Quorum sensing in halophilic bacteria: detection of N-acyl-homoserine lactones in the exopolysaccharide-producing species of Halomonas

Halomonas flagellata sp. nov., a halophilic bacterium isolated from saline soil in Xinjiang

A Gram-stain-negative, strictly aerobic, motile, slightly curved rod-shaped bacterium with multiple flagella, designated strain EGI 63088T, was isolated from a bulk soil of Kalidium foliatum, collected from Wujiaqu in Xinjiang Uighur Autonomous Region, PR China. The optimal growth temperature, salinity, and pH for strain EGI 63088T growth were 30 °C, 3% (w/v) NaCl and 8, respectively. Phylogenetic analysis using 16S rRNA gene sequences indicated that strain EGI 63088T showed the highest sequence similarities to Halomonas heilongjiangensis 9-2T (97.94%), H. lysinitropha 3(2)T (97.51%), and H. daqiaonensis CGMCC 1.9150T (97.08%). The average nucleotide identity and digital DNA-DNA hybridization values between the strain EGI 63088T and H. heilongjiangensis 9-2T were 89.03 and 41.10%, respectively. The DNA G + C content of the genome for strain EGI 63088T was 66.3 mol%. The most prevalent antibiotic resistance and virulence-related genes in Halomonas genomes were Streptomyces cinnamoneu EF-Tu mutant, pilT, and cheY, respectively. The predominant fatty acids of strain EGI 63088T were summed feature 8 (C18: 1 ω6c and/or C18: 1 ω7c), summed feature 3 (C16: 1 ω6c and/or C16: 1 ω7c), and C16: 0; its major respiratory quinone was ubiquinone-9 (Q-9), and the major polar lipids were diphosphatidylglycerol, phosphatidylglycerol, and phosphatidylethanolamine. According to the above results, strain EGI 63088T is considered a novel species of the genus Halomonas, for which the name Halomonas flagellata sp. nov. is proposed. The type strain is EGI 63088T (= KCTC 92047T = CGMCC 1.19133T).

Unraveling the Diverse Profile of N-Acyl Homoserine Lactone Signals and Their Role in the Regulation of Biofilm Formation in Porphyra haitanensis-Associated Pseudoalteromonas galatheae

N-acyl homoserine lactones (AHLs) are small, diffusible chemical signal molecules that serve as social interaction tools for bacteria, enabling them to synchronize their collective actions in a density-dependent manner through quorum sensing (QS). The QS activity from epiphytic bacteria of the red macroalgae Porphyra haitanensis, along with its involvement in biofilm formation and regulation, remains unexplored in prior scientific inquiries. Therefore, this study explores the AHL signal molecules produced by epiphytic bacteria. The bacterium isolated from the surface of P. haitanensis was identified as Pseudoalteromonas galatheae by 16s rRNA gene sequencing and screened for AHLs using two AHL reporter strains, Agrobacterium tumefaciens A136 and Chromobacterium violaceum CV026. The crystal violet assay was used for the biofilm-forming phenotype. The inferences revealed that P. galatheae produces four different types of AHL molecules, i.e., C4-HSL, C8-HSL, C18-HSL, and 3-oxo-C16-HSL, and it was observed that its biofilm formation phenotype is regulated by QS molecules. This is the first study providing insights into the QS activity, diverse AHL profile, and regulatory mechanisms that govern the biofilm formation phenotype of P. galatheae. These findings offer valuable insights for future investigations exploring the role of AHL producing epiphytes and biofilms in the life cycle of P. haitanensis.

Anianabacter salinae gen. nov., sp. nov. ASV31T, a Facultative Alkaliphilic and Extremely Halotolerant Bacterium Isolated from Brine of a Millennial Continental Saltern

During a prokaryotic diversity study in Añana Salt Valley, a new Rhodobacteraceae member, designated ASV31T, was isolated from Santa Engracia spring water. It was extremely halotolerant, tolerating up to 23% NaCl, and facultatively alkaliphilic, growing at pH 6.5–9.5 (optimum at 7.0–9.5). The isolate was a Gram-negative, rod-shaped, aerobic and non-motile bacterium that formed beige-to-pink colonies on marine agar. According to a 16S rRNA gene-based phylogenetic analysis, strain ASV31T forms a distinct branch of the family Rhodobacteraceae, with Thioclava pacifica DSM 10166T being its closest type strain (95.3%). This was confirmed with a phylogenomic tree and the values of ANI (73.9%), dDDH (19.3%), AAI (63.5%) and POCP (56.0%), which were below the genus/species level boundary. Additionally, an ability to degrade aromatic compounds and biosynthesise secondary metabolites was suggested by the genome of strain ASV31T. Distinguishing fatty acid profiles and polar lipid content were also observed. The genome size was 3.6 Mbp, with a DNA G+C content of 65.7%. Based on the data obtained, it was considered that strain ASV31T (=CECT 30309T = LMG 32242T) represents a new species of a new genus in the family Rhodobacteraceae, for which the name Anianabacter salinae gen. nov., sp. nov. is proposed.

