Bioreporter Strains Research Articles

The impact of plant volatiles on bacterial quorum sensing.

Studies describing the use of essential oil constituents as antimicrobial agents have steadily increased; however, some phyto-constituents are often overlooked due to unfavourable minimum inhibitory concentration (MIC) values. Virulence depends on transcriptional factors which are regulated by cell-to-cell communication called quorum sensing (QS). This study was undertaken to evaluate the antimicrobial and anti-QS properties of 29 compounds commonly found in essential oils using two bioreporter strains. QS-inhibitory activity was assessed qualitatively by agar diffusion and quantitatively by spectrophotometric assays. MICs of all the tested compounds ranged from 0·032 to >5 mg ml(-1). Twenty-two compounds displayed varying levels of QS inhibitory activity with zones of violacein inhibition ranging from 9 to 16 mm. Majority of tested molecules inhibited violacein and pyocyanin production in Chromobacterium violaceum and Pseudomonas aeruginosa, while seven compounds increased violacein and pyocyanin production. Interestingly, it was observed that the (+)-enantiomers of carvone, limonene and borneol increased violacein and pyocyanin production, while their levorotary analogues inhibited this production. α-Terpineol and cis-3-nonen-1-ol exhibited >90% violacein inhibition, suggesting their potential as QS inhibitors. This preliminary study indicates that plant volatiles have the potential to impede or promote bacterial communication and further studies need to be undertaken to explore the contribution of structural analogues and stereochemistry of molecules in this process. Antimicrobial and antiquorum sensing (QS) properties of 29 common essential oil compounds were evaluated. Interruption of QS may lead to the development of therapeutic, antivirulence agents to control disease-causing pathogens which are preferable over antimicrobial agents as the latter drives selection pressure on microbial communities to acquire resistance. Twenty-two compounds inhibited QS, while seven promoted the QS to a variable extent in Chromobacterium violaceum and Pseudomonas aeruginosa. Preliminary results suggest that QS-inhibitory compounds of natural origin may inspire the formulation of new generation of antimicrobial agents to control infectious pathogens.

Effects of binary mixtures of inducers (toluene analogs) and of metals on bioluminescence induction of a recombinant bioreporter strain.

This paper investigated the effects of binary mixtures of bioluminescence inducers (toluene, xylene isomers, m-toluate) and of metals (Cu, Cd, As(III), As(V), and Cr) on bioluminescence activity of recombinant (Pm-lux) strain KG1206. Different responses and sensitivities were observed depending on the types and concentrations of mixtures of inducers or metals. In the case of inducer mixtures, antagonistic and synergistic modes of action were observed, whereas metal mixtures showed all three modes of action. Antagonistic mode of action was most common for mixtures of indirect inducers, which showed bioluminescence ranging from 29% to 62% of theoretically expected effects (P(E)). On the other hand, synergistic mode of action was observed for mixtures of direct and indirect inducers, which showed bioluminescence between 141% and 243% of P(E).In the case of binary metal mixtures, bioluminescence activities were ranged from 62% to 75% and 113% to 164% of P(E) for antagonistic and synergistic modes of action, respectively (p-values 0.0001–0.038). Therefore, mixture effects could not be generalized since they were dependent on both the types and concentrations of chemicals, suggesting that biomonitoring may constitute a better strategy by investigating types and concentrations of mixture pollutants at contaminated sites.

