Beta-ketoacyl acyl carrier protein reductase (FabG) activity of the fatty acid biosynthetic pathway is a determining factor of 3-oxo-homoserine lactone acyl chain lengths

Novel extracellular lipase gene Lip1728 influences nutrient-dependent performance bacterial quorum sensing of Burkholderia pyrrocinia WZ10-3

Quorum sensing (QS) is a cellular communication mechanism in which bacteria secrete and recognize signaling molecules to regulate group behavior. Lipases provide energy for bacterial cell growth but it is unknown whether they influence nutrient-dependent QS by hydrolyzing substrate. A high-yield lipase-producing strain, Burkholderia pyrrocinia WZ10-3, was previously identified in our laboratory, but the composition of its crude enzymes was not elucidated. Here, we identified a key extracellular lipase, Lip1728, in WZ10-3, which accounts for 99 % of the extracellular lipase activity. Lip1728 prefers to hydrolyze triglycerides at sn-1,3 positions, with pNP-C16 being its optimal substrate. Lip1728 exhibited activity at pH 5.0-10.0 and regardless of the presence of metal ions. It had strong resistance to sodium dodecyl sulfate and short-chain alcohols and was activated by phenylmethanesulfonylfluoride (PMSF). Lip1728 knockout significantly affected lipid metabolism and biofilm formation in the presence of olive oil. Finally, oleic acid, a hydrolysate of Lip1728, influenced the production of the signal molecule N-acyl homoserine lactone (AHL) and biofilm formation by downregulating the AHL synthetase gene pyrI. In conclusion, Lip1728, as a key extracellular lipase in B. pyrrocinia WZ10-3, exhibits superior properties that make it suitable for biodiesel production and plays a crucial role in QS.

Protein Structure Inspired Drug Discovery

Drug discovery starts with known function, either of a compound or a protein, in-turn prompting investigations to probe 3D structure of the compound-protein interface. As protein structure determines function, we hypothesized that unique 3D structural motifs represent primary information denoting unique function that can drive discovery of novel agents. Using a physics-based protein structure analysis platform developed by us, designed to conduct computationally intensive analysis at supercomputing speeds, we probed a high-resolution protein x-ray crystallographic library developed by us. We selected 3D structural motifs whose function was not otherwise established, that offered environments supporting binding of drug-like chemicals and were present on proteins that were not established therapeutic targets. For each of eight potential binding pockets on six different proteins we accessed a 60 million compound library and used our analysis platform to evaluate binding. Using eight-day colony formation assays acquired compounds were screened for efficacy against human breast, prostate, colon and lung cancer cells and toxicity against human bone marrow stem cells. Compounds selectively inhibiting cancer growth segregated to two pockets on separate proteins. The compound, Dxr2-017, exhibited selective activity against human melanoma cells in the NCI-60 cell line screen, had an IC50 of 19 nM against human melanoma M14 cells in our eight-day assay, while over 2100-fold higher concentrations inhibited stem cells by less than 30%. We show that Dxr2-017 induces anoikis, a unique form of programmed cell death in need of targeted therapeutics. The predicted target protein for Dxr2-017 is expressed in bacteria, not in humans. This supports our strategy of focusing on unique 3D structural motifs. It is known that functionally important 3D structures are evolutionarily conserved. Here we demonstrate proof-of-concept that protein structure represents high value primary data to support discovery of novel therapeutics. This approach is widely applicable.

Strain-specific quorum-sensing responses determine virulence properties in Vibrio anguillarum

Bacterial populations communicate using quorum-sensing (QS) molecules and switch on QS regulation to engage in coordinated behaviour such as biofilm formation or virulence. The marine fish pathogen Vibrio anguillarum harbours several QS systems, and our understanding of its QS regulation is still fragmentary. Here, we identify the VanT-QS regulon and explore the diversity and trajectory of traits under QS regulation in Vibrio anguillarum through comparative transcriptomics of two wildtype strains and their corresponding mutants artificially locked in QS-on (ΔvanO) or QS-off (ΔvanT) states. Intriguingly, the two wildtype populations showed different QS responses to cell density changes and operated primarily in the QS-on and QS-off spectrum, respectively. Examining 27 V. anguillarum strains revealed that ~11% were QS-negative, and GFP-reporter measurements of nine QS-positive strains revealed a highly strain-specific nature of the QS responses. We showed that QS controls a plethora of genes involved in processes such as central metabolism, biofilm formation, competence, T6SS, and virulence properties in V. anguillarum, with large strain-specific differences. Moreover, we demonstrated that the QS state is an important driver of virulence towards fish larvae in one of two V. anguillarum strains. We speculate that infections by mixed-strain communities spanning diverse QS strategies optimize the infection efficiency of the pathogen.

Functional Annotation of Hypothetical Proteins From the Enterobacter cloacae B13 Strain and Its Association With Pathogenicity

Enterobacter cloacae B13 strain is a rod-shaped gram-negative bacterium that belongs to the Enterobacteriaceae family. It can cause respiratory and urinary tract infections, and is responsible for several outbreaks in hospitals. E. cloacae has become an important pathogen and an emerging global threat because of its opportunistic and multidrug resistant ability. However, little knowledge is present about a large portion of its proteins and functions. Therefore, functional annotation of the hypothetical proteins (HPs) can provide an improved understanding of this organism and its virulence activity. The workflow in the study included several bioinformatic tools which were utilized to characterize functions, family and domains, subcellular localization, physiochemical properties, and protein-protein interactions. The E. cloacae B13 strain has overall 604 HPs, among which 78 were functionally annotated with high confidence. Several proteins were identified as enzymes, regulatory, binding, and transmembrane proteins with essential functions. Furthermore, 23 HPs were predicted to be virulent factors. These virulent proteins are linked to pathogenesis with their contribution to biofilm formation, quorum sensing, 2-component signal transduction or secretion. Better knowledge about the HPs’ characteristics and functions will provide a greater overview of the proteome. Moreover, it will help against E. cloacae in neonatal intensive care unit (NICU) outbreaks and nosocomial infections.

Quorum Sensing Inhibitors to Quench P. aeruginosa Pathogenicity

The emergence and the dissemination of multidrug-resistant bacteria constitute a major public health issue. Among incriminated Gram-negative bacteria, Pseudomonas aeruginosa has been designated by the WHO as a critical priority threat. During the infection process, this pathogen secretes various virulence factors in order to adhere and colonize host tissues. Furthermore, P. aeruginosa has the capacity to establish biofilms that reinforce its virulence and intrinsic drug resistance. The regulation of biofilm and virulence factor production of this micro-organism is controlled by a specific bacterial communication system named Quorum Sensing (QS). The development of anti-virulence agents targeting QS that could attenuate P. aeruginosa pathogenicity without affecting its growth seems to be a promising new therapeutic strategy. This could prevent the selective pressure put on bacteria by the conventional antibiotics that cause their death and promote resistant strain survival. This review describes the QS-controlled pathogenicity of P. aeruginosa and its different specific QS molecular pathways, as well as the recent advances in the development of innovative QS-quenching anti-virulence agents to fight anti-bioresistance.

