreted effector proteins important for intracellular survival and virulence are under the control of CsrA. Combined with transcriptomics, whole shotgun proteomics of a wild type and a CsrA mutant strain and functional analyses of several CsrA-targeted RNAs we identified the first riboswitch in L. pneumophila, a thiamine pyrophosphate riboswitch, and discovered a new mode of regulation by CsrA that allows L. pneumophila to regulate the pentose phosphate pathway and the glycolysis combined or individually although they share genes in a single operon. Our results further underline the indispensable role of CsrA in the life cycle of L. pneumophila and provide new insights into its regulatory roles and mechanisms.

Intra-Species and Inter-Kingdom Signaling of Legionella pneumophila

The ubiquitous Gram-negative bacterium Legionella pneumophila parasitizes environ mental amoebae and, upon inhalation, replicates in alveolar macrophages, thus causing a life-threatening pneumonia called “Legionnaires’ disease.” The opportunistic pathogen employs a bi-phasic life cycle, alternating between a replicative, non-virulent phase and a stationary, transmissive/virulent phase. L. pneumophila employs the Lqs (Legionella quorum sensing) system as a major regulator of the growth phase switch. The Lqs system comprises the autoinducer synthase LqsA, the homologous sensor kinases LqsS and LqsT, as well as a prototypic response regulator termed LqsR. These components produce, detect, and respond to the α-hydroxyketone signaling molecule LAI-1 (Legionella autoinducer-1, 3-hydroxypentadecane-4-one). LAI-1-mediated signal transduction through the sensor kinases converges on LqsR, which dimerizes upon phosphorylation. The Lqs system regulates the bacterial growth phase switch, pathogen-host cell interactions, motility, natural competence, filament production, and expression of a chromosomal “fitness island.” Yet, LAI-1 not only mediates bacterial intra-species signaling, but also modulates the motility of eukaryotic cells through the small GTPase Cdc42 and thus promotes inter-kingdom signaling. Taken together, the low molecular weight compound LAI-1 produced by L. pneumophila and sensed by the bacteria as well as by eukaryotic cells plays a major role in pathogen-host cell interactions.

Small Non-Coding RNAs: New Insights in Modulation of Host Immune Response by Intracellular Bacterial Pathogens

Pathogenic bacteria possess intricate regulatory networks that temporally control the production of virulence factors and enable the bacteria to survive and proliferate within host cell. Small non-coding RNAs (sRNAs) have been identified as important regulators of gene expression in diverse biological contexts. Recent research has shown bacterial sRNAs involved in growth and development, cell proliferation, differentiation, metabolism, cell signaling, and immune response through regulating protein–protein interactions or via their ability to base pair with RNA and DNA. In this review, we provide a brief overview of mechanism of action employed by immune-related sRNAs, their known functions in immunity, and how they can be integrated into regulatory circuits that govern virulence, which will facilitate our understanding of pathogenesis and the development of novel, more effective therapeutic approaches to treat infections caused by intracellular bacterial pathogens.

Legionella pneumophila strain associated with the first evidence of person-to-person transmission of Legionnaires' disease: a unique mosaic genetic backbone

A first strong evidence of person-to-person transmission of Legionnaires’ Disease (LD) was recently reported. Here, we characterize the genetic backbone of this case-related Legionella pneumophila strain (“PtVFX/2014”), which also caused a large outbreak of LD. PtVFX/2014 is phylogenetically divergent from the most worldwide studied outbreak-associated L. pneumophila subspecies pneumophila serogroup 1 strains. In fact, this strain is also from serogroup 1, but belongs to the L. pneumophila subspecies fraseri. Its genomic mosaic backbone reveals eight horizontally transferred regions encompassing genes, for instance, involved in lipopolysaccharide biosynthesis or encoding virulence-associated Dot/Icm type IVB secretion system (T4BSS) substrates. PtVFX/2014 also inherited a rare ~65 kb pathogenicity island carrying virulence factors and detoxifying enzymes believed to contribute to the emergence of best-fitted strains in water reservoirs and in human macrophages, as well as a inter-species transferred (from L. oakridgensis) ~37.5 kb genomic island (harboring a lvh/lvr T4ASS cluster) that had never been found intact within L. pneumophila species. PtVFX/2014 encodes another lvh/lvr cluster near to CRISPR-associated genes, which may boost L. pneumophila transition from an environmental bacterium to a human pathogen. Overall, this unique genomic make-up may impact PtVFX/2014 ability to adapt to diverse environments, and, ultimately, to be transmitted and cause human disease.