Exploring the Function of Quorum Sensing Regulated Biofilms in Biological Wastewater Treatment: A Review

Quorum sensing (QS), a type of bacterial cell–cell communication, produces autoinducers which help in biofilm formation in response to cell population density. In this review, biofilm formation, the role of QS in biofilm formation and development with reference to biological wastewater treatment are discussed. Autoinducers, for example, acyl-homoserine lactones (AHLs), auto-inducing oligo-peptides (AIPs) and autoinducer 2, present in both Gram-negative and Gram-positive bacteria, with their mechanism, are also explained. Over the years, wastewater treatment (WWT) by QS-regulated biofilms and their optimization for WWT have gained much attention. This article gives a comprehensive review of QS regulation methods, QS enrichment methods and QS inhibition methods in biological waste treatment systems. Typical QS enrichment methods comprise adding QS molecules, adding QS accelerants and cultivating QS bacteria, while typical QS inhibition methods consist of additions of quorum quenching (QQ) bacteria, QS-degrading enzymes, QS-degrading oxidants, and QS inhibitors. Potential applications of QS regulated biofilms for WWT have also been summarized. At last, the knowledge gaps present in current researches are analyzed, and future study requirements are proposed.

Specific quorum sensing molecules are possibly associated with responses to herbicide toxicity in a Pseudomonas strain

Pesticides contribute to pest control and increase agricultural production; however, they are toxic to non-target organisms, and they contaminate the environment. The exposure of bacteria to these substances can lead to the need for physiological and structural changes for survival, which can be determined by genes whose expression is regulated by quorum sensing (QS). However, it is not yet clear whether these processes can be induced by herbicides. Thus, the aim of this work was to determine whether there is a QS response system in the Pseudomonas fluorescens CMA55 strain that is modulated by herbicides. This strain was isolated from water storage tanks used for washing pesticide packaging and was tested against herbicides containing saflufenacil, glyphosate, sulfentrazone, 2,4-D, and dicamba as active molecules. Our results showed that in the presence of herbicides containing saflufenacil and glyphosate (the latter was not present at the bacterial isolation site) the strain had a profile of QS signaling molecules that may be involved in controlling the production of reactive oxygen species. Alternatively, the same strain, in the presence of sulfentrazone (it was not present at the bacterial isolation site), 2,4-D and dicamba-containing herbicides, presented another profile of molecules that may be involved in different stages of biofilm formation. These findings, as a first screening, suggest that this strain used strategies to activate antioxidant enzymes and biofilm production under the signaling of QS molecules to respond to herbicides, regardless of previous contact, representing a model of phenotypic plasticity for adaptation to agricultural environments that can be used in studies of herbicide bioremediation.

Piscirickettsia salmonis Produces a N-Acetyl-L-Homoserine Lactone as a Bacterial Quorum Sensing System-Related Molecule

Piscirickettsia salmonis is the etiological agent of piscirickettsiosis, the most prevalent disease in salmonid species in Chilean salmonids farms. Many bacteria produce N-acyl-homoserine lactones (AHLs) as a quorum-sensing signal molecule to regulate gene expression in a cell density-dependent manner, and thus modulate physiological characteristics and several bacterial mechanisms. In this study, a fluorescent biosensor system method and gas chromatography-tandem mass spectrometry (GC/MS) were combined to detect AHLs produced by P. salmonis. These analyses revealed an emitted fluorescence signal when the biosensor P. putida EL106 (RPL4cep) was co-cultured with both, P. salmonis LF-89 type strain and an EM-90-like strain Ps007, respectively. Furthermore, the production of an AHL-type molecule was confirmed by GC/MS by both P. salmonis strains, which identified the presence of a N-acetyl-L-homoserine Lactone in the supernatant extract. However, It is suggested that an alternate pathway could synthesizes AHLs, which should be address in future experiments in order to elucidate this important bacterial process. To the best of our knowledge, the present report is the first to describe the type of AHLs produced by P. salmonis.