Functional characterization of a StyS sensor kinase reveals distinct domains associated with intracellular and extracellular sensing of styrene in P. putida CA-3

Bacterial two-component systems (TCSs) are of vital importance in the translation of rapidly changing environmental conditions into appropriate cellular regulatory responses enabling adaptation, growth, and survival. The diverse range of environmental signals that TCSs can process, coupled with discrete modular domains within TCS proteins, offers considerable potential for the rational design of bio-sensor and/or bio-reporter strains. In this study we functionally characterize the multi-domain StyS sensor kinase associated with sensing of the aromatic pollutant styrene by Pseudomonas putida CA-3. Deletion analysis of discrete domains was performed and the ability of the truncated StyS sensor proteins to activate a cognate reporter system in an E. coli host assessed. The essential histidine kinase and PAS input domains were identified for StyS dependent activation of the reporter system. However, co-expression of an ABC-transporter protein StyE, previously linked to styrene transport in P. putida CA-3, enabled activation of the reporter system with a StyS construct containing a non-essential PAS input domain, suggesting a novel role for intracellular detection and/or activation. Site directed mutagenesis and amino acid deletions were employed to further characterize the PAS sensing domains of both input regions. The potential implications of these findings in the use of multi-domain sensor kinases in rational design strategies and the potential link between transport and intracellular sensing are discussed.

Environmentally friendly pretreatment of plant biomass by planetary and attrition milling

This study evaluated the use of planetary and attrition milling as pretreatment processes for lignocellulosic biomass using rice straw. Planetary milling reduced the rice straw crystallinity from 0.48 to 0.11. Since the samples could be milled and enzymatically treated using the same media, loss of the biomass due to washing was effectively eliminated. In contrast, conventional sodium hydroxide and soaking in aqueous ammonia (SAA) processes showed a loss of 34.2% and 14.8%, respectively. Furthermore, milling produced significantly lower concentrations of soluble phenolics than the alkali treatments. Using a bioluminescent bioreporter strain that is sensitive to these phenolics, neither of the milled samples elicited a response while the sodium hydroxide and SAA samples led to a 25.8 and 4.7 -fold induction, respectively. Although planetary milling produced more reducing sugars than attrition milling before saccharification, both had similar monosaccharide yields, i.e., 0.38 and 0.34 g/g-biomass, respectively, when 40 g/l rice straw was treated.

Coral-associated bacteria, quorum sensing disrupters, and the regulation of biofouling

Marine biofouling, the settlement of microorganisms and macroorganisms on structures submerged in seawater, although economically detrimental, is a successful strategy for survival in hostile environments, where coordinated bacterial communities establish biofilms via the regulation of quorum sensing (QS) communication systems. The inhibition of QS activity among bacteria isolated from different coral species was investigated to gain further insight into its potency in the attenuation, or even the prevention, of undesirable biofouling on marine organisms. It is hypothesized that coral mucus/microorganism interactions are competitive, suggesting that the dominant communities secrete QS disruptive compounds. One hundred and twenty bacterial isolates were collected from healthy coral species and screened for their ability to inhibit QS using three bioreporter strains. Approximately 12, 11, and 24% of the isolates exhibited anti-QS activity against Escherichia coli pSB1075, Chromobacterium violaceum CV026, and Agrobacterium tumefaciens KYC55 indicator strains, respectively. Isolates with positive activity against the bioluminescent monitor strains were scanned via a cytotoxic/genotoxic, E. coli TV1061 and DPD2794 antimicrobial panel. Isolates detected by C. violaceum CV026 and A. tumefaciens KYC55 reporter strains were tested for their ability to inhibit the growth of these reporter strains, which were found to be unaffected. Tests of the Favia sp. coral isolate Fav 2-50-7 (>98% similarity to Vibrio harveyi) for its ability to attenuate the formation of biofilm showed extensive inhibitory activity against biofilms of Pseudomonas aeruginosa and Acinetobacter baumannii. To ascertain the stability and general structure of the active compound, cell-free culture supernatants exposed to an increasing temperature gradient or to digestion by proteinase K, were shown to maintain potent QS attenuation and the ability to inhibit the growth of biofilms. Mass spectrometry confirmed the presence of a low molecular mass compound. The anti-QS strategy exemplified in the coral mucus is a model with potentially wide applications, including countering the ecological threat posed by biofilms. Manipulating synchronized bacterial behavior by detecting new QS inhibitors will facilitate the discovery of new antifouling compounds.