Molecular epidemiology, phylogeny and evolution of Legionella

Legionella are opportunistic pathogens that develop in aquatic environments where they multiply in protozoa. When infected aerosols reach the human respiratory tract they may accidentally infect the alveolar macrophages leading to a severe pneumonia called Legionnaires' disease (LD). The ability of Legionella to survive within host-cells is strictly dependent on the Dot/Icm Type 4 Secretion System that translocates a large repertoire of effectors into the host cell cytosol. Although Legionella is a large genus comprising nearly 60 species that are worldwide distributed, only about half of them have been involved in LD cases. Strikingly, the species Legionella pneumophila alone is responsible for 90% of all LD cases. The present review summarizes the molecular approaches that are used for L. pneumophila genotyping with a major focus on the contribution of whole genome sequencing (WGS) to the investigation of local L. pneumophila outbreaks and global epidemiology studies. We report the newest knowledge regarding the phylogeny and the evolution of Legionella and then focus on virulence evolution of those Legionella species that are known to have the capacity to infect humans. Finally, we discuss the evolutionary forces and adaptation mechanisms acting on the Dot/Icm system itself as well as the role of mobile genetic elements (MGE) encoding T4ASSs and of gene duplications in the evolution of Legionella and its adaptation to different hosts and lifestyles.

Inflammasome Recognition and Regulation of the Legionella Flagellum

The Gram-negative bacterium Legionella pneumophila colonizes extracellular environmental niches and infects free-living protozoa. Upon inhalation into the human lung, the opportunistic pathogen grows in macrophages and causes a fulminant pneumonia termed Legionnaires' disease. L. pneumophila employs a biphasic life cycle, comprising a replicative, non-virulent, and a stationary, virulent form. In the latter phase, the pathogen produces a plethora of so-called effector proteins, which are injected into host cells, where they subvert pivotal processes and promote the formation of a distinct membrane-bound compartment, the Legionella-containing vacuole. In the stationary phase, the bacteria also produce a single monopolar flagellum and become motile. L. pneumophila flagellin is recognized by and triggers the host's NAIP5 (Birc1e)/NLRC4 (Ipaf) inflammasome, which leads to caspase-1 activation, pore formation, and pyroptosis. The production of L. pneumophila flagellin and pathogen-host interactions are controlled by a complex stationary phase regulatory network, detecting nutrient availability as well as the Legionella quorum sensing (Lqs) signaling compound LAI-1 (3-hydroxypentadecane-4-one). Thus, the small molecule LAI-1 coordinates L. pneumophila flagellin production and motility, inflammasome activation, and virulence.

csrR, a Paralog and Direct Target of CsrA, Promotes Legionella pneumophila Resilience in Water

Critical to microbial versatility is the capacity to express the cohort of genes that increase fitness in different environments. Legionella pneumophila occupies extensive ecological space that includes diverse protists, pond water, engineered water systems, and mammalian lung macrophages. One mechanism that equips this opportunistic pathogen to adapt to fluctuating conditions is a switch between replicative and transmissive cell types that is controlled by the broadly conserved regulatory protein CsrA. A striking feature of the legionellae surveyed is that each of 14 strains encodes 4 to 7 csrA-like genes, candidate regulators of distinct fitness traits. Here we focus on the one csrA paralog (lpg1593) that, like the canonical csrA, is conserved in all 14 strains surveyed. Phenotypic analysis revealed that long-term survival in tap water is promoted by the lpg1593 locus, which we name csrR (for "CsrA-similar protein for resilience"). As predicted by its GGA motif, csrR mRNA was bound directly by the canonical CsrA protein, as judged by electromobility shift and RNA-footprinting assays. Furthermore, CsrA repressed translation of csrR mRNA in vivo, as determined by analysis of csrR-gfp reporters, csrR mRNA stability in the presence and absence of csrA expression, and mutation of the CsrA binding site identified on the csrR mRNA. Thus, CsrA not only governs the transition from replication to transmission but also represses translation of its paralog csrR when nutrients are available. We propose that, during prolonged starvation, relief of CsrA repression permits CsrR protein to coordinate L. pneumophila's switch to a cell type that is resilient in water supplies.

IMPORTANCE

Persistence of L. pneumophila in water systems is a public health risk, and yet there is little understanding of the genetic determinants that equip this opportunistic pathogen to adapt to and survive in natural or engineered water systems. A potent regulator of this pathogen's intracellular life cycle is CsrA, a protein widely distributed among bacterial species that is understood quite well. Our finding that every sequenced L. pneumophila strain carries several csrA paralogs-including two common to all isolates--indicates that the legionellae exploit CsrA regulatory switches for multiple purposes. Our discovery that one paralog, CsrR, is a target of CsrA that enhances survival in water is an important step toward understanding colonization of the engineered environment by pathogenic L. pneumophila.