Pseudomonas aeruginosa Biofilm Formation and Its Control

Microbes are hardly seen as planktonic species and are most commonly found as biofilm communities in cases of chronic infections. Biofilms are regarded as a biological condition, where a large group of microorganisms gets adhered to a biotic or abiotic surface. In this context, Pseudomonas aeruginosa, a Gram-negative nosocomial pathogen is the main causative organism responsible for life-threatening and persistent infections in individuals affected with cystic fibrosis and other lung ailments. The bacteria can form a strong biofilm structure when it adheres to a surface suitable for the development of a biofilm matrix. These bacterial biofilms pose higher natural resistance to conventional antibiotic therapy due to their multiple tolerance mechanisms. This prevailing condition has led to an increasing rate of treatment failures associated with P. aeruginosa biofilm infections. A better understanding of the effect of a diverse group of antibiotics on established biofilms would be necessary to avoid inappropriate treatment strategies. Hence, the search for other alternative strategies as effective biofilm treatment options has become a growing area of research. The current review aims to give an overview of the mechanisms governing biofilm formation and the different strategies employed so far in the control of biofilm infections caused by P. aeruginosa. Moreover, this review can also help researchers to search for new antibiofilm agents to tackle the effect of biofilm infections that are currently imprudent to conventional antibiotics.

Methylotroph Quorum Sensing Signal Identification by Inverse Stable Isotopic Labeling

Natural products are an essential source of bioactive compounds. Isotopic labeling is an effective way to identify natural products that incorporate a specific precursor; however, this approach is limited by the availability of isotopically enriched precursors. We used an inverse stable isotopic labeling approach to identify natural products by growing bacteria on a ¹³C-carbon source and then identifying ¹²C-precursor incorporation by mass spectrometry. We applied this approach to methylotrophs, ecologically important bacteria predicted to have significant yet underexplored biosynthetic potential. We demonstrate that this method identifies N-acyl homoserine lactone quorum sensing signals produced by diverse methylotrophs grown on three different one-carbon compounds. We then apply this approach to simultaneously detect five previously unidentified signals produced by a methylotroph and link these compounds to their synthases. We envision that this method can be used to identify other natural product classes synthesized by methylotrophs and other organisms that grow on relatively inexpensive ¹³C-carbon sources.

Genomic and biochemical characterization of antifungal compounds produced by Bacillus subtilis PMB102 against Alternaria brassicicola

Bacillus subtilis is ubiquitous and capable of producing various metabolites, which make the bacterium a good candidate as a biocontrol agent for managing plant diseases. In this study, a phyllosphere bacterium B. subtilis PMB102 isolated from tomato leaf was found to inhibit the growth of Alternaria brassicicola ABA-31 on PDA and suppress Alternaria leaf spot on Chinese cabbage (Brassica rapa). The genome of PMB102 (Accession no. CP047645) was completely sequenced by Nanopore and Illumina technology to generate a circular chromosome of 4,103,088 bp encoding several gene clusters for synthesizing bioactive compounds. PMB102 and the other B. subtilis strains from different sources were compared in pangenome analysis to identify a suite of conserved genes involved in biocontrol and habitat adaptation. Two predicted gene products, surfactin and fengycin, were extracted from PMB102 culture filtrates and verified by LC-MS/MS. The antifungal activity of fengycin was tested on A. brassicicola ABA-31 in bioautography to inhibit hyphae growth, and in co-culturing assays to elicit the formation of swollen hyphae. Our data revealed that B. subtilis PMB102 suppresses Alternaria leaf spot by the production of antifungal metabolites, and fengycin plays an important role to inhibit the vegetative growth of A. brassicicola ABA-31.

Oxygen availability affects the synthesis of quorum sensing signal in the facultative anaerobe Novosphingobium pentaromativorans US6-1

Bacterial populations rely on quorum sensing (QS) to coordinate their behaviors and are often challenged by the fluctuation in oxygen concentrations in their habitats. Oxygen is a crucial factor that affects bacterial metabolism in multiple ways. However, little is known about whether and how oxygen availability affects QS activities. To fill this gap, we used the facultative anaerobe Novosphingobium pentaromativorans US6-1 as a model system, and observed that the QS signal acyl homoserine-lactones (AHLs) were produced only in anoxic environments, such as biofilm, or liquid medium that initially contained less than 2 mg/L dissolved oxygen, but not in highly oxic environments. Comparative transcriptome analysis revealed that oxygen availability significantly affected the physiological activities in US6-1, including fatty acid metabolism, oxidative phosphorylation, citrate cycle, QS activities, and flagellar assembly. The absence of AHLs in the oxic culture was not due to degradation, but to the very low expression of the AHL synthase gene novI. High concentration of NADH during the middle log phase under static cultivation may be a trigger for AHL synthesis. This is the first report that production of AHLs is coupled with anoxic metabolism in a facultative anaerobe, which extends our knowledge on factors affecting bacterial QS occurrence. KEY POINTS: • AHL production is anoxic cultivation related. • Oxygen availability affects AHL synthesis by influencing novI expression. • Oxygen availability changes many metabolism activities including NADH production.

Identification of a Novel Lipid Metabolism-Associated Hepatic Gene Family Induced by Estrogen via ERα in Chicken (Gallus gallus)

Liver is the main organ of lipid metabolism in chicken, especially for laying hens. To explore the molecular mechanism of lipid metabolism in chicken, five novel genes discovered in chicken liver tissue were systematically studied. Bioinformatic analysis was used to analyze the gene characteristics. The expression patterns and regulatory molecular mechanism of the five genes were examined. Our results showed that all five novel genes contain a common NADP-binding site that belongs to the NADB-Rossmann superfamily, and the genes were designated NADB-LER1-5. Phylogenetic tree of the NADB-LERs gene family in different species suggested these five genes originated from the same ancestor. Tissue distributions showed that NADB-LER1-4 genes were highly expressed in lipid metabolism organs, including liver, kidney and duodenum, and that the NADB-LER5 gene was highly expressed in liver and kidney. The spatiotemporal expression indicated that the expression levels of NADB-LER1-5 genes in liver tissue were significantly greater in sexually mature hens than that of immature pullets (P-value ≤ 0.05). The expression levels of NADB-LER1-5 were significantly induced by 17β-estradiol in primary cultured chicken embryo hepatocytes (P-value ≤ 0.05), and 17β-estradiol regulated the expression of NADB-LER1-5 mediated by ERα. Individual assays verified that under induction of 17β-estradiol, the five novel genes were significantly upregulated, with subsequent alteration in serum TG, TC, and VLDLs in 10-week-old pullets. This study proved NADB-LERs family mainly expressed in liver, kidney, and duodenum tissues. 17β-estradiol induces the expression of NADB-LER1-5 genes predominantly mediated via ERα. They likely involved in lipid metabolism in the liver of chicken.

