Green fluorescent protein-based reporter systems for genetic analysis of bacteria including monocopy applications

Aluminum-Immobilizing Rhizobacteria Modulate Root Exudation and Nutrient Uptake and Increase Aluminum Tolerance of Pea Mutant E107 (brz)

It is well known that plant-growth-promoting rhizobacteria (PGPRs) increase the tolerance of plants to abiotic stresses; however, the counteraction of Al toxicity has received little attention. The effects of specially selected Al-tolerant and Al-immobilizing microorganisms were investigated using pea cultivar Sparkle and its Al-sensitive mutant E107 (brz). The strain Cupriavidus sp. D39 was the most-efficient in the growth promotion of hydroponically grown peas treated with 80 µM AlCl₃, increasing the plant biomass of Sparkle by 20% and of E107 (brz) by two-times. This strain immobilized Al in the nutrient solution and decreased its concentration in E107 (brz) roots. The mutant showed upregulated exudation of organic acids, amino acids, and sugars in the absence or presence of Al as compared with Sparkle, and in most cases, the Al treatment stimulated exudation. Bacteria utilized root exudates and more actively colonized the root surface of E107 (brz). The exudation of tryptophan and the production of IAA by Cupriavidus sp. D39 in the root zone of the Al-treated mutant were observed. Aluminum disturbed the concentrations of nutrients in plants, but inoculation with Cupriavidus sp. D39 partially restored such negative effects. Thus, the E107 (brz) mutant is a useful tool for studying the mechanisms of plant-microbe interactions, and PGPR plays an important role in protecting plants against Al toxicity.

A Methylotrophic Bacterium Growing with the Antidiabetic Drug Metformin as Its Sole Carbon, Nitrogen and Energy Source

Metformin is one of the most prescribed antidiabetic agents worldwide and is also considered for other therapeutic applications including cancer and endocrine disorders. It is largely unmetabolized by human enzymes and its presence in the environment has raised concern, with reported toxic effects on aquatic life and potentially also on humans. We report on the isolation and characterisation of strain MD1, an aerobic methylotrophic bacterium growing with metformin as its sole carbon, nitrogen and energy source. Strain MD1 degrades metformin into dimethylamine used for growth, and guanylurea as a side-product. Sequence analysis of its fully assembled genome showed its affiliation to Aminobacter niigataensis. Differential proteomics and transcriptomics, as well as mini-transposon mutagenesis of the strain, point to genes and proteins essential for growth with metformin and potentially associated with hydrolytic C-N cleavage of metformin or with cellular transport of metformin and guanylurea. The obtained results suggest the recent evolution of the growth-supporting capacity of strain MD1 to degrade metformin. Our results identify candidate proteins of the enzymatic system for metformin transformation in strain MD1 and will inform future research on the fate of metformin and its degradation products in the environment and in humans.

Rhizobacteria Mitigate the Negative Effect of Aluminum on Pea Growth by Immobilizing the Toxicant and Modulating Root Exudation

High soil acidity is one of the main unfavorable soil factors that inhibit the growth and mineral nutrition of plants. This is largely due to the toxicity of aluminum (Al), the mobility of which increases significantly in acidic soils. Symbiotic microorganisms have a wide range of beneficial properties for plants, protecting them against abiotic stress factors. This report describes the mechanisms of positive effects of plant growth-promoting rhizobacteria Pseudomonas fluorescens SPB2137 on four pea (Pisum sativum L.) genotypes grown in hydroponics and treated with 80 µM AlCl3. In batch culture, the bacteria produced auxins, possessed 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, alkalized the medium and immobilized Al, forming biofilm-like structures and insoluble phosphates. Inoculation with Ps. fluorescens SPB2137 increased root and/or shoot biomass of Al-treated plants. The bacteria alkalized the nutrient solution and transferred Al from the solution to the residue, which contained phosphorus that was exuded by roots. As a result, the Al concentration in roots decreased, while the amount of precipitated Al correlated negatively with its concentration in the solution, positively with the solution pH and negatively with Al concentration in roots and shoots. Treatment with Al induced root exudation of organic acids, amino acids and sugars. The bacteria modulated root exudation via utilization and/or stimulation processes. The effects of Al and bacteria on plants varied depending on pea genotype, but all the effects had a positive direction and the variability was mostly quantitative. Thus, Ps. fluorescens SPB2137 improved the Al tolerance of pea due to immobilization and exclusion of toxicants from the root zone.

Generation of Synthetic Shuttle Vectors Enabling Modular Genetic Engineering of Cyanobacteria

Cyanobacteria have raised great interest in biotechnology due to their potential for a sustainable, photosynthesis-driven production of fuels and value-added chemicals. This has led to a concomitant development of molecular tools to engineer the metabolism of those organisms. In this regard, however, even cyanobacterial model strains lag behind compared to their heterotrophic counterparts. For instance, replicative shuttle vectors that allow gene transfer independent of recombination into host DNA are still scarce. Here, we introduce the pSOMA shuttle vector series comprising 10 synthetic plasmids for comprehensive genetic engineering of Synechocystis sp. PCC 6803. The series is based on the small endogenous plasmids pCA2.4 and pCB2.4, each combined with a replicon from Escherichia coli, different selection markers as well as features facilitating molecular cloning and the insulated introduction of gene expression cassettes. We made use of genes encoding green fluorescent protein (GFP) and a Baeyer-Villiger monooxygenase (BVMO) to demonstrate functional gene expression from the pSOMA plasmids in vivo. Moreover, we demonstrate the expression of distinct heterologous genes from individual plasmids maintained in the same strain and thereby confirmed compatibility between the two pSOMA subseries as well as with derivatives of the broad-host-range plasmid RSF1010. We also show that gene transfer into the filamentous model strain Anabaena sp. PCC 7120 is generally possible, which is encouraging to further explore the range of cyanobacterial host species that could be engineered via pSOMA plasmids. Altogether, the pSOMA shuttle vector series displays an attractive alternative to existing plasmid series and thus meets the current demand for the introduction of complex genetic setups and to perform extensive metabolic engineering of cyanobacteria.

Creation of Markerless Genome Modifications in a Nonmodel Bacterium by Fluorescence-Aided Recombineering

Metabolic engineering of nonmodel bacteria is often challenging because of the paucity of genetic tools for iterative genome modification necessary to equip bacteria with pathways to produce high-value products. Here, we outline a homologous recombination-based method developed to delete or add genes to the genome of a nonmodel bacterium, Zymomonas mobilis, at the desired locus using a suicide plasmid that contains gfp as a fluorescence marker to track its presence in cells. The suicide plasmid is engineered to contain two 500 bp regions homologous to the DNA sequence immediately flanking the target locus. A single crossover event at one of the two homologous regions facilitates insertion of the plasmid into the genome and subsequent homologous recombination events excise the plasmid from the genome, leaving either the original genotype or the desired modified genotype. A key feature of this plasmid is that Green Fluorescent Protein (GFP) expressed from the suicide plasmid allows easy identification and sorting of cells that have lost the plasmid by use of a fluorescence activated cell sorter. Subsequent PCR amplification of genomic DNA from strains lacking GFP allows rapid identification of the desired genotype, which is confirmed by DNA sequencing. This method provides an efficient and flexible platform for improved genetic engineering of Z. mobilis, which can be easily adapted to other nonmodel bacteria.