Campylobacter-Acanthamoeba interactions

Campylobacter jejuni is a foodborne pathogen recognized as the major cause of human bacterial enteritis. Undercooked poultry products and contaminated water are considered as the most important sources of infection. Some studies suggest transmission and survival of this bacterial pathogen may be assisted by the free-living protozoa Acanthamoeba. The latter is known to play the role of a host for various pathogenic bacteria, protecting them from harsh environmental conditions. Importantly, there is a similarity between the mechanisms of bacterial survival within amoebae and macrophages, making the former a convenient tool for the investigation of the survival of pathogenic bacteria in the environment. However, the molecular mechanisms involved in the interaction between Campylobacter and Acanthamoeba are not well understood. Whilst some studies suggest the ability of C. jejuni to survive within the protozoa, the other reports support an extracellular mode of survival only. In this review, we focus on the studies investigating the interaction between Campylobacter and Acanthamoeba, address some reasons for the contradictory results, and discuss possible implications of these results for epidemiology. Additionally, as the molecular mechanisms involved remain unknown, we also suggest possible factors that may be involved in this process. Deciphering the molecular mechanisms of pathogen-protozoa interaction will assist in a better understanding of Campylobacter lifestyle and in the development of novel antibacterial drugs.

Exposure to synthetic gray water inhibits amoeba encystation and alters expression of Legionella pneumophila virulence genes

Water conservation efforts have focused on gray water (GW) usage, especially for applications that do not require potable water quality. However, there is a need to better understand environmental pathogens and their free-living amoeba (FLA) hosts within GW, given their growth potential in stored gray water. Using synthetic gray water (sGW) we examined three strains of the water-based pathogen Legionella pneumophila and its FLA hosts Acanthamoeba polyphaga, A. castellanii, and Vermamoeba vermiformis. Exposure to sGW for 72 h resulted in significant inhibition (P < 0.0001) of amoebal encystation versus control-treated cells, with the following percentages of cysts in sGW versus controls: A. polyphaga (0.6 versus 6%), A. castellanii (2 versus 62%), and V. vermiformis (1 versus 92%), suggesting sGW induced maintenance of the actively feeding trophozoite form. During sGW exposure, L. pneumophila culturability decreased as early as 5 h (1.3 to 2.9 log10 CFU, P < 0.001) compared to controls (Δ0 to 0.1 log10 CFU) with flow cytometric analysis revealing immediate changes in membrane permeability. Furthermore, reverse transcription-quantitative PCR was performed on total RNA isolated from L. pneumophila cells at 0 to 48 h after sGW incubation, and genes associated with virulence (gacA, lirR, csrA, pla, and sidF), the type IV secretion system (lvrB and lvrE), and metabolism (ccmF and lolA) were all shown to be differentially expressed. These results suggest that conditions within GW may promote interactions between water-based pathogens and FLA hosts, through amoebal encystment inhibition and alteration of bacterial gene expression, thus warranting further exploration into FLA and L. pneumophila behavior in GW systems.

The many forms of a pleomorphic bacterial pathogen-the developmental network of Legionella pneumophila

Legionella pneumophila is a natural intracellular bacterial parasite of free-living freshwater protozoa and an accidental human pathogen that causes Legionnaires' disease. L. pneumophila differentiates, and does it in style. Recent experimental data on L. pneumophila's differentiation point at the existence of a complex network that involves many developmental forms. We intend readers to: (i) understand the biological relevance of L. pneumophila's forms found in freshwater and their potential to transmit Legionnaires' disease, and (ii) learn that the common depiction of L. pneumophila's differentiation as a biphasic developmental cycle that alternates between a replicative and a transmissive form is but an oversimplification of the actual process. Our specific objectives are to provide updates on the molecular factors that regulate L. pneumophila's differentiation (Section The Differentiation Process and Its Regulation), and describe the developmental network of L. pneumophila (Section Dissecting Lp's Developmental Network), which for clarity's sake we have dissected into five separate developmental cycles. Finally, since each developmental form seems to contribute differently to the human pathogenic process and the transmission of Legionnaires' disease, readers are presented with a challenge to develop novel methods to detect the various L. pneumophila forms present in water (Section Practical Implications), as a means to improve our assessment of risk and more effectively prevent legionellosis outbreaks.