Involvement of a Quorum Sensing Signal Molecule in the Extracellular Amylase Activity of the Thermophilic Anoxybacillus amylolyticus

Anoxybacillus amylolyticus is a moderate thermophilic microorganism producing an exopolysaccharide and an extracellular α-amylase able to hydrolyze starch. The synthesis of several biomolecules is often regulated by a quorum sensing (QS) mechanism, a chemical cell-to-cell communication based on the production and diffusion of small molecules named “autoinducers”, most of which belonging to the N-acyl homoserine lactones’ (AHLs) family. There are few reports about this mechanism in extremophiles, in particular thermophiles. Here, we report the identification of a signal molecule, the N-butanoyl-homoserine lactone (C4-HSL), from the milieu of A. amylolyticus. Moreover, investigations performed by supplementing a known QS inhibitor, trans-cinnamaldehyde, or exogenous C4-HSL in the growth medium of A. amylolyticus suggested the involvement of QS signaling in the modulation of extracellular α-amylase activity. The data showed that the presence of the QS inhibitor trans-cinnamaldehyde in the medium decreased amylolytic activity, which, conversely, was increased by the effect of exogenous C4-HSL. Overall, these results represent the first evidence of the production of AHLs in thermophilic microorganisms, which could be responsible for a communication system regulating thermostable α-amylase activity.

The Use of Electroactive Halophilic Bacteria for Improvements and Advancements in Environmental High Saline Biosensing

Halophilic bacteria are remarkable organisms that have evolved strategies to survive in high saline concentrations. These bacteria offer many advances for microbial-based biotechnologies and are commonly used for industrial processes such as compatible solute synthesis, biofuel production, and other microbial processes that occur in high saline environments. Using halophilic bacteria in electrochemical systems offers enhanced stability and applications in extreme environments where common electroactive microorganisms would not survive. Incorporating halophilic bacteria into microbial fuel cells has become of particular interest for renewable energy generation and self-powered biosensing since many wastewaters can contain fluctuating and high saline concentrations. In this perspective, we highlight the evolutionary mechanisms of halophilic microorganisms, review their application in microbial electrochemical sensing, and offer future perspectives and directions in using halophilic electroactive microorganisms for high saline biosensing.

AhaP, A Quorum Quenching Acylase from Psychrobacter sp. M9-54-1 That Attenuates Pseudomonas aeruginosa and Vibrio coralliilyticus Virulence

Although Psychrobacter strain M9-54-1 had been previously isolated from the microbiota of holothurians and shown to degrade quorum sensing (QS) signal molecules C6 and C10-homoserine lactone (HSL), little was known about the gene responsible for this activity. In this study, we determined the whole genome sequence of this strain and found that the full 16S rRNA sequence shares 99.78-99.66% identity with Psychrobacter pulmonis CECT 5989^T and P. faecalis ISO-46^T. M9-54-1, evaluated using the agar well diffusion assay method, showed high quorum quenching (QQ) activity against a wide range of synthetic N-acylhomoserine lactone (AHLs) at 4, 15, and 28 °C. High-performance liquid chromatography-mass-spectrometry (HPLC-MS) confirmed that QQ activity was due to an AHL-acylase. The gene encoding for QQ activity in strain M9-54-1 was identified from its genome sequence whose gene product was named AhaP. Purified AhaP degraded substituted and unsubstituted AHLs from C4- to C14-HSL. Furthermore, heterologous expression of ahaP in the opportunistic pathogen Pseudomonas aeruginosa PAO1 reduced the expression of the QS-controlled gene lecA, encoding for a cytotoxic galactophilic lectin and swarming motility protein. Strain M9-54-1 also reduced brine shrimp mortality caused by Vibrio coralliilyticus VibC-Oc-193, showing potential as a biocontrol agent in aquaculture.

Growth-Stimulatory Effect of Quorum Sensing Signal Molecule N-Acyl-Homoserine Lactone-Producing Multi-Trait Aeromonas spp. on Wheat Genotypes Under Salt Stress

Salinity is one of the major threats to agricultural productivity worldwide. Soil and plant management practices, along with inoculation with plant-beneficial bacteria, play a key role in the plant’s tolerance toward salinity stress. The present study demonstrates the potential of acyl homoserine lactone (AHL)-producing plant growth promoting rhizobacteria (PGPR) strains of Aeromonas sp., namely, SAL-17 (accession no. HG763857) and SAL-21 (accession no. HG763858), for growth promotion of two wheat genotypes inherently different for salt tolerance potential. AHLs are the bacterial signal molecules that regulate the expression of various genes in bacteria and plants. Both Aeromonas spp., along with innate plant-growth-promoting (PGP) and salt tolerance traits, showed AHL production which was identified on tandem mass spectrometry as C6-HSL, 3-OH-C5-HSL, 3-OH-C6-HSL, 3-oxo-C7-HSL C10-HSL, 3-oxo-C10-HSL, 3-OH-C10-HSL, 3-oxo-C12-HSL and C6-HSL, and 3-oxo-C10-HSL. The exogenous application of purified AHLs (mix) significantly improved various root parameters at 200 mM NaCl in both salt-sensitive (SSG) and salt-tolerant (STG) genotypes, where the highest increase (≈80%) was observed where a mixture of both strains of AHLs was used. Confocal microscopic observations and root overlay assay revealed a strong root colonization potential of the two strains under salt stress. The inoculation response of both STG and SSG genotypes was evaluated with two AHL-producing strains (SAL-17 and SAL-21) and compared to non-AHL-producing Aeromonas sp. SAL-12 (accession no. HG763856) in saline (EC = 7.63 ms/cm²) and non-saline soil. The data reveal that plants inoculated with the bacterial consortium (SAL-21 + SAL-17) showed a maximum increase in leaf proline content, nitrate reductase activity, chlorophyll a/b, stomatal conductance, transpiration rate, root length, shoot length, and grain weight over non-inoculated plants grown in saline soil. Both STG and SSG showed relative effectiveness toward inoculation (percent increase for STG: 165–16%; SSG: 283–14%) and showed a positive correlation of grain yield with proline and nitrate reductase activity. Furthermore, principal component analysis (PCA) and categorical PCA analysis clearly showed an inoculation response in both genotypes, revealing the effectiveness of AHL-producing Aeromonas spp. than the non-AHL-producing strain. The present study documents that the consortium of salt-tolerant AHL-producing Aeromonas spp. is equally effective for sustaining the growth of STG as well as SSG wheat genotypes in saline soil, but biosafety should be fully ensured before field release.