Bacterial Long-Range Warfare: Aerial Killing of Legionella pneumophila by Pseudomonas fluorescens

Legionella pneumophila, the causative agent of Legionnaires’ disease, is mostly found in man-made water systems and is one of the most closely monitored waterborne pathogens. With the aim of finding natural ways to control waterborne pathogens and thus further reduce the impact of disinfection by-products on human health, some studies have demonstrated the ability of bacteria to kill Legionella through the production of secondary metabolites or antimicrobial compounds. Here, we describe an unexpected growth inhibition of L. pneumophila when exposed to a physically separated strain of Pseudomonas fluorescens, designated as MFE01. Most of the members of the Legionellaceae family are sensitive to the volatile substances emitted by MFE01, unlike other bacteria tested. Using headspace solid-phase microextraction GC-MS strategy, a volatilome comparison revealed that emission of 1-undecene, 2-undecanone, and 2-tridecanone were mainly reduced in a Tn5-transposon mutant unable to inhibit at distance the growth of L. pneumophila strain Lens. We showed that 1-undecene was mainly responsible for the inhibition at distance in vitro, and led to cell lysis in small amounts, as determined by gas chromatography-mass spectrometry (GC-MS). Collectively, our results provide new insights into the mode of action of bacterial volatiles and highlight them as potent anti-Legionella agents to focus research on novel strategies to fight legionellosis.

IMPORTANCE Microbial volatile compounds are molecules whose activities are increasingly attracting the attention of researchers. Indeed, they can act as key compounds in long-distance intrakingdom and interkingdom communication, but also as antimicrobials in competition and predation. In fact, most studies to date have focused on their antifungal activities and only a few have reported on their antibacterial properties. Here, we describe that 1-undecene, naturally produced by P. fluorescens, is a volatile with potent activity against bacteria of the genus Legionella. In small amounts, it is capable of inducing cell lysis even when the producing strain is physically separated from the target. This is the first time that such activity is described. This molecule could therefore constitute an efficient compound to counter bacterial pathogens whose treatment may fail, particularly in pulmonary diseases. Indeed, inhalation of these volatiles should be considered as a possible route of therapy in addition to antibiotic treatment.

Caffeine and Theophylline Inhibit β-Galactosidase Activity and Reduce Expression in Escherichia coli

The β-galactosidase enzyme is a common reporter enzyme that has been used extensively in microbiological and synthetic biology research. Here, we demonstrate that caffeine and theophylline, common natural methylxanthine products found in many foods and pharmaceuticals, negatively impact both the expression and activity of β-galactosidase in Escherichia coli. The β-galactosidase activity in E. coli grown with increasing concentrations of caffeine and theophylline was reduced over sixfold in a dose-dependent manner. We also observed decreasing lacZ mRNA transcript levels with increasing methylxanthine concentrations in the growth media. Similarly, caffeine and theophylline inhibit the activity of the purified β-galactosidase enzyme, with an approximately 1.7-fold increase in K M toward o-nitrophenyl-β-galactoside and a concomitant decrease in v max. The authors recommend the use of alternative reporter systems when such methylxanthines are expected to be present.

Development of Strong Anaerobic Fluorescent Reporters for Clostridium acetobutylicum and Clostridium ljungdahlii Using HaloTag and SNAP-tag Proteins

One of the biggest limitations in the study and engineering of anaerobic Clostridium organisms is the lack of strong fluorescent reporters capable of strong and real-time fluorescence. Recently, we developed a strong fluorescent reporter system for Clostridium organisms based on the FAST protein. Here, we report the development of two new strong fluorescent reporter systems for Clostridium organisms based on the HaloTag and SNAP-tag proteins, which produce strong fluorescent signals when covalently bound to fluorogenic ligands. These new fluorescent reporters are orthogonal to the FAST ligands and to each other, allowing for simultaneous labeling and visualization. We used HaloTag and SNAP-tag to label the strictly anaerobic organisms Clostridium acetobutylicum and Clostridium ljungdahlii We have also identified a new strong promoter for protein expression in C. acetobutylicum, based on the phosphotransacetylase gene (pta) from C. ljungdahlii Furthermore, the HaloTag and the SNAP-tag, in combination with the previously described FAST system, were successfully used to measure cell populations in bacterial mixed cultures and showed the simultaneous orthogonal labeling of HaloTag and SNAP-tag together with the FAST protein reporter. Finally, we show the expression of recombinant fusion protein of FAST and the ZapA division protein (from C. acetobutylicum) in C. ljungdahlii. The availability of multiple strong fluorescent reporters is a major addition to the genetic toolkit of Clostridium and other anaerobes that will lead to better understanding of these unique organisms.IMPORTANCE Up to this point, assays and methods involving fluorescent reporter proteins were unavailable or limited in Clostridium organisms and other strict anaerobes. Green fluorescent protein (GFP), mCherry, and flavin-binding proteins (and their derivatives) have been used only in a few clostridia with limited success and yielded low fluorescence compared to aerobic microbial systems. Recently, we reported a new strong fluorescent reporter system based on the FAST protein as a first step in expanding the fluorescence-based reporters for Clostridium and other anaerobic microbial platforms. Additional strong orthogonal fluorescent proteins, with distinct emission spectra are needed to allow for (i) multispecies tracking within the growing field of microbial cocultures and microbiomes, (ii) protein localization and tracking in anaerobes, and (iii) identification and development of natural and synthetic promoters, ribosome-binding sites (RBS), and terminators for optimal protein expression in anaerobes. Here, we present two new strong fluorescent reporter systems based on the HaloTag and SNAP-tag proteins.

Chromatic Bacteria - A Broad Host-Range Plasmid and Chromosomal Insertion Toolbox for Fluorescent Protein Expression in Bacteria

Differential fluorescent labeling of bacteria has become instrumental for many aspects of microbiological research, such as the study of biofilm formation, bacterial individuality, evolution, and bacterial behavior in complex environments. We designed a variety of plasmids, each bearing one of eight unique, constitutively expressed fluorescent protein genes in conjunction with one of four different antibiotic resistance combinations. The fluorophores mTagBFP2, mTurquoise2, sGFP2, mClover3, sYFP2, mOrange2, mScarlet-I, and mCardinal, encoding for blue, cyan, green, green-yellow, yellow, orange, red, and far-red fluorescent proteins, respectively, were combined with selectable markers conferring tetracycline, gentamicin, kanamycin, and/or chloramphenicol resistance. These constructs were cloned into three different plasmid backbones: a broad host-range plasmid, a Tn5 transposon delivery plasmid, and a Tn7 transposon delivery plasmid. The utility of the plasmids and transposons was tested in bacteria from the phyla Actinobacteria, Proteobacteria, and Bacteroidetes. We were able to tag representatives from the phylum Proteobacteria at least via our Tn5 transposon delivery system. The present study enables labeling bacteria with a set of plasmids available to the community. One potential application of fluorescently-tagged bacterial species is the study of bacteria-bacteria, bacteria-host, and bacteria-environment interactions.