The role of ClpP, RpoS and CsrA in growth and filament formation of Salmonella enterica serovar Typhimurium at low temperature

BACKGROUND

Salmonellae are food-borne pathogens of great health and economic importance. To pose a threat to humans, Salmonellae normally have to cope with a series of stressful conditions in the food chain, including low temperature. In the current study, we evaluated the importance of the Clp proteolytic complex and the carbon starvation protein, CsrA, for the ability of Salmonella Typhimurium to grow at low temperature.

RESULTS

A clpP mutant was severely affected in growth and formed pin point colonies at 10°C. Contrary to this, rpoS and clpP/rpoS mutants were only slightly affected. The clpP mutant formed cold resistant suppressor mutants at a frequency of 2.5 × 10(-3) and these were found not to express RpoS. Together these results indicated that the impaired growth of the clpP mutant was caused by high level of RpoS. Evaluation by microscopy of the clpP mutant revealed that it formed filamentous cells when grown at 10°C, and this phenotype too, disappered when rpoS was mutated in parallel indicating a RpoS-dependency. A csrA (sup) mutant was also growth attenuated a low temperature. An rpoS/csrA (sup) double mutant was also growth attenuated, indicating that the phenotype of the csrA mutant was independent from RpoS.

CONCLUSIONS

The cold sensitivity of clpP mutant was associated with increased levels of RpoS and probably caused by toxic levels of RpoS. Although a csrA mutant also accumulated high level of RpoS, growth impairment caused by lack of csrA was not related to RpoS levels in a similar way.

Identification of Legionella pneumophila effectors regulated by the LetAS-RsmYZ-CsrA regulatory cascade, many of which modulate vesicular trafficking

Legionella pneumophila, the causative agent of Legionnaires' disease, is an intracellular human pathogen that utilizes the Icm/Dot type IVB secretion system to translocate a large repertoire of effectors into host cells. To find coregulated effectors, we performed a bioinformatic genomic screen with the aim of identifying effector-encoding genes containing putative CsrA regulatory elements. The regulation of these genes by the LetAS-RsmYZ-CsrA regulatory cascade was experimentally validated by examining their levels of expression in deletion mutants of relevant regulators and by site-directed mutagenesis of the putative CsrA sites. These analyses resulted in the identification of 26 effector-encoding genes regulated by the LetAS-RsmYZ-CsrA regulatory cascade, all of which were expressed at higher levels during the stationary phase. To determine if any of these effectors is involved in modulating the secretory pathway, they were overexpressed in wild-type yeast as well as in a yeast sec22 deletion mutant, which encodes an R-SNARE that participates in the endoplasmic reticulum (ER)-Golgi trafficking. This examination identified many novel LetAS-RsmYZ-CsrA regulated effectors which are involved in this process. To further characterize the role of these 26 effectors in vesicular trafficking, they were examined in yeast arf and arl deletion mutants, which encode small GTPases that regulate ER-Golgi trafficking. This analysis revealed that the effectors examined manipulate different processes of the secretory pathway. Collectively, our results demonstrate that several of the L. pneumophila effectors which are coregulated in the bacterial cell are involved in the modulation of the same eukaryotic pathway.

The Legionella pneumophila two-component regulatory systems that participate in the regulation of Icm/Dot effectors

Legionella pneumophila, the causative agent of Legionnaires' disease, actively manipulates intracellular processes to establish a replication niche inside their host cells. The establishment of its replication niche requires a functional Icm/Dot type IV secretion system which translocates about 300 effector proteins into the host cells during infection. This enormous number of effectors should be coordinated at the level of gene expression, in order to be expressed and translocated at the correct time and appropriate amounts. One of the predominant ways in bacteria to regulate virulence gene expression is by the use of two-component systems (TCSs). To date, four TCSs have been shown to be involved in the regulation of Icm/Dot effector-encoding genes: The PmrAB and CpxRA TCSs that directly control, and the LetAS and LqsRS TCSs that indirectly control the level of expression of effector-encoding genes. According to our current knowledge, these four TCSs control the expression of about 70 effector-encoding genes. The regulation by different TCSs divides the effectors into groups of co-regulated effector-encoding genes that are probably co-expressed at a similar time during infection and might perform related functions. In addition, examples of interplay between these TCSs were already reported indicating that they form part of a regulatory network that orchestrates the expression of L. pneumophila effector-encoding genes during infection.