Detection of furfural and 5-hydroxymethylfurfural with a yhcN::luxCDABE bioreporter strain

RT-qPCR data demonstrated that the yhcN gene was highly expressed when Escherichia coli was exposed to 500 ppm of ferulic acid (9.8-fold), vanillin (7.3-fold) or furfural (2.2-fold). Consequently, an E. coli yhcN::lux bioreporter strain (DMY1) was constructed. This strain is very responsive to furfural and 5-hydroxymethylfurfural, showing a maximum induced bioluminescence of 29.4- and 17.8-fold, respectively. The responses to different phenolics were also significant, with relative responses of between 4- and 16-fold. In tests with rice straw hydrolysate samples, a dual induction of DMY1 was observed and it was demonstrated that this corresponded to the activities of the phenolics and HMF within the sample. The maximum response seen with the hydrolysate sample was 3.5 and the minimum phenolic concentration detected was 39 mg/L. These results illustrate that this strain can be used to monitor for the presence of furans and phenolics commonly found within plant hydrolysate samples.

Quorum quenching by Bacillus cereus U92: a double-edged sword in biological control of plant diseases

In the present survey, quorum quenching activity was examined from a biocontrol point of view. Acyl-homoserine lactone (AHL) degrading bacteria were isolated from tomato rhizosphere using two standard bioreporter strains and different synthetic AHLs and then identified according to 16S rDNA sequences. Five isolates capable of inactivating both short and long 3oxo-substituted AHLs showed high similarity with the genera Bacillus, Microbacterium and Arthrobacter, and thereby Bacillus cereus U92 was determined as the most efficient quorum quencher strain. In the quantitative experiments, this strain remarkably inactivated all synthetic AHLs up to 80%. In the laboratory co-cultures, B. cereus U92 efficiently quenched QS-regulated phenotypes in Agrobacterium tumefaciens, Pseudomonas aeruginosa, Pseudomonas chlororaphis and Chromobacterium violaceum. The strain successfully reduced the frequency of Ti-plasmid conjugal transfer in A. tumefaciens by about 99% in the binary cultures. Meanwhile, in a more natural environment, this strain acted as a biocontrol agent, efficient in alleviating QS-regulated crown gall incidence on tomato roots (up to 90%) as well as attenuating Pectobacterium soft rot on potato tubers (up to 60%). On the other hand, reducing phenazine production in P. chlororaphis operated as a suppressor of its QS-regulated biocontrol activity and also inhibited pyocyanin production in P. aeruginosa, a plant growth-promoting bacterium, by 75%. In general, B. cereus U92 seems very promising in the biological control of pathogenic bacteria; however, its broad AHL-degrading activity has a detrimental role on beneficial microbes which should not be neglected.

Escherichia coli bioreporters for the detection of 2,4-dinitrotoluene and 2,4,6-trinitrotoluene

The primary explosive found in most land mines, 2,4,6-trinitrotoluene (2,4,6-TNT), is often accompanied by 2,4-dinitrotoluene (2,4-DNT) and 1,3-dinitrobenzene (1,3-DNB) impurities. The latter two compounds, being more volatile, have been reported to slowly leak through land mine covers and permeate the soil under which they are located, thus serving as potential indicators for buried land mines. We report on the construction of genetically engineered Escherichia coli bioreporter strains for the detection of these compounds, based on a genetic fusion between two gene promoters, yqjF and ybiJ, to either the green fluorescent protein gene GFPmut2 or to Photorhabdus luminescens bioluminescence luxCDABE genes. These two gene promoters were identified by exposing to 2,4-DNT a comprehensive library of about 2,000 E. coli reporter strains, each harboring a different E. coli gene promoter controlling a fluorescent protein reporter gene. Both reporter strains detected 2,4-DNT in an aqueous solution as well as in vapor form or when buried in soil. Performance of the yqjF-based sensor was significantly improved in terms of detection threshold, response time, and signal intensity, following two rounds of random mutagenesis in the promoter region. Both yqjF-based and ybiJ-based reporters were also induced by 2,4,6-TNT and 1,3-DNB. It was further demonstrated that both 2,4,6-TNT and 2,4-DNT are metabolized by E. coli and that the actual induction of both yqjF and ybiJ is caused by yet unidentified degradation products. This is the first demonstration of an E. coli whole-cell sensor strain for 2,4-DNT and 2,4,6-TNT, constructed using its own endogenous sensing elements.