Stable Expression of Modified Green Fluorescent Protein in Group B Streptococci To Enable Visualization in Experimental Systems

Group B streptococcus (GBS) is a Gram-positive bacterium associated with various diseases in humans and animals. Many studies have examined GBS physiology, virulence, and microbe-host interactions using diverse imaging approaches, including fluorescence microscopy. Strategies to label and visualize GBS using fluorescence biomarkers have been limited to antibody-based methods or nonspecific stains that bind DNA or protein; an effective plasmid-based system to label GBS with a fluorescence biomarker would represent a useful visualization tool. In this study, we developed and validated a green fluorescent protein (GFP)-variant-expressing plasmid, pGU2664, which can be applied as a marker to visualize GBS in experimental studies. The synthetic constitutively active CP25 promoter drives strong and stable expression of the GFPmut3 biomarker in GBS strains carrying pGU2664. GBS maintains GFPmut3 activity at different phases of growth. The application of fluorescence polarization enables easy discrimination of GBS GFPmut3 activity from the autofluorescence of culture media commonly used to grow GBS. Differential interference contrast microscopy, in combination with epifluorescence microscopy to detect GFPmut3 in GBS, enabled visualization of bacterial attachment to live human epithelial cells in real time. Plasmid pGU2664 was also used to visualize phenotypic differences in the adherence of wild-type GBS and an isogenic gene-deficient mutant strain lacking CovR (the control of virulence regulator) in adhesion assays. The system for GFPmut3 expression in GBS described in this study provides a new tool for the visualization of this organism in diverse research applications. We discuss the advantages and consider the limitations of this fluorescent biomarker system developed for GBS.IMPORTANCE Group B streptococcus (GBS) is a bacterium associated with various diseases in humans and animals. This study describes the development of a strategy to label and visualize GBS using a fluorescence biomarker, termed GFPmut3. We show that this biomarker can be successfully applied to track the growth of bacteria in liquid medium, and it enables the detailed visualization of GBS in the context of live human cells in real-time microscopic analysis. The system for GFPmut3 expression in GBS described in this study provides a new tool for the visualization of this organism in diverse research applications.

Tetranychus urticae (Acari: Tetranychidae) transmits Acidovorax citrulli, causal agent of bacterial fruit blotch of watermelon

The two-spotted spider mite (TSSM) Tetranychus urticae is one of the most important pests of cucurbit plants. If TSSM can act as vector for Acidovorax citrulli (Acc), causal agent of bacterial fruit blotch (BFB), then the movement of mites from infected to healthy plants may represent a potential source of inocula for BFB outbreaks. To confirm the association between Acc and TSSM, we generated a green fluorescent protein-tagged mutant strain (Acc02rf) by transposon mutagenesis and demonstrated that TSSM can transmit Acc from infected to non-infected watermelon plants. Challenge with 10 TSSMs carrying Acc02rf population densities of 1.3 × 10(3) CFU each on freshly grown individual watermelon plants caused disease transmission to 53 %. Incubation periods ranged 7-9 days. Bacteria recovered from symptoms typical of those associated with leaf necrosis were characterized and identified as Acc. To our knowledge, this is the first report showing that TSSM can be a vector of Acc. The results reported here support that the strong association of TSSM with Acc is of particular importance in controlling BFB.

BEEP: An assay to detect bio-energetic and envelope permeability alterations in Pseudomonas aeruginosa

We developed an effective and rapid assay to detect both bio-energetic and envelope permeability (BEEP) alterations of Pseudomonas aeruginosa. The assay is based on quantification of extracellular ATP in bacterial cultures using luciferase as a reporter. To demonstrate the validity of our assay we conducted a biased screen of a transposon insertion library in P. aeruginosa strain PAO1 in order to expedite the isolation of mutants with defects in bioenergetic pathways. We successfully isolated insertion mutants that were reduced for extracellular ATP accumulation and identified the corresponding mutations that caused the phenotype. Most of the genes identified from this analysis were associated with energy metabolism and several appeared to be potentially novel bioenergetic targets. In addition, we show that treatment of P. aeruginosa strain PAO1 with antibiotics that disrupt the bacterial cell envelope leads to greater extracellular ATP accumulation. In summary, increases in extracellular ATP accumulation above wild type levels indicated a perturbation of membrane permeability while decreases in extracellular ATP accumulation indicated defects in bioenergetics.

Strain ŽP - the first bacterial conjugation-based "kill"-"anti-kill" antimicrobial system

As multidrug resistant bacteria pose one of the greatest risks to human health new alternative antibacterial agents are urgently needed. One possible mechanism that can be used as an alternative to traditional antibiotic therapy is transfer of killing agents via conjugation. Our work was aimed at providing a proof of principle that conjugation-based antimicrobial systems are possible. We constructed a bacterial conjugation-based "kill"-"anti-kill" antimicrobial system employing the well known Escherichia coli probiotic strain Nissle 1917 genetically modified to harbor a conjugative plasmid carrying the "kill" gene (colicin ColE7 activity gene) and a chromosomally encoded "anti-kill" gene (ColE7 immunity gene). The constructed strain acts as a donor in conjugal transfer and its efficiency was tested in several types of conjugal assays. Our results clearly demonstrate that conjugation-based antimicrobial systems can be highly efficient.

A set of versatile brick vectors and promoters for the assembly, expression, and integration of synthetic operons in Methylobacterium extorquens AM1 and other alphaproteobacteria

The discipline of synthetic biology requires standardized tools and genetic elements to construct novel functionalities in microorganisms; yet, many model systems still lack such tools. Here, we describe a novel set of vectors that allows the convenient construction of synthetic operons in Methylobacterium extorquens AM1, an important alphaproteobacterial model organism for methylotrophy and a promising platform organism for methanol-based biotechnology. In addition, we provide a set of constitutive alphaproteobacterial promoters of different strengths that were characterized in detail by two approaches: on the single-cell scale and on the cell population level. Finally, we describe a straightforward strategy to deliver synthetic constructs to the genome of M. extorquens AM1 and other Alphaproteobacteria. This study defines a new standard to systematically characterize genetic parts for their use in M. extorquens AM1 by using single-cell fluorescence microscopy and opens the toolbox for synthetic biological applications in M. extorquens AM1 and other alphaproteobacterial model systems.