Impact of the structure-activity relationship of AHL analogues on quorum sensing in Gram-negative bacteria

With the emergence of microbial resistance pathogens, recent research aims at studying new mechanisms of action of antibiotics. This review discusses the mechanisms and types of quorum sensing (QS) inhibitors in Gram negative bacteria. It illustrates all published data available in literature pertaining to novel compounds that showed activity against different targets in the quorum sensing pathways in Gram negative bacteria. A systemic overview has been conducted by searching PubMed, Medline, and the Cochrane Library and data extraction of all quorum sensing inhibitors with their mechanisms of action have been collected. This review will focus on signaling autoinducer AI-1 in Gram negative bacteria. The biological activity of the antagonists is mainly reported as IC₅₀ (the concentration of an inhibitor where the response is reduced by half).

Comparative metabolomics shows the metabolic profiles fluctuate in multi-drug resistant Escherichia coli strains

In this study, two susceptible strains and two multi-drug resistant clinical Escherichia coli strains were obtained by Kirby-Bauer method, and then a GC-MS-based metabolomics method was used to compare the differential expression of metabolites between two drug sensitive (CK1 and CK2) and two multidrug-resistant (MDR1 and MDR2) clinical strains of E. coli. We characterized a total of 273 metabolites, including 77 commonly altered metabolites, between MDR vs. antibiotic sensitive strains. Interestingly, the PCA score plot clearly discriminated drug sensitive and MDR strains. The following bioinformatics analysis showed that biosynthesis of amino acids, biosynthesis of phenylpropanoids and purine metabolism were commonly enriched in MDR strains. Moreover, microbial metabolism in diverse environments, carbon metabolism，and pyrimidine metabolism pathways were more likely to be enriched MDR1 strain, while ABC transporters, and cysteine and methionine metabolism pathways were enriched in MDR2 strains. The enzyme activities in several involved metabolic pathways were further measured and metabolite candidates were validated by GC-MS-SIM method. These results indicated that antibiotic resistance affects the metabolite profiles of bacteria. In general, our study provides evidence on the study and prediction of MDR characteristics and mechanisms in bacteria at the metabolite level. BIOLOGICAL SIGNIFICANCE: Overuse and abuse of antibiotics has led to the emergence of antibiotic-resistant strains of bacteria; however, relatively little is known about their resistance mechanisms. In this study, metabolomics method was used to compare the differential expression of metabolites between sensitive and multidrug-resistant clinical strains of E. coli. Results show that the PCA score plot clearly discriminated sensitive and MDR strains, indicating that they had different metabolic profiles. Further, bioinformatics analysis showed that biosynthesis of amino acids, biosynthesis of phenylpropanoids and purine metabolism may be related to resistance. Finally, the enzyme activities in several involved metabolic pathways were further measured and metabolite candidates were validated by GC-MS-SIM method. In general, our study provides evidence on the study and prediction of MDR characteristics and mechanisms in bacteria at the metabolite level.

Quorum sensing in Vibrio spp.: the complexity of multiple signalling molecules in marine and aquatic environments

Quorum sensing (QS) is a density-dependent mechanism enabling bacteria to coordinate their actions via the release of small diffusible molecules named autoinducers (AIs). Vibrio spp. are able to adapt to changing environmental conditions by using a wide range of physiological mechanisms and many species pose a threat for human health and diverse marine and estuarine ecosystems worldwide. Cell-to-cell communication controls many of their vital functions such as niche colonization, survival strategies, or virulence. In this review, I summarize (1) the different known QS pathways (2) the diversity of AIs as well as their biological functions, and (3) the QS-mediated interactions between Vibrio and other organisms. However, the current knowledge is limited to a few pathogenic or bioluminescent species and in order to provide a genus-wide view an inventory of QS genes among 87 Vibrio species has been made. The large diversity of signal molecules and their differential effects on a particular physiological function suggest that the complexity of multiple signalling systems within bacterial communities is far from being fully understood. I question here the real level of specificity of such communication in the environment and discuss the different perspectives in order to better apprehend QS in natural habitats.

Characterization of 3-Oxacyl-Acyl Carrier Protein Reductase Homolog Genes in Pseudomonas aeruginosa PAO1

Bacterial 3-oxoacyl-ACP reductase (OAR) catalyzes the 3-oxoacyl-ACP reduction step in the fatty acid synthesis pathway. At least 12 genes in the Pseudomonas aeruginosa genome are annotated as OAR-encoding genes. In this study, we characterized the functions of these genes with biochemical and genetic techniques. With the exception of PA2967, which encodes FabG, an essential protein in fatty acid synthesis, only the PA4389 and PA4786 gene products had OAR activity, and the single deletion of these two genes reduced the ability of P. aeruginosa to produce several specific quorum-sensing (QS) signals. However, PA4389 and PA4786 do not have key roles in fatty acid synthesis. Moreover, although most OAR homologs had no OAR activity, some may function in carbon utilization. The PA3128 product may play a role in the TCA cycle, and PA0182 and PA1470 seem to be required for the utilization of several amino acids. The rest of the OAR homologs have no roles in carbon utilization, but the deletion of one of these genes might affect the production of virulence factors by P. aeruginosa. We conclude that most OAR homolog genes do not encode OAR enzymes, and that these proteins do not function in fatty acid synthesis.

FabG: from a core to circumstantial catalyst

Core biochemical pathways such as Fatty-acid synthesis II (FAS II) is ascribed to the synthesis of fatty-acids, biotin and lipoic acid in prokaryotes. It has two dehydrogenases namely, FabG and FabI which interact with the fatty-acid chain bound to Acyl-carrier protein (ACP), a well-studied enzyme which binds to substrates of varying lengths. This protein-protein interaction 'broadens' the active site of these dehydrogenases thus, contributing to their flexible nature. This property is exploited for catalysing numerous chiral synthons, alkanes, long-chain alcohols and secondary metabolites in industries especially with FabG. FASI relegates FASII in eukaryotes making it a 'relic gene pool' and an antibacterial drug target with diverse inhibitor and substrate markush. FabG often substitutes other dehydrogenases for producing secondary metabolites in nature. This redundancy is probably due to gene duplication or addition events possibly making FabG, a progenitor to some of the complex short-chain dehydrogenases used in organisms and industries today.