Serum complement enhances the responses of genotoxin- and oxidative stress-sensitive Escherichia coli bioreporters

Bacterial bioreporters are limited in their abilities to detect large polar molecules due to their membrane selectivity. In this study, the activity of serum complement was used to bypass this undesired selectivity. Initially, the serum complement activity was assessed using the responses of a bacterial bioreporter harboring a recA::luxCDABE transcriptional fusion when exposed to the chemotherapy drug, mitomycin C (MMC). Using 50°C-treated serum, the limit of detection for this bacterial sensor was lowered by nearly 450-fold, from 31μg/L to 0.07μg/L MMC. Real-time quantitative PCR demonstrated that serum-treated cultures responded more strongly to 100μg/L MMC, with 3.1-fold higher recA expression levels. Subsequent experiments with other bioreporter strains also found enhanced sensitivities and responses. Finally, combining each of the above findings, tests were performed to demonstrate the potential application of the recA::luxCDABE bioreporter within a lab-on-a-CD platform as a point-of-care diagnostic to measure chemotherapeutic drug concentrations within blood.

Evaluation of Anti-Quorum Sensing Activity of 97 Indigenous Plant Extracts From Korea through Bioreporter Bacterial Strains Chromobacterium violaceum and Pseudomonas aeruginosa

Quorum sensing (QS) is a recently discovered chemical communication system that enhances survival of bacteria, as a group allowing resident bacteria to assume specialized roles vital for intra- and inter bacterial gene regulation, and for keeping bacterial colonies intact. Furthermore, with the continuing emergence and spread of multidrug-resistant bacteria, antipathogenic strategy to combat bacterial infections through the interruption of quorum sensing controlled virulence factors had been shown to receive increased attention. With this prospect in the current study, we attempt to screen anti-quorum sensing activity of 97 indigenous plant extracts from Korea, through biomonitor bacterial strains, Chromobacterium violaceum (CV12472) and Pseudomonas aeruginosa (PAO1). Standard disc-diffusion assays were used to detect anti-quorum sensing activity (ring of colorless, but viable cells around the disk), of the plant extracts for CV12472. A special swarm media that allow swarming motility growth of POA1 was used to conduct inhibition of swarming motility assay. Minimum Inhibitory Concentration (MIC) test for the 97 plant extracts against bioreporter strains (CV12472 and PAO1) revealed antibacterial activity of three plant extracts (Potentilla cryptotaeniae, Viburnum carlesii and Prunus armeniaca var. ansu). Out of the 97 plant extracts, significant inhibition of pigment production were detected by six plant extracts in CV12472, while 16 plant extracts had shown inhibition of swarming motility in POA1. In conclusion, a total of 18 plant extracts were screened for their anti-quorum sensing activity by the two bioreporter strains. Of the 18 plant extracts, four had shown anti-quorum activities in both bioreporter strains.