Facets of small RNA-mediated regulation in Legionella pneumophila

Legionella pneumophila is a water-borne pathogen that causes a severe lung infection in humans. It is able to replicate inside amoeba in the water environment, and inside lung macrophages in humans. Efficient regulation of gene expression is critical for responding to the conditions that L. pneumophila encounters and for intracellular multiplication in host cells. In the last two decades, many reports have contributed to our understanding of the critical importance of small regulatory RNAs (sRNAs) in the regulatory network of bacterial species. This report presents the current state of knowledge about the sRNAs expressed by L. pneumophila and discusses a few regulatory pathways in which sRNAs should be involved in this pathogen.

Campylobacter jejuni CsrA complements an Escherichia coli csrA mutation for the regulation of biofilm formation, motility and cellular morphology but not glycogen accumulation

Background

Although Campylobacter jejuni is consistently ranked as one of the leading causes of bacterial diarrhea worldwide, the mechanisms by which C. jejuni causes disease and how they are regulated have yet to be clearly defined. The global regulator, CsrA, has been well characterized in several bacterial genera and is known to regulate a number of independent pathways via a post transcriptional mechanism, but remains relatively uncharacterized in the genus Campylobacter. Previously, we reported data illustrating the requirement for CsrA in several virulence related phenotypes of C. jejuni strain 81–176, indicating that the Csr pathway is important for Campylobacter pathogenesis.

Results

We compared the Escherichia coli and C. jejuni orthologs of CsrA and characterized the ability of the C. jejuni CsrA protein to functionally complement an E. coli csrA mutant. Phylogenetic comparison of E. coli CsrA to orthologs from several pathogenic bacteria demonstrated variability in C. jejuni CsrA relative to the known RNA binding domains of E. coli CsrA and in several amino acids reported to be involved in E. coli CsrA-mediated gene regulation. When expressed in an E. coli csrA mutant, C. jejuni CsrA succeeded in recovering defects in motility, biofilm formation, and cellular morphology; however, it failed to return excess glycogen accumulation to wild type levels.

Conclusions

These findings suggest that C. jejuni CsrA is capable of efficiently binding some E. coli CsrA binding sites, but not others, and provide insight into the biochemistry of C. jejuni CsrA.

Legionella pneumophila transcriptional response to chlorine treatment

Legionella pneumophila is a ubiquitous environmental microorganism found in freshwater that can cause an acute form of pneumonia known as Legionnaires' disease. Despite widespread use of chlorine to ensure drinking water quality and awareness that L. pneumophila may escape these treatments, little is known about its effects on L. pneumophila. The aim of this study was to investigate the L. pneumophila transcriptional response induced by chlorine treatment. Transcriptome analysis, using DNA arrays, showed that a sublethal dose of chlorine induces a differential expression of 391 genes involved in stress response, virulence, general metabolism, information pathways and transport. Many of the stress response genes were significantly upregulated, whereas a significant number of virulence genes were repressed. In particular, exposure of L. pneumophila to chlorine induced the expression of cellular antioxidant proteins, stress proteins and transcriptional regulators. In addition, glutathione S-transferase specific activity was enhanced following chlorine treatment. Our results clearly indicate that chlorine induces expression of proteins involved in cellular defence mechanisms against oxidative stress that might be involved in adaptation or resistance to chlorine treatment.

BarA-UvrY two-component system regulates virulence of uropathogenic E. coli CFT073

Uropathogenic Escherichia coli (UPEC), a member of extraintestinal pathogenic E. coli, cause ∼80% of community-acquired urinary tract infections (UTI) in humans. UPEC initiates its colonization in epithelial cells lining the urinary tract with a complicated life cycle, replicating and persisting in intracellular and extracellular niches. Consequently, UPEC causes cystitis and more severe form of pyelonephritis. To further understand the virulence characteristics of UPEC, we investigated the roles of BarA-UvrY two-component system (TCS) in regulating UPEC virulence. Our results showed that mutation of BarA-UvrY TCS significantly decreased the virulence of UPEC CFT073, as assessed by mouse urinary tract infection, chicken embryo killing assay, and cytotoxicity assay on human kidney and uroepithelial cell lines. Furthermore, mutation of either barA or uvrY gene reduced the production of hemolysin, lipopolysaccharide (LPS), proinflammatory cytokines (TNF-α and IL-6) and chemokine (IL-8). The virulence phenotype was restored similar to that of wild-type by complementation of either barA or uvrY gene in trans. In addition, we discussed a possible link between the BarA-UvrY TCS and CsrA in positively and negatively controlling virulence in UPEC. Overall, this study provides the evidences for BarA-UvrY TCS regulates the virulence of UPEC CFT073 and may point to mechanisms by which virulence regulations are observed in different ways may control the long-term survival of UPEC in the urinary tract.