Microbial biofilm ecology, in silico study of quorum sensing receptor-ligand interactions and biofilm mediated bioremediation

Biofilms are structured microbial communities of single or multiple populations in which microbial cells adhere to a surface and get embedded in extracellular polymeric substances (EPS). This review attempts to explain biofilm architecture, development phases, and forces that drive bacteria to promote biofilm mode of growth. Bacterial chemical communication, also known as Quorum sensing (QS), which involves the production, detection, and response to small molecules called autoinducers, is highlighted. The review also provides a brief outline of interspecies and intraspecies cell-cell communication. Additionally, we have performed docking studies using Discovery Studio 4.0, which has enabled our understanding of the prominent interactions between autoinducers and their receptors in different bacterial species while also scoring their interaction energies. Receptors, such as LuxN (Phosphoreceiver domain and RecA domain), LuxP, and LuxR, interacted with their ligands (AI-1, AI-2, and AHL) with a CDocker interaction energy of - 31.6083 kcal/mole; - 34.5821 kcal/mole, - 48.2226 kcal/mole and - 41.5885 kcal/mole, respectively. Since biofilms are ideal for the remediation of contaminants due to their high microbial biomass and their potential to immobilize pollutants, this article also provides an overview of biofilm-mediated bioremediation.

Archaea join the conversation: detection of AHL-like activity across a range of archaeal isolates

Quorum sensing is a mechanism of genetic control allowing single cell organisms to coordinate phenotypic response(s) across a local population and is often critical for ecosystem function. Although quorum sensing has been extensively studied in bacteria comparatively less is known about this mechanism in Archaea. Given the growing significance of Archaea in both natural and anthropogenic settings, it is important to delineate how widespread this phenomenon of signaling is in this domain. Employing a plasmid-based AHL biosensor in conjunction with thin-layer chromatography (TLC), the present study screened a broad range of euryarchaeota isolates for potential signaling activity. Data indicated the presence of 11 new Archaeal isolates with AHL-like activity against the LuxR-based AHL biosensor, including for the first time putative AHL activity in a thermophile. The presence of multiple signals and distinct changes between growth phases were also shown via TLC. Multiple signal molecules were detected using TLC in Haloferax mucosum, Halorubrum kocurii, Natronococcus occultus and Halobacterium salinarium. The finding of multiple novel signal producers suggests the potential for quorum sensing to play an important role not only in the regulation of complex phenotypes within Archaea but the potential for cross-talk with bacterial systems.

Quorum sensing mediated response of Achromobacter denitrificans SP1 towards prodigiosin production under phthalate stress

Quorum sensing is a density-dependent chemical process between bacteria, which may be intergenus or intragenus. N-acyl homoserine lactones (HSLs) are a type of small signaling molecules associated with Gram-negative bacteria for monitoring their own population density. The present study unveils the mechanism of HSLs in Achromobacter denitrificans SP1 while transforming di(2-ethylhexyl) phthalate (DEHP) into prodigiosin in a simple basal salt medium. The primary detection of HSLs was done by the colorimetric method. Fourier-transform infrared spectroscopy and liquid chromatography-mass spectrometry-quadrupole time-of-flight confirmed and identified the HSLs. The maximum production of HSLs was observed between 24 and 72 h of incubation, which is noted to be a peak time of DEHP degradation. A total of 57.2% of DEHP was degraded within 30 h and complete degradation was observed within 72 h of incubation. Regulation in the synthesis of various acyl-HSL molecules, viz. 3OC6-HSL in the initial stage of DEHP stress, 3OC8-HSL, and C10-HSL during the time of degradation and 3OC12-HSL on completion of degradation was noticed. The role of HSLs on the production of prodigiosin was confirmed using vanillin as an HSL inhibitor. Through the selective activation of HSL molecules, A. denitrificans SP1 sustain the changing stressful conditions. Supplementation of acyl-HSL signal molecules may boost up the efficacy of A. denitrificans SP1 in both DEHP degradation and prodigiosin production which offers great potential towards the management of DEHP containing plastic wastes.