Sensing of plant hydrolysate-related phenolics with an aaeXAB::luxCDABE bioreporter strain of Escherichia coli

A bioluminescent Escherichia coli bioreporter strain to detect hydrolysate related phenolics was developed by cloning the aaeXAB promoter from E. coli upstream of the luxCDABE genes. E. coli str. DH5α carrying this plasmid (pDMA3) was responsive to sub-inhibitory concentrations of plant hydrolysate-related phenolics, such as ferulic and vanillic acids, responding to these compounds at concentrations as low as 9.8 and 4.9mg/L, respectively. Experiments with a mixture of the compounds showed similar responses as with single compound tests, with a minimum detectable concentration of 19.5mg/L. Finally, tests using rice straw hydrolysates were conducted, with E. coli str. DH5α/pDMA3 showing a maximum induction of 33-fold and a minimum detectable phenolic concentration of 9.3mg/L, based upon Folin–Ciocalteu’s reagent. These results demonstrate that this bioreporter maintains its sensitivity even with hydrolysate samples and that it can be potentially applied within biofuel industries to detect phenolics present within plant hydrolysates.

AiiA lactonase disrupts N-acylhomoserine lactone and attenuates quorum-sensing-related virulence in Pectobacterium carotovorum EMPCC

Enzymatic degradation of N-acyl homoserine lactone (NAHL) was found to interfere with the quorum sensing (QS) system and related functions in several soil bacteria. In this research, the NAHL lactonase gene aiiA was amplified using aiiA-7F/aiiA7R PCR primers from the quorum sensing inhibitor rhizobacterium Bacillus sp. strain DMS133, and cloned. The plasmid pME7075, carrying the DMS133 aiiA gene under the constitutive lac promoter, was introduced into the plant pathogen Pectobacterium carotovorum EMPCC, creating strain EMPCC/aiiA. Heterologous expression of the DMS133 aiiA gene in EMPCC severely reduced the accumulation of the NAHL throughout growth, and completely prevented pigmentation of the CV026 bioreporter strain. Virulence analysis revealed that the P. carotovorum strain EMPCC/aiiA expressing AiiA lactonase had drastically reduced tissue maceration activity compared with the wild type EMPCC strain. These results provide evidence that AiiA plays an important role in the quorum quenching ability of Bacillus sp. DMS133 whose AHL degradation capacity was investigated previously. In addition, the communication signal-inactivation approach represents a promising strategy for the prevention of diseases in which virulence is regulated by QS signal molecules.

Use of protein stability to develop dual luciferase toxicity bioreporter strains

This study presents a simple protocol to measure 2 promoter activities within a single culture when using both Lux and firefly luciferase (FF-Luc) reporters. To demonstrate this, 2 E. coli strains were constructed using 2 compatible plasmids, one harboring a katG::luc fusion gene and the other either a fabA::lux or grpE::lux fusion gene. To differentiate between the FF-Luc and Lux activities within E. coli, we used the instability of the V. fischeri Lux proteins. Basically, it involved a two step assay where (1) without addition of luciferin, only the Lux activity was assayed and (2) with added luciferin and a heat treatment at 42°C, the FF-Luc activity was assayed. This was possible because a shift from 28 to 42°C for 10 min was sufficient to denature/inactivate the Lux proteins to background levels. After treatment, the substrate for FF-Luc was added and the FF-Luc activity could be reliably measured. Using this protocol, it was possible to assay the activities of both bioluminescent reporter proteins and, thus, the relative activity of the different promoters. Subsequent experiments were performed using known inducers of the katG, fabA and grpE promoters where tests were successfully performed with single compound samples as well as samples causing a variety of stresses. These results clearly demonstrated that two promoter activities can be monitored in a single host with this dual-luciferase system.

A novel property of propolis (bee glue): Anti-pathogenic activity by inhibition of N-acyl-homoserine lactone mediated signaling in bacteria