Evidence of a Large Diversity of N-acyl-Homoserine Lactones in Symbiotic Vibrio fischeri Strains Associated with the Squid Euprymna scolopes

Vibrio fischeri possesses a complex AHL-mediated Quorum-sensing (QS) system including two pathways, LuxI/R (3-oxo-C6-HSL and C6-HSL) and AinS/R (C8-HSL), which are important for the regulation of physiological traits. Diverse QS-dependent functional phenotypes have been described in V. fischeri; however, AHL diversity is still underestimated. In the present study, we investigated AHL diversity in five symbiotic V. fischeri strains with distinct phenotypic properties using UHPLC-HRMS/MS. The results obtained (1) revealed an unexpectedly high diversity of signaling molecules, (2) emphasized the complexity of QS in V. fischeri, and (3) highlight the importance of understanding the specificity of AHL-mediated QS.

Genetic diversity and phenotypic plasticity of AHL-mediated Quorum sensing in environmental strains of Vibrio mediterranei

N-Acyl homoserine lactone (AHL)-mediated Quorum sensing (QS) is one of the most studied social behavior among Proteobacteria. However, despite the current knowledge on QS-associated phenotypes such as bioluminescence, biofilm formation, or pathogenesis, the characterization of environmental factors driving QS in realistic ecological settings remains scarce. We investigated the dynamics of AHL and AHL-producing Vibrio among 840 isolates  collected fortnightly from the Salses-Leucate Mediterranean lagoon in spring and summer 2015 and 2016. Vibrio isolates were characterized by gyrB gene sequencing, Enterobacterial repetitive intergenic consensus polymerase chain reaction, and genome sequencing, and AHL production was investigated by a biosensors-based UHPLC-HRMS/MS approach. Our results revealed, for the first time, a succession of V. mediterranei isolates with different AHL production phenotypes over time and this dynamics was observed in a single genotype (average genomic nucleotide identity >99.9). A multivariate DistLM analysis revealed that 83.4% of the temporal variation of V. mediterranei QS phenotypes was explained by environmental variables. Overall, our results suggest that isolates of a single genotype are able to change their QS phenotypes in response to environmental conditions, highlighting the phenotypic plasticity of bacterial communication in the environment.

Plasmid AZOBR_p1-borne fabG gene for putative 3-oxoacyl-[acyl-carrier protein] reductase is essential for proper assembly and work of the dual flagellar system in the alphaproteobacterium Azospirillum brasilense Sp245

Azospirillum brasilense can swim and swarm owing to the activity of a constitutive polar flagellum (Fla) and inducible lateral flagella (Laf), respectively. Experimental data on the regulation of the Fla and Laf assembly in azospirilla are scarce. Here, the coding sequence (CDS) AZOBR_p1160043 (fabG1) for a putative 3-oxoacyl-[acyl-carrier protein (ACP)] reductase was found essential for the construction of both types of flagella. In an immotile leaky Fla− Laf− fabG1::Omegon-Km mutant, Sp245.1610, defects in flagellation and motility were fully complemented by expressing the CDS AZOBR_p1160043 from plasmid pRK415. When pRK415 with the cloned CDS AZOBR_p1160045 (fliC) for a putative 65.2 kDa Sp245 Fla flagellin was transferred into the Sp245.1610 cells, the bacteria also became able to assemble a motile single flagellum. Some cells, however, had unusual swimming behavior, probably because of the side location of the organelle. Although the assembly of Laf was not restored in Sp245.1610 (pRK415-p1160045), this strain was somewhat capable of swarming motility. We propose that the putative 3-oxoacyl-[ACP] reductase encoded by the CDS AZOBR_p1160043 plays a role in correct flagellar location in the cell envelope and (or) in flagellar modification(s), which are also required for the inducible construction of Laf and for proper swimming and swarming motility of A. brasilense Sp245.

Vanillic acid from Actinidia deliciosa impedes virulence in Serratia marcescens by affecting S-layer, flagellin and fatty acid biosynthesis proteins

Serratia marcescens is one of the important nosocomial pathogens which rely on quorum sensing (QS) to regulate the production of biofilm and several virulence factors. Hence, blocking of QS has become a promising approach to quench the virulence of S. marcescens. For the first time, QS inhibitory (QSI) and antibiofilm potential of Actinidia deliciosa have been explored against S. marcescens clinical isolate (CI). A. deliciosa pulp extract significantly inhibited the virulence and biofilm production without any deleterious effect on the growth. Vanillic acid was identified as an active lead responsible for the QSI activity. Addition of vanillic acid to the growth medium significantly affected the QS regulated production of biofilm and virulence factors in a concentration dependent mode in S. marcescens CI, ATCC 14756 and MG1. Furthermore vanillic acid increased the survival of Caenorhabditis elegans upon S. marcescens infection. Proteomic analysis and mass spectrometric identification of differentially expressed proteins revealed the ability of vanillic acid to modulate the expression of proteins involved in S-layers, histidine, flagellin and fatty acid production. QSI potential of the vanillic acid observed in the current study paves the way for exploring it as a potential therapeutic candidate to treat S. marcescens infections.

Only Acyl Carrier Protein 1 (AcpP1) Functions in Pseudomonas aeruginosa Fatty Acid Synthesis

The genome of Pseudomonas aeruginosa contains three open reading frames, PA2966, PA1869, and PA3334, which encode putative acyl carrier proteins, AcpP1, AcpP2, and AcpP3, respectively. In this study, we found that, although these apo-ACPs were successfully phosphopantetheinylated by P. aeruginosa phosphopantetheinyl transferase (PcpS) and all holo-forms of these proteins could be acylated by Vibrio harveyi acyl-ACP synthetase (AasS), only AcpP1 could be used as a substrate for the synthesis of fatty acids, catalyzed by P. aeruginosa cell free extracts in vitro, and only acpP1 gene could restore growth in the Escherichia coliacpP mutant strain CY1877. And P. aeruginosaacpP1 could not be deleted, while disruption of acpP2 or acpP3 in the P. aeruginosa genome allowed mutant strains to grow as well as the wild type strain. These findings confirmed that only P. aeruginosa AcpP1 functions in fatty acid biosynthesis, and that acpP2 and acpP3 do not play roles in the fatty acid synthetic pathway. Moreover, disruption of acpP2 and acpP3 did not affect the ability of P. aeruginosa to produce N-acylhomoserine lactones (AHL), but replacement of P. aeruginosaacpP1 with E. coliacpP caused P. aeruginosa to reduce the production of AHL molecules, which indicated that neither P. aeruginosa AcpP2 nor AcpP3 can act as a substrate for synthesis of AHL molecules in vivo. Furthermore, replacement of acpP1 with E. coliacpP reduced the ability of P. aeruginosa to produce some exo-products and abolished swarming motility in P. aeruginosa.