CsrA and TnaB coregulate tryptophanase activity to promote exotoxin-induced killing of Caenorhabditis elegans by enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli(EPEC) requires the tnaA-encoded enzyme tryptophanase and its substrate tryptophan to synthesize diffusible exotoxins that kill the nematode Caenorhabditis elegans. Here, we demonstrate that the RNA-binding protein CsrA and the tryptophan permease TnaB coregulate tryptophanase activity, through mutually exclusive pathways, to stimulate toxin-mediated paralysis and killing of C. elegans.

Small Regulatory RNA and Legionella pneumophila

Legionella pneumophila is a gram-negative bacterial species that is ubiquitous in almost any aqueous environment. It is the agent of Legionnaires’ disease, an acute and often under-reported form of pneumonia. In mammals, L. pneumophila replicates inside macrophages within a modified vacuole. Many protein regulators have been identified that control virulence-related properties, including RpoS, LetA/LetS, and PmrA/PmrB. In the past few years, the importance of regulation of virulence factors by small regulatory RNA (sRNAs) has been increasingly appreciated. This is also the case in L. pneumophila where three sRNAs (RsmY, RsmZ, and 6S RNA) were recently shown to be important determinants of virulence regulation and 79 actively transcribed sRNAs were identified. In this review we describe current knowledge about sRNAs and their regulatory properties and how this relates to the known regulatory systems of L. pneumophila. We also provide a model for sRNA-mediated control of gene expression that serves as a framework for understanding the regulation of virulence-related properties of L. pneumophila.

Legionella Pneumophila Transcriptome during Intracellular Multiplication in Human Macrophages

Legionella pneumophila is the causative agent of Legionnaires' disease, an acute pulmonary infection. L. pneumophila is able to infect and multiply in both phagocytic protozoa, such as Acanthamoeba castellanii, and mammalian professional phagocytes. The best-known L. pneumophila virulence determinant is the Icm/Dot type IVB secretion system, which is used to translocate more than 150 effector proteins into host cells. While the transcriptional response of Legionella to the intracellular environment of A. castellanii has been investigated, much less is known about the Legionella transcriptional response inside human macrophages. In this study, the transcriptome of L. pneumophila was monitored during exponential and post-exponential phase in rich AYE broth as well as during infection of human cultured macrophages. This was accomplished with microarrays and an RNA amplification procedure called selective capture of transcribed sequences to detect small amounts of mRNA from low numbers of intracellular bacteria. Among the genes induced intracellularly are those involved in amino acid biosynthetic pathways leading to l-arginine, l-histidine, and l-proline as well as many transport systems involved in amino acid and iron uptake. Genes involved in catabolism of glycerol are also induced during intracellular growth, suggesting that glycerol could be used as a carbon source. The genes encoding the Icm/Dot system are not differentially expressed inside cells compared to control bacteria grown in rich broth, but the genes encoding several translocated effectors are strongly induced. Moreover, we used the transcriptome data to predict previously unrecognized Icm/Dot effector genes based on their expression pattern and confirmed translocation for three candidates. This study provides a comprehensive view of how L. pneumophila responds to the human macrophage intracellular environment.

Morphological responses of Legionella pneumophila biofilm to nanoparticle exposure

Legionella pneumophila is a pathogenic bacterium that forms biofilms in natural and anthropogenic habitats. This feature not only facilitates colonization but also limits the effectiveness of biocides. L. pneumophila was exposed to three sizes of citrate-capped gold nanospheres in both planktonic and biofilm stages. TEM micrographs indicated that gold nanoparticles (AuNPs) adsorbed to the bacterial cell surface, were absorbed into the cells, aggregated within the cells, and integrated into the extrapolymeric matrix of the biofilm. Both 4 and 18 nm, but not 50 nm AuNPs caused an alteration of biofilm morphology. Treatment with 20 nm polystyrene spheres did not induce these changes suggesting that the response was a result of the gold and not just the presence of the nanosphere. The morphological changes observed in the biofilm suggest that aquatic ecosystems may be affected by nanoparticle exposure. This may compromise ecosystem functions such as nutrient cycling facilitated by natural biofilms.