Oral streptococci and nitrite-mediated interference of Pseudomonas aeruginosa

The oral cavity harbors a diverse community of microbes that are physiologically unique. Oral microbes that exist in this polymicrobial environment can be pathogenic or beneficial to the host. Numerous oral microbes contribute to the formation of dental caries and periodontitis; however, there is little understanding of the role these microbes play in systemic infections. There is mounting evidence that suggests that oral commensal streptococci are cocolonized with Pseudomonas aeruginosa during cystic fibrosis pulmonary infections and that the presence of these oral streptococci contributes to improved lung function. The goal of this study was to examine the underlying mechanism by which Streptococcus parasanguinis antagonizes pathogenic P. aeruginosa. In this study, we discovered that oral commensal streptococci, including Streptococcus parasanguinis, Streptococcus sanguinis, and Streptococcus gordonii, inhibit the growth of P. aeruginosa and that this inhibition is mediated by the presence of nitrite and the production of hydrogen peroxide (H2O2) by oral streptococci. The requirement of both H2O2 and nitrite for the inhibition of P. aeruginosa is due to the generation of reactive nitrogenous intermediates (RNI), including peroxynitrite. Transposon mutagenesis showed that a P. aeruginosa mutant defective in a putative ABC transporter permease is resistant to both streptococcus/nitrite- and peroxynitrite-mediated killing. Furthermore, S. parasanguinis protects Drosophila melanogaster from killing by P. aeruginosa in a nitrite-dependent manner. Our findings suggest that the combination of nitrite and H2O2 may represent a unique anti-infection strategy by oral streptococci during polymicrobial infections.

Prevalence of streptomycin-resistance genes in bacterial populations in European habitats

The prevalence of selected streptomycin (Sm)-resistance genes, i.e. aph (3''), aph (6)-1d, aph (6)-1c, ant (3'') and ant (6), was assessed in a range of pristine as well as polluted European habitats. These habitats included bulk and rhizosphere soils, manure from farm animals, activated sludge from wastewater treatment plants and seawater. The methods employed included assessments of the prevalence of the genes in habitat-extracted DNA by PCR, followed by hybridisation with specific probes, Sm-resistant culturable bacteria and exogenous isolation of plasmids carrying Sm-resistance determinants. The direct DNA-based analysis showed that aph (6)-1d genes were most prevalent in the habitats examined. The presence of the other four Sm-modifying genes was demonstrated in 58% of the tested habitats. A small fraction of the bacterial isolates (8%) did not possess any of the selected Sm-modifying genes. These isolates were primarily obtained from activated sludge and manure. The presence of Sm-modifying genes in the isolates often coincided with the presence of IncP plasmids. Exogenous isolation demonstrated the presence of plasmids of 40-200 kb in size harbouring Sm-resistance genes from all the environments tested. Most plasmids were shown to carry the ant (3'') gene, often in combination with other Sm-resistance genes, such as aph (3'') and aph (6)-1d. The most commonly found Sm-modifying gene on mobile genetic elements was ant (3''). Multiple Sm-resistance genes on the same genetic elements appeared to be the rule rather than the exception. It is concluded that Sm-resistance genes are widespread in the environmental habitats studied and often occur on mobile genetic elements and ant (3'') was most often encountered.

Construction and characterization of a Streptococcus suis serotype 2 recombinant expressing enhanced green fluorescent protein

Streptococcus suis serotype 2 (S. suis 2) is an important pathogen, responsible for diverse diseases in swine and humans. To obtain a S. suis 2 strain that can be tracked in vitro and in vivo, we constructed the Egfp-HA9801 recombinant S. suis 2 strain with egfp and spc^r genes inserted via homologous recombination. To assess the effects of the egfp and spc^r genes in HA9801, the biochemical characteristics, growth features and virulence in Balb/C mice were compared between the recombinant and the parent HA9801 strain. We detected the EGFP expression from Egfp-HA9801 by epifluorescence microscopy. The results showed that the biochemical characterization and growth features of the Egfp-HA9801 recombinant were highly similar to that of the parent HA9801. We did not find significant differences in lethality (50% lethal dose), morbidity and mortality between the two strains. Furthermore, the bacterial counts in each various tissues of Egfp-HA9801-infected mice displayed similar dynamic compared with the HA9801-infected mice. Our results also showed that the Egfp-HA9801 cells grown at 37°C for 36 h displayed greater green fluorescence signals than the cells grown at 28°C for 36 h and 37°C for 24 h. The fluorescence in the tissue cryosections of Egfp-HA9801-injected mice was also stronger than that of the HA9801 group. Together, these results indicate that the egfp and spc^r insertions into the Egfp-HA9801 recombinant did not significantly change the virulence when compared with HA980, and this EGFP labeled strain can be used for future S. suis 2 pathogenesis research.

Forward genetic in planta screen for identification of plant-protective traits of Sphingomonas sp. strain Fr1 against Pseudomonas syringae DC3000

Sphingomonas sp. strain Fr1 has recently been shown to protect Arabidopsis thaliana against the bacterial leaf pathogen Pseudomonas syringae DC3000. Here, we describe a forward genetic in planta screen to identify genes in Sphingomonas sp. Fr1 necessary for this effect. About 5,000 Sphingomonas sp. Fr1 mini-Tn5 mutants were assayed for a defect in plant protection against a luxCDABE-tagged P. syringae DC3000 derivative in a space-saving 24-well plate system. The bioluminescence of the pathogen was used as the indicator of pathogen proliferation and allowed for the identification of Sphingomonas sp. Fr1 mutants that had lost the ability to restrict pathogen growth before disease symptoms were visible. Potential candidates were validated using the same miniaturized experimental system. Of these mutants, 10 were confirmed as plant protection defective yet colonization competent. The mutants were subsequently evaluated in a previously described standard microbox system, and plants showed enhanced disease phenotypes after pathogen infection relative to those inoculated with the parental strain as a control. However, the disease severities were lower than those observed for control plants that were grown axenically prior to pathogen challenge, which suggests that several traits may contribute to plant protection. Transposon insertion sites of validated mutants with defects in plant protection were determined and mapped to 7 distinct genomic regions. In conclusion, the established screening protocol allowed us to identify mutations that affect plant protection, and it opens the possibility to uncover traits important for in planta microbe-microbe interactions.