Insights into the Psychrophilic and Sea Ice-Specific Lifestyle of Marinobacter sp. Strain AC-23: a Genomic Approach

Marinobacter sp. strain AC-23 was isolated from Kongsfjorden in the Arctic. Here, we report the first draft genome sequence of a putative novel species of the genus Marinobacter comprising 4,149,715 bp, with a mean G+C content of 54.4%. The draft genome sequence will aid in understanding the psychrophilic and sea ice-specific lifestyle.

Functional Characterization of Triclosan-Resistant Enoyl-acyl-carrier Protein Reductase (FabV) in Pseudomonas aeruginosa

Pseudomonas aeruginosa is extremely resistant to triclosan. Previous studies have shown that P. aeruginosa encodes a triclosan-resistant enoyl-acyl-carrier protein reductase (ENR), FabV, and that deletion of fabV causes P. aeruginosa to be extremely sensitive to triclosan. In this report, we complemented a P. aeruginosa fabV deletion strain with several triclosan-resistant ENR encoding genes, including Vibrio cholerae fabV, Bacillus subtilis fabL and Enterococcus faecalis fabK. All complemented strains restored triclosan resistance to the level of the wild-type strain, which confirmed that triclosan-resistant ENR allows P. aeruginosa to be extremely resistant to triclosan. Moreover, fabV exhibits pleiotropic effects. Deletion of fabV led P. aeruginosa to show attenuated swarming motility, decreased rhamnolipid, pyoverdine and acyl-homoserine lactones (AHLs) production. Complementation of the fabV mutant with any one ENR encoding gene could restore these features to some extent, in comparison with the wild-type strain. Furthermore, we found that addition of exogenous AHLs could restore the fabV mutant strain to swarm on semisolid plates and to produce more virulence factors than the fabV mutant strain. These findings indicate that deletion of fabV reduced the activity of ENR in P. aeruginosa, decreased fatty acid synthesis, and subsequently depressed the production of AHLs and other virulence factors, which finally may led to a reduction in the pathogenicity of P. aeruginosa. Therefore, fabV should be an ideal target for the control of P. aeruginosa infectivity.

Metagenomic and metaproteomic analyses of Accumulibacter phosphatis-enriched floccular and granular biofilm

Biofilms are ubiquitous in nature, forming diverse adherent microbial communities that perform a plethora of functions. Here we operated two laboratory-scale sequencing batch reactors enriched with Candidatus Accumulibacter phosphatis (Accumulibacter) performing enhanced biological phosphorus removal. Reactors formed two distinct biofilms, one floccular biofilm, consisting of small, loose, microbial aggregates, and one granular biofilm, forming larger, dense, spherical aggregates. Using metagenomic and metaproteomic methods, we investigated the proteomic differences between these two biofilm communities, identifying a total of 2022 unique proteins. To understand biofilm differences, we compared protein abundances that were statistically enriched in both biofilm states. Floccular biofilms were enriched with pathogenic secretion systems suggesting a highly competitive microbial community. Comparatively, granular biofilms revealed a high-stress environment with evidence of nutrient starvation, phage predation pressure, and increased extracellular polymeric substance and cell lysis. Granular biofilms were enriched in outer membrane transport proteins to scavenge the extracellular milieu for amino acids and other metabolites, likely released through cell lysis, to supplement metabolic pathways. This study provides the first detailed proteomic comparison between Accumulibacter-enriched floccular and granular biofilm communities, proposes a conceptual model for the granule biofilm, and offers novel insights into granule biofilm formation and stability.

Unique and conserved genome regions in Vibrio harveyi and related species in comparison with the shrimp pathogen Vibrio harveyi CAIM 1792

Vibrio harveyi CAIM 1792 is a marine bacterial strain that causes mortality in farmed shrimp in north-west Mexico, and the identification of virulence genes in this strain is important for understanding its pathogenicity. The aim of this work was to compare the V. harveyi CAIM 1792 genome with related genome sequences to determine their phylogenic relationship and explore unique regions in silico that differentiate this strain from other V. harveyi strains. Twenty-one newly sequenced genomes were compared in silico against the CAIM 1792 genome at nucleotidic and predicted proteome levels. The proteome of CAIM 1792 had higher similarity to those of other V. harveyi strains (78%) than to those of the other closely related species Vibrio owensii (67%), Vibrio rotiferianus (63%) and Vibrio campbellii (59%). Pan-genome ORFans trees showed the best fit with the accepted phylogeny based on DNA-DNA hybridization and multi-locus sequence analysis of 11 concatenated housekeeping genes. SNP analysis clustered 34/38 genomes within their accepted species. The pangenomic and SNP trees showed that V. harveyi is the most conserved of the four species studied and V. campbellii may be divided into at least three subspecies, supported by intergenomic distance analysis. blastp atlases were created to identify unique regions among the genomes most related to V. harveyi CAIM 1792; these regions included genes encoding glycosyltransferases, specific type restriction modification systems and a transcriptional regulator, LysR, reported to be involved in virulence, metabolism, quorum sensing and motility.

Acyl-ACP substrate recognition in Burkholderia mallei BmaI1 acyl-homoserine lactone synthase

The acyl-homoserine lactone (AHL) autoinducer mediated quorum sensing regulates virulence in several pathogenic bacteria. The hallmark of an efficient quorum sensing system relies on the tight specificity in the signal generated by each bacterium. Since AHL signal specificity is derived from the acyl-chain of the acyl-ACP (ACP = acyl carrier protein) substrate, AHL synthase enzymes must recognize and react with the native acyl-ACP with high catalytic efficiency while keeping reaction rates with non-native acyl-ACPs low. The mechanism of acyl-ACP substrate recognition in these enzymes, however, remains elusive. In this study, we investigated differences in catalytic efficiencies for shorter and longer chain acyl-ACP substrates reacting with an octanoyl-homoserine lactone synthase Burkholderia mallei BmaI1. With the exception of two-carbon shorter hexanoyl-ACP, the catalytic efficiencies of butyryl-ACP, decanoyl-ACP, and octanoyl-CoA reacting with BmaI1 decreased by greater than 20-fold compared to the native octanoyl-ACP substrate. Furthermore, we also noticed kinetic cooperativity when BmaI1 reacted with non-native acyl-donor substrates. Our kinetic data suggest that non-native acyl-ACP substrates are unable to form a stable and productive BmaI1·acyl-ACP·SAM ternary complex and are thus effectively discriminated by the enzyme. These results offer insights into the molecular basis of substrate recognition for the BmaI1 enzyme.