Two small ncRNAs jointly govern virulence and transmission in Legionella pneumophila

To transit from intra- to extracellular environments, Legionella pneumophila differentiates from a replicative/non-virulent to a transmissive/virulent form using the two-component system LetA/LetS and the global repressor protein CsrA. While investigating how both regulators act co-ordinately we characterized two ncRNAs, RsmY and RsmZ, that link the LetA/LetS and CsrA regulatory networks. We demonstrate that LetA directly regulates their expression and show that RsmY and RsmZ are functional in Escherichia coli and are able to bind CsrA in vitro. Single mutants have no (ΔrsmY) or a little (ΔrsmZ) impact on virulence, but the ΔrsmYZ strain shows a drastic defect in intracellular growth in Acanthamoeba castellanii and THP-1 monocyte-derived macrophages. Analysis of the transcriptional programmes of the ΔletA, ΔletS and ΔrsmYZ strains revealed that the switch to the transmissive phase is partially blocked. One major difference between the ΔletA, ΔletS and ΔrsmYZ strains was that the latter synthesizes flagella. Taken together, LetA activates transcription of RsmY and RsmZ, which sequester CsrA and abolish its post-transcriptional repressive activity. However, the RsmYZ-CsrA pathway appears not to be the main or only regulatory circuit governing flagella synthesis. We suggest that rather RpoS and LetA, by influencing LetE and probably cyclic-di-GMP levels, regulate motility in L. pneumophila.

Post-transcriptional global regulation by CsrA in bacteria

Global regulation allows bacteria to rapidly modulate the expression of a large variety of unrelated genes in response to environmental changes. Global regulators act at different levels of gene expression. This review focuses on CsrA, a post-transcriptional regulator that affects translation of its gene targets by binding mRNAs. CsrA controls a large variety of physiological processes such as central carbon metabolism, motility and biofilm formation. The activity of CsrA is itself tightly regulated by the CsrB and CsrC small RNAs and the BarA-UvrY two-component system.

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.

The RNA binding protein CsrA is a pleiotropic regulator of the locus of enterocyte effacement pathogenicity island of enteropathogenic Escherichia coli

The attaching and effacing (A/E) pathogen enteropathogenic Escherichia coli (EPEC) forms characteristic actin-filled membranous protrusions upon infection of host cells termed pedestals. Here we examine the role of the RNA binding protein CsrA in the expression of virulence genes and proteins that are necessary for pedestal formation. The csrA mutant was defective in forming actin pedestals on epithelial cells and in disrupting transepithelial resistance across polarized epithelial cells. Consistent with reduced pedestal formation, secretion of the translocators EspA, EspB, and EspD and the effector Tir was substantially reduced in the csrA mutant. Purified CsrA specifically bound to the sepL espADB mRNA leader, and the corresponding transcript levels were reduced in the csrA mutant. In contrast, Tir synthesis was unaffected in the csrA mutant. Reduced secretion of Tir appeared to be in part due to decreased synthesis of EscD, an inner membrane architectural protein of the type III secretion system (TTSS) and EscF, a protein that forms the protruding needle complex of the TTSS. These effects were not mediated through the locus of enterocyte effacement (LEE) transcriptional regulator GrlA or Ler. In contrast to the csrA mutant, multicopy expression of csrA repressed transcription from LEE1, grlRA, LEE2, LEE5, escD, and LEE4, an effect mediated by GrlA and Ler. Consistent with its role in other organisms, CsrA also regulated flagellar motility and glycogen levels. Our findings suggest that CsrA governs virulence factor expression in an A/E pathogen by regulating mRNAs encoding translocators, effectors, or transcription factors.

SigmaS controls multiple pathways associated with intracellular multiplication of Legionella pneumophila

Legionella pneumophila is the causative agent of the severe and potentially fatal pneumonia Legionnaires' disease. L. pneumophila is able to replicate within macrophages and protozoa by establishing a replicative compartment in a process that requires the Icm/Dot type IVB secretion system. The signals and regulatory pathways required for Legionella infection and intracellular replication are poorly understood. Mutation of the rpoS gene, which encodes sigma(S), does not affect growth in rich medium but severely decreases L. pneumophila intracellular multiplication within protozoan hosts. To gain insight into the intracellular multiplication defect of an rpoS mutant, we examined its pattern of gene expression during exponential and postexponential growth. We found that sigma(S) affects distinct groups of genes that contribute to Legionella intracellular multiplication. We demonstrate that rpoS mutants have a functional Icm/Dot system yet are defective for the expression of many genes encoding Icm/Dot-translocated substrates. We also show that sigma(S) affects the transcription of the cpxR and pmrA genes, which encode two-component response regulators that directly affect the transcription of Icm/Dot substrates. Our characterization of the L. pneumophila small RNA csrB homologs, rsmY and rsmZ, introduces a link between sigma(S) and the posttranscriptional regulator CsrA. We analyzed the network of sigma(S)-controlled genes by mutational analysis of transcriptional regulators affected by sigma(S). One of these, encoding the L. pneumophila arginine repressor homolog gene, argR, is required for maximal intracellular growth in amoebae. These data show that sigma(S) is a key regulator of multiple pathways required for L. pneumophila intracellular multiplication.