Effects of Halophyte Root Exudates and Their Components on Chemotaxis, Biofilm Formation and Colonization of the Halophilic Bacterium Halomonas Anticariensis FP35T

Increase in soil salinity poses an enormous problem for agriculture and highlights the need for sustainable crop production solutions. Plant growth-promoting bacteria can be used to boost the growth of halophytes in saline soils. Salicornia is considered to be a promising salt-accumulating halophyte for capturing large amounts of carbon from the atmosphere. In addition, colonization and chemotaxis could play an important role in Salicornia-microbe interactions. In this study, the role of chemotaxis in the colonization of the halophilic siredophore-producing bacteria, Halomonas anticariensis FP35T, on Salicornia hispanica plants was investigated. The chemotactic response of FP35T to Salicornia root exudates showed optimum dependence at a salt concentration of 5 % NaCl (w/v). Oleanolic acid, the predominant compound in the exudates detected by HPLC and identified by UPLC-HRMS Q-TOF, acts as a chemoattractant. In vitro experiments demonstrated the enhanced positive effects of wild-type H. anticariensis strain FP35T on root length, shoot length, germination and the vigour index of S. hispanica. Furthermore, these positive effects partially depend on an active chemotaxis system, as the chemotaxis mutant H. anticariensis FP35 ΔcheA showed reduced plant growth promotion for all the parameters tested. Overall, our results suggest that chemotaxis responses to root exudates play an important role in interactions between Salicornia and halophilic bacteria, enhance their colonization and boost plant growth promotion. Preliminary results also indicate that root exudates have a positive impact on H. anticariensis FP35T biofilm formation under saline conditions, an effect which totally depends on the presence of the cheA gene.

Plant growth-promoting activity and quorum quenching-mediated biocontrol of bacterial phytopathogens by Pseudomonas segetis strain P6

Given the major threat of phytopathogenic bacteria to food production and ecosystem stability worldwide, novel alternatives to conventional chemicals-based agricultural practices are needed to combat these bacteria. The objective of this study is to evaluate the ability of Pseudomonas segetis strain P6, which was isolated from the Salicornia europaea rhizosphere, to act as a potential biocontrol agent given its plant growth-promoting (PGP) and quorum quenching (QQ) activities. Seed biopriming and in vivo assays of tomato plants inoculated with strain P6 resulted in an increase in seedling height and weight. We detected QQ activity, involving enzymatic degradation of signal molecules in quorum sensing communication systems, against a broad range of N-acylhomoserine lactones (AHLs). HPLC-MRM data and phylogenetic analysis indicated that the QQ enzyme was an acylase. The QQ activity of strain P6 reduced soft rot symptoms caused by Dickeya solani, Pectobacterium atrosepticum and P. carotovorum on potato and carrot. In vivo assays showed that the PGP and QQ activities of strain P6 protect tomato plants against Pseudomonas syringae pv. tomato, indicating that strain P6 could have biotechnological applications. To our knowledge, this is the first report to show PGP and QQ activities in an indigenous Pseudomonas strain from Salicornia plants.

Silencing of Phytopathogen Communication by the Halotolerant PGPR Staphylococcus equorum Strain EN21

Increasing world food demand together with soil erosion and indiscriminate use of chemical fertilization highlight the need to adopt sustainable crop production strategies. In this context, a combination of plant growth-promoting rhizobacteria (PGPR) and pathogen management represents a sustainable and efficient alternative. Though little studied, halophilic and halotolerant PGPR could be a beneficial plant growth promotion strategy for saline and non-saline soils. The virulence of many bacterial phytopathogens is regulated by quorum sensing (QS) systems. Quorum quenching (QQ) involves the enzymatic degradation of phytopathogen-generated signal molecules, mainly N-acyl homoserine lactones (AHLs). In this study, we investigate plant growth-promoting (PGP) activity and the capacity of the halotolerant bacterium Staphylococcus equorum strain EN21 to attenuate phytopathogens virulence through QQ. We used biopriming and in vivo tomato plant experiments to analyse the PGP activity of strain EN21. AHL inactivation was observed to reduce Pseudomonas syringae pv. tomato infections in tomato and Arabidopsis plants. Our study of Dickeya solani, Pectobacterium carotovorum subsp. carotovorum and Erwinia amylovora bacteria in potato tubers, carrots and pears, respectively, also demonstrated the effectiveness of QS interruption by EN21. Overall, this study highlights the potential of strain S. equorum EN21 in plant growth promotion and QQ-driven bacterial phytopathogen biocontrol.