Phytohormone production and colonization of canola (Brassica napus L.) roots by Pseudomonas fluorescens 6-8 under gnotobiotic conditions

Pseudomonas fluorescens 6-8, a rhizosphere isolate previously shown to enhance root elongation of canola ( Brassica napus L.), was characterized for its ability to produce indole-3-acetic acid and cytokinins in pure culture and in the rhizosphere of canola under gnotobiotic conditions in comparison with the cytokinin-producing strain P. fluorescens G20-18 and its mutant CNT2. Strain 6-8 produced isopentenyl adenosine, zeatin riboside, and dihydroxyzeatin riboside at levels similar to those of G20-18, but only very low concentrations of indole-3-acetic acid. In a gnotobiotic assay canola inoculated with 6-8 and G20-18 had higher concentrations of isopentenyl adenosine and zeatin riboside in the rhizosphere and greater root length than the noninoculated control. The ability of strain 6-8 to colonize canola roots was assessed following transformation with the green fluorescent protein and inoculation onto canola seed in a gnotobiotic assay. Higher populations of strain 6-8 were observed on the proximal region of the root closest to the seed than on the mid and distal portions 9 days after seed inoculation. The ability of P. fluorescens 6-8 to produce cytokinins, colonize the roots of canola seedlings, and enhance root elongation may contribute to its ability to survive in the rhizosphere and may benefit seedling growth.

Dispersal of Salmonella Typhimurium by rain splash onto tomato plants

Outbreaks of Salmonella enterica have increasingly been associated with tomatoes and traced back to production areas, but the spread of Salmonella from a point source onto plants has not been described. Splash dispersal by rain could be one means of dissemination. Green fluorescent protein-labeled, kanamycin-resistant Salmonella enterica sv. Typhimurium dispensed on the surface of plastic mulch, organic mulch, or soil at 10⁸ CFU/cm² was used as the point source in the center of a rain simulator. Tomato plants in soil with and without plastic or organic mulch were placed around the point source, and rain intensities of 60 and 110 mm/h were applied for 5, 10, 20, and 30 min. Dispersal of Salmonella followed a negative exponential model with a half distance of 3 cm at 110 mm/h. Dispersed Salmonella survived for 3 days on tomato leaflets, with a total decline of 5 log and an initial decimal reduction time of 10 h. Recovery of dispersed Salmonella from plants at the maximum observed distance ranged from 3 CFU/g of leaflet after a rain episode of 110 mm/h for 10 min on soil to 117 CFU/g of leaflet on plastic mulch. Dispersal of Salmonella on plants with and without mulch was significantly enhanced by increasing rain duration from 0 to 10 min, but dispersal was reduced when rainfall duration increased from 10 to 30 min. Salmonella may be dispersed by rain to contaminate tomato plants in the field, especially during rain events of 10 min and when plastic mulch is used.

Colonization and bioherbicidal activity on green foxtail byPseudomonas fluorescensBRG100 in a pesta formulation

Pseudomonas fluorescens BRG100 produces secondary metabolites with herbicidal activity on green foxtail ( Setaria viridis ), an important weed pest in Canadian agriculture. Five gfp transformants of P. fluorescens BRG100 were compared with the wild-type isolate for green foxtail root herbicide activity, i.e., root growth suppression, doubling time, carbon utilization, and colonization of green foxtail root (proximal and distal regions). The most revealing comparison between the wild type and its gfp transformants was herbicidal activity on green foxtail. Herbicidal activity of transformant gfp-7 was not significantly different from the uninoculated control, suggesting that insertion of the gfp gene may have interfered with a gene, or genes, vital to the bioherbicide process. Doubling time, carbon utilization, and colonization of green foxtail did not differ to a great extent between the wild type and the gfp transformants, indicating their suitability as conservatively tagged organisms for subsequent colonization–herbicidal activity studies. Accordingly, a pesta granule formulation delivered transformant gfp-2 to the seed coat and roots of green foxtail. Epifluorescent and confocal laser scanning microscopy revealed the transformant gfp-2 colonized the ventral portion of the seed coat, root hairs, and all areas of the root except the root cap region, where gfp-2 presumably exerted herbicidal effects. These results suggest that P. fluorescens BRG100 has considerable potential as a bioherbicide because of its successful colonization and suppressive activity on green foxtail root growth.

Internal colonization of Salmonella enterica serovar Typhimurium in tomato plants

Several Salmonella enterica outbreaks have been traced back to contaminated tomatoes. In this study, the internalization of S. enterica Typhimurium via tomato leaves was investigated as affected by surfactants and bacterial rdar morphotype, which was reported to be important for the environmental persistence and attachment of Salmonella to plants. Surfactants, especially Silwet L-77, promoted ingress and survival of S. enterica Typhimurium in tomato leaves. In each of two experiments, 84 tomato plants were inoculated two to four times before fruiting with GFP-labeled S. enterica Typhimurium strain MAE110 (with rdar morphotype) or MAE119 (without rdar). For each inoculation, single leaflets were dipped in 10(9) CFU/ml Salmonella suspension with Silwet L-77. Inoculated and adjacent leaflets were tested for Salmonella survival for 3 weeks after each inoculation. The surface and pulp of ripe fruits produced on these plants were also examined for Salmonella. Populations of both Salmonella strains in inoculated leaflets decreased during 2 weeks after inoculation but remained unchanged (at about 10(4) CFU/g) in week 3. Populations of MAE110 were significantly higher (P<0.05) than those of MAE119 from day 3 after inoculation. In the first year, nine fruits collected from one of the 42 MAE119 inoculated plants were positive for S. enterica Typhimurium. In the second year, Salmonella was detected in adjacent non-inoculated leaves of eight tomato plants (five inoculated with strain MAE110). The pulp of 12 fruits from two plants inoculated with MAE110 was Salmonella positive (about 10(6) CFU/g). Internalization was confirmed by fluorescence and confocal laser microscopy. For the first time, convincing evidence is presented that S. enterica can move inside tomato plants grown in natural field soil and colonize fruits at high levels without inducing any symptoms, except for a slight reduction in plant growth.

Functional genomics of dichloromethane utilization in Methylobacterium extorquens DM4

Dichloromethane (CH(2)Cl(2) , DCM) is a chlorinated solvent mainly produced by industry, and a common pollutant. Some aerobic methylotrophic bacteria are able to grow with this chlorinated methane as their sole carbon and energy source, using a DCM dehalogenase/glutathione S-transferase encoded by dcmA to transform DCM into two molecules of HCl and one molecule of formaldehyde, a toxic intermediate of methylotrophic metabolism. In Methylobacterium extorquens DM4 of known genome sequence, dcmA lies on a 126 kb dcm genomic island not found so far in other DCM-dechlorinating strains. An experimental search for the molecular determinants involved in specific cellular responses of strain DM4 growing with DCM was performed. Random mutagenesis with a minitransposon containing a promoterless reporter gfp gene yielded 25 dcm mutants with a specific DCM-associated phenotype. Differential proteomic analysis of cultures grown with DCM and with methanol defined 38 differentially abundant proteins. The 5.5 kb dcm islet directly involved in DCM dehalogenation is the only one of seven gene clusters specific to the DCM response to be localized within the dcm genomic island. The DCM response was shown to involve mainly the core genome of Methylobacterium extorquens, providing new insights on DCM-dependent adjustments of C1 metabolism and gene regulation, and suggesting a specific stress response of Methylobacterium during growth with DCM. Fatty acid, hopanoid and peptidoglycan metabolisms were affected, hinting at the membrane-active effects of DCM due to its solvent properties. A chloride-induced efflux transporter termed CliABC was also newly identified. Thus, DCM dechlorination driven by the dcm islet elicits a complex adaptive response encoded by the core genome common to dechlorinating as well as non-dechlorinating Methylobacterium strains.