The Pseudomonas aeruginosa AlgZR two-component system coordinates multiple phenotypes

Pseudomonas aeruginosa is an opportunistic pathogen that causes a multitude of infections. These infections can occur at almost any site in the body and are usually associated with a breach of the innate immune system. One of the prominent sites where P. aeruginosa causes chronic infections is within the lungs of cystic fibrosis patients. P. aeruginosa uses two-component systems that sense environmental changes to differentially express virulence factors that cause both acute and chronic infections. The P. aeruginosa AlgZR two component system is one of its global regulatory systems that affects the organism's fitness in a broad manner. This two-component system is absolutely required for two P. aeruginosa phenotypes: twitching motility and alginate production, indicating its importance in both chronic and acute infections. Additionally, global transcriptome analyses indicate that it regulates the expression of many different genes, including those associated with quorum sensing, type IV pili, type III secretion system, anaerobic metabolism, cyanide and rhamnolipid production. This review examines the complex AlgZR regulatory network, what is known about the structure and function of each protein, and how it relates to the organism's ability to cause infections.

Combining functional and structural genomics to sample the essential Burkholderia structome

Background

The genus Burkholderia includes pathogenic gram-negative bacteria that cause melioidosis, glanders, and pulmonary infections of patients with cancer and cystic fibrosis. Drug resistance has made development of new antimicrobials critical. Many approaches to discovering new antimicrobials, such as structure-based drug design and whole cell phenotypic screens followed by lead refinement, require high-resolution structures of proteins essential to the parasite.

Methodology/Principal Findings

We experimentally identified 406 putative essential genes in B. thailandensis, a low-virulence species phylogenetically similar to B. pseudomallei, the causative agent of melioidosis, using saturation-level transposon mutagenesis and next-generation sequencing (Tn-seq). We selected 315 protein products of these genes based on structure-determination criteria, such as excluding very large and/or integral membrane proteins, and entered them into the Seattle Structural Genomics Center for Infection Disease (SSGCID) structure determination pipeline. To maximize structural coverage of these targets, we applied an “ortholog rescue” strategy for those producing insoluble or difficult to crystallize proteins, resulting in the addition of 387 orthologs (or paralogs) from seven other Burkholderia species into the SSGCID pipeline. This structural genomics approach yielded structures from 31 putative essential targets from B. thailandensis, and 25 orthologs from other Burkholderia species, yielding an overall structural coverage for 49 of the 406 essential gene families, with a total of 88 depositions into the Protein Data Bank. Of these, 25 proteins have properties of a potential antimicrobial drug target i.e., no close human homolog, part of an essential metabolic pathway, and a deep binding pocket. We describe the structures of several potential drug targets in detail.

Conclusions/Significance

This collection of structures, solubility and experimental essentiality data provides a resource for development of drugs against infections and diseases caused by Burkholderia. All expression clones and proteins created in this study are freely available by request.

Attenuation of quorum sensing in the pathogen Acinetobacter baumannii using non-native N-Acyl homoserine lactones

Many bacterial pathogens use quorum sensing (QS) to control virulence. As a result, the development of methods to intercept QS has attracted significant interest as a potential anti-infective therapy. Acinetobacter baumannii has emerged as a pan-drug-resistant pathogen and displays a remarkable ability to persist in hospital settings despite desiccation and antimicrobial treatment. Recent studies have shown that A. baumannii QS mutants have limited motility and fail to form mature biofilms; these phenotypes are linked to its ability to persist on biotic and abiotic surfaces and increase its pathogenicity. A. baumannii uses N-(3-hydroxydodecanoyl)-l-homoserine lactone (OH-dDHL) and its putative cognate receptor, AbaR, for QS. We sought to identify non-native ligands capable of blocking or promoting AbaR activity in A. baumannii for use as chemical probes to modulate QS phenotypes in this pathogen. We screened a focused library of synthetic, non-native N-acyl homoserine lactones (AHLs) to identify such compounds, and several highly potent antagonists and agonists were uncovered, with IC(50) and EC(50) values in the low micromolar range, respectively. The strongest AbaR antagonists largely contained aromatic acyl groups, whereas the AbaR agonists closely resembled OH-dDHL. Notably, the 10 most potent AbaR antagonists also strongly inhibited A. baumannii motility, and five antagonists reduced biofilm formation in A. baumannii by up to 40%. The discovery of these compounds is significant, as they represent, to our knowledge, the first non-native modulators of QS in A. baumannii to be reported and could find utility as new tools to study the role and timing of QS phenotypes in A. baumannii infections.

ExpI and PhzI are descendants of the long lost cognate signal synthase for SdiA

SdiA of E. coli and Salmonella is a LuxR homolog that detects N-acyl homoserine lactones (AHLs). Most LuxR homologs function together with a cognate AHL synthase (a LuxI homolog), but SdiA does not. Instead, SdiA detects AHLs produced by other bacterial species. In this report, we performed a phylogenetic analysis of SdiA. The results suggest that one branch of the Enterobacteriaceae obtained a rhlR/rhlI pair by horizontal transfer. The Erwinia and Pantoea branches still contain the complete pair where it is known as expR/expI and phzR/phzI, respectively. A deletion event removed the luxI homolog from the remainder of the group, leaving just the luxR homolog known as sdiA. Thus ExpR and PhzR are SdiA orthologs and ExpI and PhzI are descendants of the long lost cognate signal synthase of SdiA.

Fatty acid cosubstrates provide β-oxidation precursors for rhamnolipid biosynthesis in Pseudomonas aeruginosa, as evidenced by isotope tracing and gene expression assays

Rhamnolipids have multiple potential applications as "green" surfactants for industry, remediation, and medicine. As a result, they have been intensively investigated to add to our understanding of their biosynthesis and improve yields. Several studies have noted that the addition of a fatty acid cosubstrate increases rhamnolipid yields, but a metabolic explanation has not been offered, partly because biosynthesis studies to date have used sugar or sugar derivatives as the carbon source. The objective of this study was to investigate the role of fatty acid cosubstrates in improving rhamnolipid biosynthesis. A combination of stable isotope tracing and gene expression assays was used to identify lipid precursors and potential lipid metabolic pathways used in rhamnolipid synthesis when fatty acid cosubstrates are present. To this end, we compared the rhamnolipids produced and their yields using either glucose alone or glucose and octadecanoic acid-d(35) as cosubstrates. Using a combination of sugar and fatty acids, the rhamnolipid yield was significantly higher (i.e., doubled) than when glucose was used alone. Two patterns of deuterium incorporation (either 1 or 15 deuterium atoms) in a single Rha-C(10) lipid chain were observed for octadecanoic acid-d(35) treatment, indicating that in the presence of a fatty acid cosubstrate, both de novo fatty acid synthesis and β-oxidation are used to provide lipid precursors for rhamnolipids. Gene expression assays showed a 200- to 600-fold increase in the expression of rhlA and rhlB rhamnolipid biosynthesis genes and a more modest increase of 3- to 4-fold of the fadA β-oxidation pathway gene when octadecanoic acid was present. Taken together, these results suggest that the simultaneous use of de novo fatty acid synthesis and β-oxidation pathways allows for higher production of lipid precursors, resulting in increased rhamnolipid yields.