AhlX, an N-acylhomoserine Lactonase with Unique Properties

N-Acylhomoserine lactonase degrades the lactone ring of N-acylhomoserine lactones (AHLs) and has been widely suggested as a promising candidate for use in bacterial disease control. While a number of AHL lactonases have been characterized, none of them has been developed as a commercially available enzymatic product for in vitro AHL quenching due to their low stability. In this study, a highly stable AHL lactonase (AhlX) was identified and isolated from the marine bacterium Salinicola salaria MCCC1A01339. AhlX is encoded by a 768-bp gene and has a predicted molecular mass of 29 kDa. The enzyme retained approximately 97% activity after incubating at 25 °C for 12 days and ~100% activity after incubating at 60 °C for 2 h. Furthermore, AhlX exhibited a high salt tolerance, retaining approximately 60% of its activity observed in the presence of 25% NaCl. In addition, an AhlX powder made by an industrial spray-drying process attenuated Erwinia carotovora infection. These results suggest that AhlX has great potential for use as an in vitro preventive and therapeutic agent for bacterial diseases.

Isolation of novel quorum-sensing active bacteria from microbial mats in Shark Bay Australia

Quorum sensing is a potent system of genetic control allowing phenotypes to be coordinated across localized communities. In this study, quorum sensing systems in Shark Bay microbial mats were delineated using a targeted approach analyzing whole mat extractions as well as the creation of an isolate library. A library of 165 isolates from different mat types were screened using the AHL biosensor E. coli MT102. Based on sequence identity 30 unique isolates belonging to Proteobacteria, Actinobacteria and Firmicutes were found to activate the AHL biosensor, suggesting AHLs or analogous compounds were potentially present. Several of the isolates have not been shown previously to produce signal molecules, particularly the members of the Actinobacteria and Firmicutes phyla including Virgibacillus, Halobacillius, Microbacterium and Brevibacterium. These active isolates were further screened using thin-layer chromatography (TLC) providing putative identities of AHL molecules present within the mat communities. Nine isolates were capable of producing several spots of varying sizes after TLC separation, suggesting the presence of multiple signalling molecules. This study is the first to delineate AHL-based signalling in the microbial mats of Shark Bay, and suggests quorum sensing may play a role in the ecosphysiological coordination of complex phenotypes across microbial mat communities.

Stenotrophomonas maltophilia AHL-Degrading Strains Isolated from Marine Invertebrate Microbiota Attenuate the Virulence of Pectobacterium carotovorum and Vibrio coralliilyticus

Many Gram-negative aquacultural and agricultural pathogens control virulence factor expression through a quorum-sensing (QS) mechanism involving the production of N-acylhomoserine (AHL) signalling molecules. Thus, the interruption of QS systems by the enzymatic degradation of signalling molecules, known as quorum quenching (QQ), has been proposed as a novel strategy to combat these infections. Given that the symbiotic bacteria of marine invertebrates are considered to be an important source of new bioactive molecules, this study explores the presence of AHL-degrading bacteria among 827 strains previously isolated from the microbiota of anemones and holothurians. Four of these strains (M3-1, M1-14, M3-13 and M9-54-2), belonging to the species Stenotrophomonas maltophilia, were selected on the basis of their ability to degrade a broad range of AHLs, and the enzymes involved in their activity were identified. Strain M9-54-2, which showed the strongest AHL-degrading activity, was selected for further study. High-performance liquid chromatography-mass-spectrometry confirmed that the QQ enzyme is not a lactonase. Strain M9-54-2 degraded AHL accumulation and reduced the production of enzymatic activity in Pectobacterium carotovorum CECT 225^T and Vibrio coralliilyticus VibC-Oc-193 in in vitro co-cultivation experiments. The effect of AHL inactivation was confirmed by a reduction in potato tuber maceration and brine shrimp (Artemia salina) mortality caused by P. carotovorum and Vibrio coralliilyticus, respectively. This study strengthens the evidence of marine organisms as an underexplored and promising source of QQ enzymes, useful to prevent infections in aquaculture and agriculture. To our knowledge, this is the first time that anemones and holothurians have been studied for this purpose.