Production of the chiral compound (R)-3-hydroxybutyrate by a genetically engineered methylotrophic bacterium

In this study, a methylotrophic bacterium, Methylobacterium rhodesianum MB 126, was used for the production of the chiral compound (R)-3-hydroxybutyrate (R-3HB) from methanol. R-3HB is formed during intracellular degradation of the storage polymer (R)-3-polyhydroxybutyrate (PHB). Since the monomer R-3HB does not accumulate under natural conditions, M. rhodesianum was genetically modified. The gene (hbd) encoding the R-3HB-degrading enzyme, R-3HB dehydrogenase, was inactivated in M. rhodesianum. The resulting hbd mutant still exhibited low growth rates on R-3HB as the sole source of carbon and energy, indicating the presence of alternative pathways for R-3HB utilization. Therefore, transposon mutagenesis was carried out with the hbd mutant, and a double mutant unable to grow on R-3HB was obtained. This mutant was shown to be defective in lipoic acid synthase (LipA), resulting in an incomplete citric acid cycle. Using the hbd lipA mutant, we produced 3.2 to 3.5 mM R-3HB in batch and 27 mM (2,800 mg liter(-1)) in fed-batch cultures. This was achieved by sequences of cultivation conditions initially favoring growth, then PHB accumulation, and finally PHB degradation.

Enhanced biofilm formation and 3-chlorobenzoate degrading activity by the bacterial consortium of Burkholderia sp. NK8 and Pseudomonas aeruginosa PAO1

AIMS

To characterize biofilm formation of a chlorobenzoates (CBs) degrading bacterium, Burkholderia sp. NK8, with another bacterial species, and the biodegradation activity against CBs in the mixed-species biofilm.

METHODS AND RESULTS

Burkholderia sp. NK8 was solely or co-cultured with each of five other representative bacteria in microtitre dishes. Biofilm formation involving the strain NK8 was synergistically promoted by co-culturing with only Pseudomonas aeruginosa PAO1. Epifluorescent microscopy revealed that cells of the bacterial strain NK8 were viable and distributed randomly in the mixed-species biofilms. Enumeration of the attached cells on the surface of wells revealed that cells of the strain NK8 increased approx. 10-fold by the co-culture with the strain PAO1 compared to those by monoculture of the strain NK8, and the degradation activity of 3-chlorobenzoate by the dual-species biofilms was more promoted than that by the strain NK8-monocultured biofilms.

CONCLUSIONS

Enhanced biofilm formation of Burkholderia sp. NK8 by the bacterial consortium occurred, but is determined by the partner bacterial species. The mixed-species biofilms have the advantage to degrade CBs on a solid surface.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides a significance of bacterial consortia on the biofilm formation and the degradation activity of Burkholderia sp. NK8, which contribute for complete degradation of chlorinated aromatics.

Virulence determinants from a cystic fibrosis isolate of Pseudomonas aeruginosa include isocitrate lyase

Chronic lung infections caused by Pseudomonas aeruginosa are the leading cause of morbidity and mortality for cystic fibrosis (CF) patients. Adaptation of P. aeruginosa to the CF lung results in the loss of acute virulence determinants and appears to activate chronic virulence strategies in this pathogen. In order to identify such strategies, a random transposon mutagenesis was performed and 18 genes that were required for optimal infection of alfalfa seedlings by FRD1, a CF isolate of P. aeruginosa, were recognized. The largest subset of genes (seven of the 18), were associated with central carbon metabolism, including the gene that encodes isocitrate lyase (ICL), aceA. Because FRD1 is avirulent in animal infection models, we constructed an ICL mutant in P. aeruginosa strain PAO1 in order to assess the requirement of ICL in mammalian infection. The PAO1 ICL mutant was less virulent in the rat lung infection model, indicating that ICL is required for the pathogenesis of P. aeruginosa in mammals. Furthermore, FRD1 showed increased ICL activity and expression of an aceA : : lacZ fusion compared to PAO1. We suggest that upregulation of ICL occurred during adaptation of FRD1 to the CF lung and that some of the novel virulence mechanisms employed by FRD1 to infect alfalfa seedlings may be the same mechanisms P. aeruginosa relies upon to persist within human niches.

Monitoring of phytopathogenic Ralstonia solanacearum cells using green fluorescent protein-expressing plasmid derived from bacteriophage phiRSS1

A green fluorescent protein (GFP)-expressing plasmid was constructed from a filamentous bacteriophage phiRSS1 that infects the phytopathogen Ralstonia solanacearum. This plasmid designated as pRSS12 (4.7 kbp in size) consists of an approximately 2248 bp region of the phiRSS1 RF DNA, including ORF1-ORF3 and the intergenic region (IG), and a Km cassette in addition to the GFP gene. It was easily introduced by electroporation and stably maintained even without selective pressure in strains of R. solanacearum of different races and biovars. Strong green fluorescence emitted from pRSS12-transformed bacterial cells was easily monitored in tomato tissues (stem, petiole, and root) after infection as well as from soil samples. These results suggest that pRSS12 can serve as an easy-to-use GFP-tagging tool for any given strain of R. solanacearum in cytological as well as field studies.

Abscisic acid deficiency leads to rapid activation of tomato defence responses upon infection with Erwinia chrysanthemi

In addition to the important role of abscisic acid (ABA) in abiotic stress signalling, basal and high ABA levels appear to have a negative effect on disease resistance. Using the ABA-deficient sitiens tomato (Solanum lycopersicum) mutant and different application methods of exogenous ABA, we demonstrated the influence of this plant hormone on disease progression of Erwinia chrysanthemi. This necrotrophic plant pathogenic bacterium is responsible for soft rot disease on many plant species, causing maceration symptoms mainly due to the production and secretion of pectinolytic enzymes. On wild-type (WT) tomato cv. Moneymaker E. chrysanthemi leaf inoculation resulted in maceration both within and beyond the infiltrated zone of the leaf, but sitiens showed a very low occurrence of tissue maceration, which never extended the infiltrated zone. A single ABA treatment prior to infection eliminated the effect of pathogen restriction in sitiens, while repeated ABA spraying during plant development rendered both WT and sitiens very susceptible. Quantification of E. chrysanthemi populations inside the leaf did not reveal differences in bacterial growth between sitiens and WT. Sitiens was not more resistant to pectinolytic cell-wall degradation, but upon infection it showed a faster and stronger activation of defence responses than WT, such as hydrogen peroxide accumulation, peroxidase activation and cell-wall fortifications. Moreover, the rapid activation of sitiens peroxidases was also observed after application of bacteria-free culture filtrate containing E. chrysanthemi cell-wall-degrading enzymes and was absent during infection with an out E. chrysanthemi mutant impaired in secretion of these extracellular enzymes.