Ethnopharmacological RelevanceAn alternative approach to antibiotics is the development of anti-pathogenic agents to control the bacterial virulome. Such anti-pathogenic agents could target a phenomena known as quorum sensing (QS). Materials and methodsSix bacterial N-acyl-homoserine lactone (AHL)-dependent bioreporter strains were used to evaluate if bee hive glue also known as propolis contains constituents capable of inhibiting QS-controlled AHL signaling. In addition, the effect of propolis on the QS-dependent swarming motility was evaluated with the opportunisitic pathogen, Pseudomonas aeruginosa. ResultsDifferences in the propolis tincture samples were identified by physiochemical profiles and absorption spectra. Propolis tinctures at 0.0005% (v/v) that do not affect bacteria biosensor growth or the reporter system monitored were exposed to biosensors with and without the addition an AHL. No AHL signal mimics were found to be present in the propolis tinctures. However, when propolis and an inducer AHL signal were together exposed to five Escherichia coli and a Chromobacterium violaceum biosensor, propolis disrupted the QS bacterial signaling system in liquid- and agar-based bioassays and in C18 reverse-phase thin-layer plate assays. Swarming motility in the opportunistic pathogen, Pseudomonas aeruginosa PAO1 and its AHL-dependent LasR- and RhlR-based QS behaviors were also inhibited by propolis. ConclusionsTogether, we present evidence that propolis contain compounds that suppress QS responses. In this regard, anti-pathogenic compounds from bee harvested propolis could be identified and isolated and thus will be valuable for the further development of therapeutics to disrupt QS signaling systems which regulate the virulome in many pathogenic bacteria.

Characterization of Quorum Sensing Signals in Coral-Associated Bacteria

Marine environment habitats, such as the coral mucus layer, are abundant in nutrients and rich with diverse populations of microorganisms. Since interactions among microorganisms found in coral mucus can be either mutualistic or competitive, understanding quorum sensing-based acyl homoserine lactone (AHL) language may shed light on the interaction between coral-associated microbial communities in the native host. More than 100 bacterial isolates obtained from different coral species were screened for their ability to produce AHL. When screening the isolated coral bacteria for AHL induction activity using the reporter strains Escherichia coli K802NR-pSB1075 and Agrobacterium tumefaciens KYC55, we found that approximately 30% of the isolates tested positive. Thin layer chromatography separation of supernatant extracts revealed different AHL profiles, with detection of at least one active compound in the supernatant of those bacterial extracts being able to induce AHL activity in the two different bioreporter strains. The active extract of bacterial isolate 3AT 1-10-4 was subjected to further analysis by preparative thin layer chromatography and liquid chromatography tandem mass spectrometry. One of the compounds was found to correspond with N-(3-hydroxydecanoyl)-L-homoserine lactone. 16S rRNA gene sequencing of the isolates with positive AHL activity affiliated them with the Vibrio genus. Understanding the ecological role of AHL in the coral environment and its regulatory circuits in the coral holobiont-associated microbial community will further expand our knowledge of such interactions.

Internal arsenite bioassay calibration using multiple bioreporter cell lines

SummaryBioassays with bioreporter bacteria are usually calibrated with analyte solutions of known concentrations that are analysed along with the samples of interest. This is done as bioreporter output (the intensity of light, fluorescence or colour) does not only depend on the target concentration, but also on the incubation time and physiological activity of the cells in the assay. Comparing the bioreporter output with standardized colour tables in the field seems rather difficult and error‐prone. A new approach to control assay variations and improve application ease could be an internal calibration based on the use of multiple bioreporter cell lines with drastically different reporter protein outputs at a given analyte concentration. To test this concept, different Escherichia coli‐based bioreporter strains expressing either cytochrome c peroxidase (CCP, or CCP mutants) or β‐galactosidase upon induction with arsenite were constructed. The reporter strains differed either in the catalytic activity of the reporter protein (for CCP) or in the rates of reporter protein synthesis (for β‐galactosidase), which, indeed, resulted in output signals with different intensities at the same arsenite concentration. Hence, it was possible to use combinations of these cell lines to define arsenite concentration ranges at which none, one or more cell lines gave qualitative (yes/no) visible signals that were relatively independent of incubation time or bioreporter activity. The discriminated concentration ranges would fit very well with the current permissive (e.g. World Health Organization) levels of arsenite in drinking water (10 µg l−1).