Saline Environments as a Source of Potential Quorum Sensing Disruptors to Control Bacterial Infections: A Review

Saline environments, such as marine and hypersaline habitats, are widely distributed around the world. They include sea waters, saline lakes, solar salterns, or hypersaline soils. The bacteria that live in these habitats produce and develop unique bioactive molecules and physiological pathways to cope with the stress conditions generated by these environments. They have been described to produce compounds with properties that differ from those found in non-saline habitats. In the last decades, the ability to disrupt quorum-sensing (QS) intercellular communication systems has been identified in many marine organisms, including bacteria. The two main mechanisms of QS interference, i.e., quorum sensing inhibition (QSI) and quorum quenching (QQ), appear to be a more frequent phenomenon in marine aquatic environments than in soils. However, data concerning bacteria from hypersaline habitats is scarce. Salt-tolerant QSI compounds and QQ enzymes may be of interest to interfere with QS-regulated bacterial functions, including virulence, in sectors such as aquaculture or agriculture where salinity is a serious environmental issue. This review provides a global overview of the main works related to QS interruption in saline environments as well as the derived biotechnological applications.

Quorum sensing inhibitors as antipathogens: biotechnological applications

The mechanisms through which microbes communicate using signal molecules has inspired a great deal of research. Microbes use this exchange of information, known as quorum sensing (QS), to initiate and perpetuate infectious diseases in eukaryotic organisms, evading the eukaryotic defense system by multiplying and expressing their pathogenicity through QS regulation. The major issue to arise from such networks is increased bacterial resistance to antibiotics, resulting from QS-dependent mediation of the formation of biofilm, the induction of efflux pumps, and the production of antibiotics. QS inhibitors (QSIs) of diverse origins have been shown to act as potential antipathogens. In this review, we focus on the use of QSIs to counter diseases in humans as well as plants and animals of economic importance. We also discuss the challenges encountered in the potential applications of QSIs.

AHL-lactonase expression in three marine emerging pathogenic Vibrio spp. reduces virulence and mortality in brine shrimp (Artemia salina) and Manila clam (Venerupis philippinarum)

Bacterial infectious diseases produced by Vibrio are the main cause of economic losses in aquaculture. During recent years it has been shown that the expression of virulence genes in some Vibrio species is controlled by a population-density dependent gene-expression mechanism known as quorum sensing (QS), which is mediated by the diffusion of signal molecules such as N-acylhomoserine lactones (AHLs). QS disruption, especially the enzymatic degradation of signalling molecules, known as quorum quenching (QQ), is one of the novel therapeutic strategies for the treatment of bacterial infections. In this study, we present the detection of AHLs in 34 marine Vibrionaceae strains. Three aquaculture-related pathogenic Vibrio strains, V. mediterranei VibC-Oc-097, V. owensii VibC-Oc-106 and V. coralliilyticus VibC-Oc-193 were selected for further studies based on their virulence and high production of AHLs. This is the first report where the signal molecules have been characterized in these emerging marine pathogens and correlated to the expression of virulence factors. Moreover, the results of AHL inactivation in the three selected strains have been confirmed in vivo against brine shrimps (Artemia salina) and Manila clams (Venerupis philippinarum). This research contributes to the development of future therapies based on AHL disruption, the most promising alternatives for fighting infectious diseases in aquaculture.

HqiA, a novel quorum-quenching enzyme which expands the AHL lactonase family

The screening of a metagenomic library of 250,000 clones generated from a hypersaline soil (Spain) allowed us to identify a single positive clone which confers the ability to degrade N-acyl homoserine lactones (AHLs). The sequencing of the fosmid revealed a 42,318 bp environmental insert characterized by 46 ORFs. The subcloning of these ORFs demonstrated that a single gene (hqiA) allowed AHL degradation. Enzymatic analysis using purified HqiA and HPLC/MS revealed that this protein has lactonase activity on a broad range of AHLs. The introduction of hqiA in the plant pathogen Pectobacterium carotovorum efficiently interfered with both the synthesis of AHLs and quorum-sensing regulated functions, such as swarming motility and the production of maceration enzymes. Bioinformatic analyses highlighted that HqiA showed no sequence homology with the known prototypic AHL lactonases or acylases, thus expanding the AHL-degrading enzymes with a new family related to the cysteine hydrolase (CHase) group. The complete sequence analysis of the fosmid showed that 31 ORFs out of the 46 identified were related to Deltaproteobacteria, whilst many intercalated ORFs presented high homology with other taxa. In this sense, hqiA appeared to be assigned to the Hyphomonas genus (Alphaproteobacteria), suggesting that horizontal gene transfer had occurred.

Selection of the N-Acylhomoserine Lactone-Degrading Bacterium Alteromonas stellipolaris PQQ-42 and of Its Potential for Biocontrol in Aquaculture

The production of virulence factors by many pathogenic microorganisms depends on the intercellular communication system called quorum sensing, which involves the production and release of signal molecules known as autoinducers. Based on this, new-therapeutic strategies have emerged for the treatment of a variety of infections, such as the enzymatic degradation of signaling molecules, known as quorum quenching (QQ). In this study, we present the screening of QQ activity amongst 450 strains isolated from a bivalve hatchery in Granada (Spain), and the selection of the strain PQQ-42, which degrades a wide range of N-acylhomoserine lactones (AHLs). The selected strain, identified as Alteromonas stellipolaris, degraded the accumulation of AHLs and reduced the production of protease and chitinase and swimming motility of a Vibrio species in co-cultivation experiments in vitro. In the bio-control experiment, strain PQQ-42 significantly reduced the pathogenicity of Vibrio mediterranei VibC-Oc-097 upon the coral Oculina patagonica showing a lower degree of tissue damage (29.25 ± 14.63%) in its presence, compared to when the coral was infected with V. mediterranei VibC-Oc-097 alone (77.53 ± 13.22%). Our results suggest that this AHL-degrading bacterium may have biotechnological applications in aquaculture.