Fatty acid biosynthesis in Pseudomonas aeruginosa is initiated by the FabY class of β-ketoacyl acyl carrier protein synthases

The prototypical type II fatty acid synthesis (FAS) pathway in bacteria utilizes two distinct classes of β-ketoacyl synthase (KAS) domains to assemble long-chain fatty acids, the KASIII domain for initiation and the KASI/II domain for elongation. The central role of FAS in bacterial viability and virulence has stimulated significant effort toward developing KAS inhibitors, particularly against the KASIII domain of the β-acetoacetyl-acyl carrier protein (ACP) synthase FabH. Herein, we show that the opportunistic pathogen Pseudomonas aeruginosa does not utilize a FabH ortholog but rather a new class of divergent KAS I/II enzymes to initiate the FAS pathway. When a P. aeruginosa cosmid library was used to rescue growth in a fabH downregulated strain of Escherichia coli, a single unannotated open reading frame, PA5174, complemented fabH depletion. While deletion of all four KASIII domain-encoding genes in the same P. aeruginosa strain resulted in a wild-type growth phenotype, deletion of PA5174 alone specifically attenuated growth due to a defect in de novo FAS. Siderophore secretion and quorum-sensing signaling, particularly in the rhl and Pseudomonas quinolone signal (PQS) systems, was significantly muted in the absence of PA5174. The defect could be repaired by intergeneric complementation with E. coli fabH. Characterization of recombinant PA5174 confirmed a preference for short-chain acyl coenzyme A (acyl-CoA) substrates, supporting the identification of PA5174 as the predominant enzyme catalyzing the condensation of acetyl coenzyme A with malonyl-ACP in P. aeruginosa. The identification of the functional role for PA5174 in FAS defines the new FabY class of β-ketoacyl synthase KASI/II domain condensation enzymes.

Stereochemical insignificance discovered in Acinetobacter baumannii quorum sensing

Stereochemistry is a key aspect of molecular recognition for biological systems. As such, receptors and enzymes are often highly stereospecific, only recognizing one stereoisomer of a ligand. Recently, the quorum sensing signaling molecules used by the nosocomial opportunistic pathogen, Acinetobacter baumannii, were identified, and the primary signaling molecule isolated from this species was N-(3-hydroxydodecanoyl)-L-homoserine lactone. A plethora of bacterial species have been demonstrated to utilize 3-hydroxy-acylhomoserine lactone autoinducers, and in virtually all cases, the (R)-stereoisomer was identified as the natural ligand and exhibited greater autoinducer activity than the corresponding (S)-stereoisomer. Using chemical synthesis and biochemical assays, we have uncovered a case of stereochemical insignificance in A. baumannii and provide a unique example where stereochemistry appears nonessential for acylhomoserine lactone-mediated quorum sensing signaling. Based on previously reported phylogenetic studies, we suggest that A. baumannii has evolutionarily adopted this unique, yet promiscuous quorum sensing system to ensure its survival, particularly in the presence of other proteobacteria.

Macromolecular fingerprinting of sulfolobus species in biofilm: a transcriptomic and proteomic approach combined with spectroscopic analysis

Microorganisms in nature often live in surface-associated sessile communities, encased in a self-produced matrix, referred to as biofilms. Biofilms have been well studied in bacteria but in a limited way for archaea. We have recently characterized biofilm formation in three closely related hyperthermophilic crenarchaeotes: Sulfolobus acidocaldarius, S. solfataricus, and S. tokodaii. These strains form different communities ranging from simple carpet structures in S. solfataricus to high density tower-like structures in S. acidocaldarius under static condition. Here, we combine spectroscopic, proteomic, and transcriptomic analyses to describe physiological and regulatory features associated with biofilms. Spectroscopic analysis reveals that in comparison to planktonic life-style, biofilm life-style has distinctive influence on the physiology of each Sulfolobus spp. Proteomic and transcriptomic data show that biofilm-forming life-style is strain specific (eg ca. 15% of the S. acidocaldarius genes were differently expressed, S. solfataricus and S. tokodaii had ∼3.4 and ∼1%, respectively). The -omic data showed that regulated ORFs were widely distributed in basic cellular functions, including surface modifications. Several regulated genes are common to biofilm-forming cells in all three species. One of the most striking common response genes include putative Lrs14-like transcriptional regulators, indicating their possible roles as a key regulatory factor in biofilm development.

S. acidocaldarius, S. solfataricus, and S. tokodaii strains were grown independently as biofilms. Comparison between planktonic and biofilm cell popupations of all three strains was performed by spectroscopic analysis (FTIR and XPS), iTRAQ proteomics, and RNA microarrays. To highlight common features in biofilm formation among the Sulfolobus strains, the data is presented as a comparative analysis. One of the most striking common response genes include putative Lrs14-like transcriptional regulators, suggesting their roles as key regulatory factor in biofilm development.

Quenching the quorum sensing system: potential antibacterial drug targets

Emergence of antibiotic and multi-drug resistant pathogenic bacteria has created the need for new drugs and drug targets. During pathogenesis bacteria release signals which regulate virulence and pathogenicity related genes. Such bacteria co-ordinate their virulent behaviour in a cell density dependent phenomenon termed as quorum sensing (QS). In contrast, microbes interfere with QS system by quenching the signals, termed quorum quenching (QQ). As a consequence of disrupted QS, pathogens become susceptible to antibiotics and drugs. In this article, the biodiversity of organisms with potential to quench QS signals and the use of QQ molecules as antibacterial drugs have been reviewed.