Construction and evaluation of a microbiological positive process internal control for PCR-based examination of food samples for Listeria monocytogenes and Salmonella enterica

PCR assays for food-borne pathogens are widely available but have had more limited application to food testing compared with their use in clinical laboratories. When testing food samples, false negative PCR results can occur and may be due to interference with target-cell lysis necessary for nucleic acid extraction, nucleic acid degradation and/or direct inhibition of the PCR. Therefore, it is essential to include appropriate controls for the application of PCR to the detection of pathogens in food samples. The purpose of this study was to develop and evaluate a novel internal control (IC), capable of monitoring the complete detection procedure, from DNA extraction through to amplification and detection. A 'positive process' IC was developed for the reliable application of real-time PCR assays for the detection of Salmonella enterica or Listeria monocytogenes in enrichment broths. Two novel real-time 5' nuclease PCR assays for the detection of a 77 bp fragment of the green fluorescent protein (gfp) gene and a 91 bp fragment of the iroB gene of S. enterica were developed. These assays were specific and had detection limits of 5+/-0.88 and 15+/-1.03 CFU per PCR for the gfp and iroB genes respectively. The gfp PCR assay was combined with the iroB PCR assay, and also with a previously published real-time 5' nuclease PCR assay for the detection of the hlyA gene of L. monocytogenes. Duplexed assays were optimised such that the target genes were simultaneously amplified at similar sensitivities to single reactions. The gfp gene was cloned into the chromosome of a non-pathogenic strain of Escherichia coli and the modified strain successfully encapsulated in LENTICULE discs. A single disc was added to 1 ml of standard enrichment broths immediately prior to DNA extraction, and used as an IC for the detection of L. monocytogenes and S. enterica by PCR. This method was evaluated using 393 naturally contaminated food or environmental samples, 267 for the detection of Salmonella spp. and 126 for Listeria spp. PCR inhibition was detected in 29 (8%) extracts, although neither pathogens were detected in these broths by culture. S. enterica was detected by PCR in 43 of 45 (96%) broths that were positive by conventional culture. The iroB gene was also detected in a further 2 broths by PCR alone. L. monocytogenes was detected in 6 broths by both PCR and conventional culture, plus an additional 7 by PCR only. Control strategies such as those described here are important tools for the interpretation of PCR assays by improving the reliability of detection of pathogens in food.

Thermoregulation of N-acyl homoserine lactone-based quorum sensing in the soft rot bacterium Pectobacterium atrosepticum

The psychrotolerant bacterium Pectobacterium atrosepticum produces four N-acyl homoserine lactones under a wide range of temperatures. Their thermoregulation differs from that of the exoenzyme production, described as being under quorum-sensing control. A mechanism involved in this thermoregulation consists of controlling N-acyl homoserine lactones synthase production at a transcriptional level.

Heterogeneity in expression of the Escherichia coli colicin K activity gene cka is controlled by the SOS system and stochastic factors

Phenotypic diversity provides populations of prokaryotic and eukaryotic organisms with the flexibility required to adapt to and/or survive environmental perturbations. Consequently, there is much interest in unraveling the molecular mechanisms of heterogeneity. A classical example of heterogeneity in Escherichia coli is the subset (3%) of the population that expresses the colicin K activity gene (cka) upon nutrient starvation. Here, we report on the mechanism underlying this variable response. As colicin synthesis is regulated by the LexA protein, the central regulator of the SOS response, we focused on the role of LexA and the SOS system in the variable cka expression. Real-time RT-PCR showed that the SOS system, without exogenous DNA damage, induces moderate levels of cka expression. The use of cka-gfp fusions demonstrated that modification of the conserved LexA boxes in the cka promoter region affected LexA binding affinity and the percentage of cka-gfp expressing cells in the population. A lexA-gfp fusion showed that the lexA gene is highly expressed in a subset of bacteria. Furthermore, cka-gfp fusions cloned into higher copy plasmid vectors increased the percentage of cka-gfp positive bacteria. Together, these results indicate that the bistability in cka expression in the bacterial population is determined by (1) basal SOS activity, (2) stochastic factors and possibly (3) the interplay of LexA dimers at cka operator. Other LexA regulated processes could exhibit similar regulation.

Pleiotropic physiological effects in the plant growth-promoting bacterium Azospirillum brasilense following chromosomal labeling in the clpX gene

Azospirillum brasilense 8-I was chromosomally labeled with green fluorescent protein (gfp) genes, using either the native promoterless gfp gene or the mutant gfpmut2 gene under the transcriptional control of the neomycin phosphate transferase (npt2) promoter inserted into Tn5 suicide plasmid vectors. One A. brasilense exconjugant, showing a steady and strong fluorescence following irradiation with 365-nm UV light was characterized in detail. This strain, A. brasilense 8-I-gfp showed increased N(2)-fixation of approximately threefold, up to a twofold increase in exopolysaccharide production, and a significant decrease in indole-3-acetic acid and poly-beta-hydroxybutyrate production over the parental strain. Sequence analysis showed that the Tn5 carrying the gfp gene was inserted in the clpX gene encoding a heat-shock protein. This data is consistent with a model in which the observed physiological changes are a consequence of pleiotropic changes that occur as a consequence of impaired heat shock (stress) protein synthesis. In summary, (i) chromosomally labelled Azospirillum brasilense was obtained carrying either native or mutant gfp genes, (ii) Pleiotropic physiological effects were caused by disruption of the clpX gene as the consequence of the insertion, (iii) a new indole-3-acetic acid-attenuated mutant of A. brasilense producing only 0.25% of the indole-3-acetic acid produced by the wild-type is presented.