Prokaryotic Gene Profiling Assays to Detect Sediment Toxicity: Evaluating the Ecotoxicological Relevance of a Cell-Based Assay

Despite their complexity, ecotoxicological measurements using higher level responses remain a major tool in the assessment of ecosystem integrity. Nevertheless, the past decade saw an increasing number of cell based testing systems have found widespread application in ecotoxicology. One such test is bacterial bioreporters carrying a stress sensitive promoter fused to an easily detectable reporter gene. In the presence of a specific toxic stress,the expression cassette is switched on and the reporter gene is produced. This study evaluated the use of 14 different Escherichia coli bioreporter strains sensitive to different types of toxicity in the assessment of the ecological status of a small river basin in Flanders, Belgium. The river is fed at two geographically separate locations by two distinct and well-described effluents, one from a household sewage treatment facility and one from the discharge of the wastewater treatment facility of a large chemical plant. The results of the bacterial gene profiling assay were related to active biomonitoring results obtained through higher-level responses of caged Dreissena polymorpha, Chironomus riparius, and Cyprinus carpio deployed at the locations sampled for the bacterial assay. The results of the gene induction assay and the active biomonitoring data correlated well and corresponded to the flow dilution data, which is used here as a surrogate forthe chemical pollution gradient present in the river basin.

Environmental Biotech

Environmental Biotech

Subinhibitory concentrations of cinnamaldehyde interfere with quorum sensing

To investigate the effect of cinnamaldehyde (CA) on transcription from selected quorum sensing (QS) promoters. The action of CA on QS was assayed using three E. coli green fluorescent protein (GFP) based bioreporters (two inducible and the other constitutive) and two Vibrio harveyi bioluminescent reporter strains. LuxR-mediated transcription from the P(luxI) promoter, which is induced by 3-oxo-C6-homoserine lactone (HSL), was reduced by 70 per cent following exposure to 200 micromol l(-1) CA (26 ppm). The bioluminescence of Vibrio harveyi BB886, which is mediated by 3-hydroxy-C4-HSL, was reduced by 55 per cent after exposure to 60 micromol l(-1) CA (8 ppm), and 100 micromol l(-1) CA (13 ppm) inhibited the bioluminescence of the autoinducer-2 (AI-2) responsive reporter strain V. harveyi BB170 by nearly 60 per cent. CA did not inhibit the growth of the bioreporter strains at these concentrations. CA had a minimal effect on LasR promoter activity, induced by 3-oxo-C12-HSL. Low concentrations of CA were effective at inhibiting two types of acyl homoserine lactone mediated QS, and also autoinducer-2 mediated QS. Because CA is widely used in the food and flavour industries, its potential to affect bacterial QS regulated processes should be recognized.

Production of quorum-sensing-related signal molecules by epiphytic bacteria inhabiting wheat heads

The production of quorum-sensing-related signal molecules (QSRMs) among culturable bacteria comprising the community on wheat heads was investigated. The taxonomic position of 186 bacterial isolates obtained from ten heads was inferred based on 16S rRNA gene sequences, and their QSRM production was determined using two bioreporter strains of N-acylhomoserine lactones. Approximately 33% of isolates produced QSRMs, though the proportion of QSRM-producing isolates on a wheat head was significantly negatively correlated with population size. Most of the producing isolates were Pantoea species, most commonly Pantoea ananatis. Furthermore, the proportion of Pantoea ananatis that produced QSRMs was significantly negatively correlated with the number of bacterial genera (community richness) on each head. Finally, community richness was positively correlated with population size. Qualitative analysis using thin-layer-chromatography revealed that the QSRMs of Pantoea isolates were composed of at least two compounds. This is the first report indicating that Pantoea ananatis isolates inhabiting wheat heads are capable of producing QSRMs. QSRM production by Pantoea spp. may contribute to the predominance of this genus on wheat heads, particularly at relatively low population densities and community diversity.