Multiple FadD acyl-CoA synthetases contribute to differential fatty acid degradation and virulence in Pseudomonas aeruginosa

A close interconnection between nutrient metabolism and virulence factor expression contributes to the pathophysiology of Pseudomonas aeruginosa as a successful pathogen. P. aeruginosa fatty acid (FA) degradation is complicated with multiple acyl-CoA synthetase homologs (FadDs) expressed in vivo in lung tissue during cystic fibrosis infections. The promoters of two genetically linked P. aeruginosa fadD genes (fadD1 and fadD2) were mapped and northern blot analysis indicated they could exist on two different transcripts. These FadDs contain ATP/AMP signature and FA-binding motifs highly homologous to those of the Escherichia coli FadD. Upon introduction into an E. coli fadD^-/fadR^- double mutant, both P. aeruginosa fadDs functionally complemented the E. coli fadD^-/fadR^- mutant, allowing degradation of different chain-length FAs. Chromosomal mutagenesis, growth analysis, induction studies, and determination of kinetic parameters suggested that FadD1 has a substrate preference for long-chain FAs while FadD2 prefers shorter-chain FAs. When compared to the wild type strain, the fadD2 mutant exhibited decreased production of lipase, protease, rhamnolipid and phospholipase, and retardation of both swimming and swarming motilities. Interestingly, fadD1 mutant showed only increased swarming motility. Growth analysis of the fadD mutants showed noticeable deficiencies in utilizing FAs and phosphatidylcholine (major components of lung surfactant) as the sole carbon source. This defect translated into decreased in vivo fitness of P. aeruginosa in a BALB/c mouse lung infection model, supporting the role of lipids as a significant nutrient source for this bacterium in vivo.

Quantitative liquid chromatography-tandem mass spectrometry profiling of activated methyl cycle metabolites involved in LuxS-dependent quorum sensing in Escherichia coli

A rapid, selective, and sensitive liquid chromatography-tandem mass spectrometry assay has been developed and validated for the simultaneous quantification of the metabolites and precursors of the activated methyl cycle, reported in preliminary form by Heurlier et al. (2009) [43]. Analytes were extracted from Escherichia coli MG1655 and chemically derivatized as N(O,S)-iso-butyloxycarbonyl iso-butyl esters using iso-butyl chloroformate in an aqueous iso-butanol/pyridine environment. S-Adenosylmethionine, S-adenosylhomocysteine, S-ribosylhomocysteine, homocysteine, methionine, cystathionine, cysteine, and homoserine were quantified by liquid chromatography-positive ion tandem electrospray ionization mass spectrometry. Internal standards were isotopically labeled [(13)CD(3)]methionine and S-adenosylcysteine. Linearity of the assay was established up to a concentration of 700 microg/g cell dry weight for each analyte. The validated assay was used to quantitatively profile the intracellular activated methyl cycle metabolites as a function of growth in E. coli MG1655 and its derivative Deltapfs and DeltaluxS mutants to determine the metabolic consequences of a disruption to the activated methyl cycle and, hence, LuxS-dependent quorum sensing.

Expression, purification and characterization of the acyl carrier protein phosphodiesterase from Pseudomonas Aeruginosa

Acyl carrier protein phosphodiesterases (AcpH) are the only enzymes known to remove the 4'-phosphopantetheinyl moiety from holo acyl carrier proteins (ACP), which are a large family of proteins essential for the biosynthesis of lipid and other cellular metabolites. Here we report that the AcpH (paAcpH) from Pseudomonas aeruginosa can be overexpressed in Escherichia coli as a soluble and stable protein after optimization of the expression and purification conditions. This marks an improvement from the aggregation-prone E. coli AcpH that could only be obtained by refolding the polypeptide obtained from the inclusion body. With the soluble recombinant protein, we found that PaAcpH exhibits preferred substrate specificity towards the ACPs from the fatty acid synthesis pathway among eight carrier proteins. We further showed that PaAcpH hydrolyzes and releases the 4'-phosphopantetheinyl group-linked products from a multidomain polyketide synthase, demonstrating that the enzyme is fully capable of hydrolyzing acylated ACP substrates.

The siderophore pyoverdine of Pseudomonas syringae pv. tabaci 6605 is an intrinsic virulence factor in host tobacco infection

To investigate the role of iron uptake mediated by the siderophore pyoverdine in the virulence of the plant pathogen Pseudomonas syringae pv. tabaci 6605, three predicted pyoverdine synthesis-related genes, pvdJ, pvdL, and fpvA, were mutated. The pvdJ, pvdL, and fpvA genes encode the pyoverdine side chain peptide synthetase III L-Thr-L-Ser component, the pyoverdine chromophore synthetase, and the TonB-dependent ferripyoverdine receptor, respectively. The Delta pvdJ and Delta pvdL mutants were unable to produce pyoverdine in mineral salts-glucose medium, which was used for the iron-depleted condition. Furthermore, the Delta pvdJ and Delta pvdL mutants showed lower abilities to produce tabtoxin, extracellular polysaccharide, and acyl homoserine lactones (AHLs), which are quorum-sensing molecules, and consequently had reduced virulence on host tobacco plants. In contrast, all of the mutants had accelerated swarming ability and increased biosurfactant production, suggesting that swarming motility and biosurfactant production might be negatively controlled by pyoverdine. Scanning electron micrographs of the surfaces of tobacco leaves inoculated with the mutant strains revealed only small amounts of extracellular polymeric matrix around these mutants, indicating disruption of the mature biofilm. Tolerance to antibiotics was drastically increased for the Delta pvdL mutant, as for the Delta psyI mutant, which is defective in AHL production. These results demonstrated that pyoverdine synthesis and the quorum-sensing system of Pseudomonas syringae pv. tabaci 6605 are indispensable for virulence in host tobacco infection and that AHL may negatively regulate tolerance to antibiotics.

Proteome of Gluconacetobacter diazotrophicus co-cultivated with sugarcane plantlets

Gluconacetobacter diazotrophicus is a micro-aerobic bacterium able to fix atmospheric nitrogen in endophytic mode. A proteomic approach was used to analyze proteins differentially expressed in the presence and absence of sugarcane plantlets. Two-dimensional gel electrophoresis (2-DE) showed 42 spots with altered levels of expression. Analysis of these spots by matrix-assisted laser desorption ionization time-of-flight in tandem (MALDI-TOF-TOF) identified 38 proteins. Differentially expressed proteins were associated with carbohydrate and energy metabolism, folding, sorting and degradation processes, and transcription and translation. Among proteins expressed in co-cultivated bacteria, four belong to membrane systems; others, like a transcription elongation factor (GreA), a 60 kDa chaperonin (GroEL), and an outer membrane lipoprotein (Omp16) have also been described in other plant-bacteria associations, indicating a common protein expression pattern as a result of symbiosis. A high protein content of 60kDa chaperonin isoforms was detected as non-differentially expressed proteins of the bacteria proteome. These results allow the assessment of the physiological significance of specific proteins to G. diazotrophicus metabolism and to the pathways involved in bacteria-host endophytic interaction.