2,4-Dichlorophenoxyacetic Acid (2,4-D) Utilization by Delftia acidovorans MC1 at Alkaline pH and in the Presence of Dichlorprop is Improved by Introduction of the tfdK Gene

Growth of Delftia acidovorans MC1 on 2,4-dichlorophenoxyacetic acid (2,4-D) and on racemic 2-(2,4-dichlorophenoxy)propanoic acid ((RS)-2,4-DP) was studied in the perspective of an extension of the strain's degradation capacity at alkaline pH. At pH 6.8 the strain grew on 2,4-D at a maximum rate (mu max) of 0.158 h(-1). The half-maximum rate-associated substrate concentration (Ks) was 45 microM. At pH 8.5 mu max was only 0.05 h(-1) and the substrate affinity was mucher lower than at pH 6.8. The initial attack of 2,4-D was not the limiting step at pH 8.5 as was seen from high dioxygenase activity in cells grown at this pH. High stationary 2,4-D concentrations and the fact that mu max with dichlorprop was around 0.2 h(-1) at both pHs rather pointed at limited 2,4-D uptake at pH 8.5. Introduction of tfdK from D. acidovorans P4a by conjugation, coding for a 2,4-D-specific transporter resulted in improved growth on 2,4-D at pH 8.5 with mu max of 0.147 h(-1) and Ks of 267 microM. Experiments with labeled substrates showed significantly enhanced 2,4-D uptake by the transconjugant TK62. This is taken as an indication of expression of the tfdK gene and proper function of the transporter. The uncoupler carbonylcyanide m-chlorophenylhydrazone (CCCP) reduced the influx of 2,4-D. At a concentration of 195 microM 2,4-D, the effect amounted to 90% and 50%, respectively, with TK62 and MC1. Cloning of tfdK also improved the utilization of 2,4-D in the presence of (RS)-2,4-DP. Simultaneous and almost complete degradation of both compounds occurred in TK62 up to D = 0.23 h(-1) at pH 6.8 and up to D = 0.2 h(-1) at pH 8.5. In contrast, MC1 left 2,4-D largely unutilized even at low dilution rates when growing on herbicide mixtures at pH 8.5.

Bacterial-associated cholera toxin and GM1 binding are required for transcytosis of classical biotype Vibrio cholerae through an in vitro M cell model system

To elucidate mechanisms involved in M cell uptake and transcytosis of Vibrio cholerae, we used an in vitro model of human M-like cells in a Caco-2 monolayer. Interspersed among the epithelial monolayer of Caco-2 cells we detect cells that display M-like features with or without prior lymphocyte treatment and we have established key parameters for V. cholerae transcytosis in this model. Cholera toxin (CT) mutants lacking the A subunit alone or both the A and B subunits were deficient for transcytosis. We explored this finding further and showed that expression of both subunits is required for binding by whole V. cholerae to immobilized CT receptor, the glycosphingolipid GM1. Confocal microscopy showed CT associated with transcytosing bacteria, and transcytosis was inhibited by pre-incubation with GM1 before infection. Finally, heat treatment of the bacterial cells caused a loss of binding to GM1 that was correlated with a significant decrease in uptake and transcytosis by the monolayer. Our data support a model in which the ability of bacteria to interact with GM1 in a CT-dependent fashion plays a critical role in transcytosis of V. cholerae by M cells.

Bioluminescent bacterial biosensors for the assessment of metal toxicity and bioavailability in soils

A major factor governing the toxicity of heavy metals in soils is their bioavailability. Traditionally, sequential extraction procedures using different extractants followed by chemical analysis have been used for determining the biologically available fraction of metals in soils. Yet, the transfer of results obtained on non-biological systems to biological ones is certainly questionable. Therefore, bioluminescence-based bacterial biosensors have been developed using genetically engineered microorganisms, constructed by fusing transcriptionally active components of metal resistance mechanisms to lux genes from naturally bioluminescent bacteria like Vibrio fischeri for the assessment of metal toxicity and bioavailability in polluted soils. As compared to chemical methods, bacterial biosensors present certain advantages, such as selectivity, sensitivity, simplicity, and low cost. Despite certain inherent limitations, bacterial bioluminescent systems have proven their usefulness in soils under laboratory and field conditions. Finally, green fluorescent protein-based bacterial biosensors are also applicable for determining with high sensitivity the bioavailability of heavy metals in soil samples.

Identification of virulence-associated genes of Pseudomonas viridiflava activated during infection by use of a novel IVET promoter probing plasmid

Analysis of virulence mechanisms of plant pathogens is often limited by the lack of genetic tools that can be used to identify genes that are preferentially expressed during their interactions with plants. In the present study, we used the newly constructed IVET (in vivo expression technique) plasmid pIviGK and the corresponding antibiotic resistance-based selection method to identify genes that encode pathogenicity factors of the soft rot-causing bacterium Pseudomonas viridiflava. These included pel, the gene encoding pectate lyase, which is responsible for the development of soft rot symptoms. We have also isolated and characterized the gene mviNpv encoding a putative novel membrane associated virulence factor of P. viridiflava. A mutation in mviNpv was shown to influence motility as well as virulence of P. viridiflava. The mviNpv gene is expressed to a moderate level in LB media and its expression increases under inducing conditions as was shown by measuring in planta expression dynamics of the fused gfp reporter gene.

Effects of cattle feeding regimen and soil management type on the fate of Escherichia coli O157:H7 and salmonella enterica serovar typhimurium in manure, manure-amended soil, and lettuce

Survival of the green fluorescent protein-transformed human pathogens Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium was studied in a laboratory-simulated lettuce production chain. Dairy cows were fed three different roughage types: high-digestible grass silage plus maize silage (6:4), low-digestible grass silage, and straw. Each was adjusted with supplemental concentrates to high and low crude protein levels. The pathogens were added to manure, which was subsequently mixed (after 56 and 28 days for E. coli O157:H7 and Salmonella serovar Typhimurium, respectively) with two pairs of organically and conventionally managed loamy and sandy soil. After another 14 days, iceberg lettuce seedlings were planted and then checked for pathogens after 21 days of growth. Survival data were fitted to a logistic decline function (exponential for E. coli O157:H7 in soil). Roughage type significantly influenced the rate of decline of E. coli O157:H7 in manure, with the fastest decline in manure from the pure straw diet and the slowest in manure from the diet of grass silage plus maize silage. Roughage type showed no effect on the rate of decline of Salmonella serovar Typhimurium, although decline was significantly faster in the manure derived from straw than in the manure from the diet of grass silage plus maize silage. The pH and fiber content of the manure were significant explanatory factors and were positively correlated with the rate of decline. With E. coli O157:H7 there was a trend of faster decline in organic than in conventional soils. No pathogens were detected in the edible lettuce parts. The results indicate that cattle diet and soil management are important factors with respect to the survival of human pathogens in the environment.

Physiological stress of intracellular Shigella flexneri visualized with a metabolic sensor fused to a surface-reporter system

When deleted of its N-terminal signal-reception domain, the broad host range sigma54-dependent transcriptional regulator XylR, along with its cognate promoter Pu, becomes a sensor of the metabolic stress of the carrier bacteria. We have employed a surface reporter system to visualize the physiological status of intracellular Shigella flexneri during infection of Henle 407 cells in culture. To this end, the xylRDeltaA gene has been engineered adjacent to a bicistronic transcriptional fusion of Pu to a lamB variant tagged with a short viral sequence (cor) and beta-galactosidase (lacZ). The accessibility of the cor epitope to the externalmost medium and the expression of Pu in the bacterial population was confirmed, respectively, with immunomagnetic beads and the sorting of Escherichia coli cells treated with a fluorescent antibody. Intracellular Shigella cells expressed the Pu-lamB/cor-lacZ reporter at high levels, suggesting that infectious cells endure a considerable metabolic constraint during the invasion process.