Conditional essentiality of the csrA gene in Escherichia coli

CsrA is a global posttranscriptional regulator of numerous physiological processes, such as glycogenesis and glycolysis. Here, we show that the csrA gene of Escherichia coli is essential for growth on LB and on synthetic medium containing glycolytic carbon sources. However, csrA is not necessary for growth on synthetic medium containing pyruvate, showing that the Krebs cycle is functional in the csrA::cat deletion mutant. Deletion of the glgCAP operon in the csrA::cat mutant restored the ability to grow on LB and on synthetic medium containing glycolytic carbon sources, showing that growth inhibition is due to an excess of glycogen synthesis.

Synergistic contribution of the Legionella pneumophila lqs genes to pathogen-host interactions

The causative agent of Legionnaires' disease, Legionella pneumophila, is a natural parasite of environmental protozoa and employs a biphasic life style to switch between a replicative and a transmissive (virulent) phase. L. pneumophila harbors the lqs (Legionella quorum sensing) cluster, which includes genes encoding the autoinducer synthase LqsA, the sensor kinase LqsS, the response regulator LqsR, and a homologue of HdeD, which is involved in acid resistance in Escherichia coli. LqsR promotes host-cell interactions as an element of the stationary-phase virulence regulatory network. Here, we characterize L. pneumophila mutant strains lacking all four genes of the lqs cluster or only the hdeD gene. While an hdeD mutant strain did not have overt physiological or virulence phenotypes, an lqs mutant showed an aberrant morphology in stationary growth phase and was defective for intracellular growth, efficient phagocytosis, and cytotoxicity against host cells. Cytotoxicity was restored upon reintroduction of the lqs genes into the chromosome of an lqs mutant strain. The deletion of the lqs cluster caused more-severe phenotypes than deletion of only lqsR, suggesting a synergistic effect of the other lqs genes. A transcriptome analysis indicated that in the stationary phase more than 380 genes were differentially regulated in the lqs mutant and wild-type L. pneumophila. Genes involved in protein production, metabolism, and bioenergetics were upregulated in the lqs mutant, whereas genes encoding virulence factors, such as effectors secreted by the Icm/Dot type IV secretion system, were downregulated. A proteome analysis revealed that a set of Icm/Dot substrates is not produced in the absence of the lqs gene cluster, which confirms the findings from DNA microarray assays and mirrors the virulence phenotype of the lqs mutant strain.

Temperature-dependent parasitic relationship between Legionella pneumophila and a free-living amoeba (Acanthamoeba castellanii)

We analyzed the effects of temperature on the interaction of Legionella pneumophila with Acanthamoeba castellanii. At <20 degrees C, overexpression of type 1 metacaspase, a stimulator of A. castellanii encystation, was associated with a reduced number of bacteria within amoeba. At low temperatures, A. castellanii seems to eliminate L. pneumophila by encystation and digestion.

The response regulator CpxR directly regulates expression of several Legionella pneumophila icm/dot components as well as new translocated substrates

Legionella pneumophila has been shown to utilize the icm/dot type IV secretion system for pathogenesis. This system was shown to be composed of icm/dot complex components and accessory proteins, as well as a large number of translocated substrates. Bioinformatic analysis of the regulatory regions of all the genes revealed that several icm/dot genes, as well as two genes encoding icm/dot translocated substrates, contain the conserved CpxR regulatory element, a regulator that has been shown previously to control the expression of the icmR gene. An experimental analysis, which included a comparison of gene expression in a L. pneumophila wild-type strain and gene expression in a cpxR deletion mutant, construction of mutants with mutations in the CpxR conserved regulatory elements, controlled expression studies, and mobility shift assays, demonstrated the direct relationship between the CpxR regulator and the expression of the genes. Furthermore, genomic analysis identified nine additional genes that contain a putative CpxR regulatory element; five of these genes (two legA genes and three ceg genes) were suggested previously to be putative icm/dot translocated substrates. The three ceg genes identified, which were shown previously to contain a putative PmrA regulatory element, were found here to be regulated by both CpxR and PmrA. The other six genes (two legA genes and four new genes products were found to be regulated by CpxR. Moreover, using the CyaA translocation assay, these nine gene products were found to be translocated into host cells in an Icm/Dot-dependent manner. Our results establish that the CpxR regulator is a fundamental regulator of the icm/dot type IV secretion system in L. pneumophila.