Potential Emergence of Multi-quorum Sensing Inhibitor Resistant (MQSIR) Bacteria

Expression of certain bacterial genes only at a high bacterial cell density is termed as quorum-sensing (QS). Here bacteria use signaling molecules to communicate among themselves. QS mediated genes are generally involved in the expression of phenotypes such as bioluminescence, biofilm formation, competence, nodulation, and virulence. QS systems (QSS) vary from a single in Vibrio spp. to multiple in Pseudomonas and Sinorhizobium species. The complexity of QSS is further enhanced by the multiplicity of signals: (1) peptides, (2) acyl-homoserine lactones, (3) diketopiperazines. To counteract this pathogenic behaviour, a wide range of bioactive molecules acting as QS inhibitors (QSIs) have been elucidated. Unlike antibiotics, QSIs don't kill bacteria and act at much lower concentration than those of antibiotics. Bacterial ability to evolve resistance against multiple drugs has cautioned researchers to develop QSIs which may not generate undue pressure on bacteria to develop resistance against them. In this paper, we have discussed the implications of the diversity and multiplicity of QSS, in acting as an arsenal to withstand attack from QSIs and may use these as reservoirs to develop multi-QSI resistance.

Type 2 quorum sensing monitoring, inhibition and biofilm formation in marine microrganisms

The quorum sensing (QS) dependent behaviour of micro-organisms, in particular expression of virulence genes, biofilm formation and dispersal, have provided impetus for investigating practical approaches to interfere with microbial QS. This study tests Halomonas pacifica and Marinobacter hydrocarbonoclasticus, two halophilic marine micro-organism, for their AI-2 dependent QS signalling and the effect of two well-known quorum-sensing inhibitors (QSIs), patulin and penicillic acid, on biofilm formation. We report, for the first time, the successful amplification of a putative luxS gene in H. pacifica using degenerated primers and AI-2 dependent QS as well as inhibition using QSIs. Penicillic acid had a strong inhibitory effect on AI-2 induction of H. pacifica at non-growth inhibitory concentrations, while patulin has an adverse effect only at the highest concentration (25 μM). QSIs effect on biofilm forming capability was isolate specific, with maximum inhibition at 25 μM of patulin in H. pacifica. In M. hydrocarbonoclasticus, no adverse effects were noted at any tested concentration of either QSIs. Detection of bioluminescence and the presence of a putative luxS gene provide biochemical and genetic evidence for the production of a signalling molecule(s) which is the essential first step in characterizing H. pacifica QS. This study highlights the importance of AI-2 dependent QS in a marine setting, not previously reported. It further suggests that QSI compounds must be selected in the specific system in which they are to function, and they cannot easily be transferred from one QS system to another.

Draft Genome Sequence of the Moderately Halophilic Gammaproteobacterium Halomonas anticariensis FP35T

Halomonas anticariensis strain FP35^T is a moderately halophilic bacterium isolated from a soil sample taken from Fuente de Piedra, a saline wetland in the province of Málaga (Spain), which produces an exopolysaccharide and quorum-sensing signaling molecules of the type N-acylhomoserine lactone. We report here the draft genome sequence of this gammaproteobacterium.

N-acylhomoserine lactone-degrading bacteria isolated from hatchery bivalve larval cultures

Quorum sensing (QS) systems, which depend on N-acylhomoserine lactone (AHL) signal molecules, mediate the production of virulence factors in many pathogenic microorganisms. One hundred and forty-six bacterial strains, isolated from a bivalve hatchery, were screened for their capacity to degrade five synthetic AHLs [N-butyryl-DL-homoserine lactone (C4-HSL), N-hexanoyl-DL-homoserine lactone (C6-HSL), N-octanoyl-DL-homoserine lactone (C8-HSL), N-decanoyl-DL-homoserine lactone (C10-HSL) and N-dodecanoyl-DL-homoserine lactone (C12-HSL)] using well diffusion agar-plate assays with three biosensors, Chromobacterium violaceum CV026, C. violaceum VIR07 and Agrobacterium tumefaciens NTL4 (pZLR4). The results of these assays led to our choosing four strains (PP2-67, PP2-459, PP2-644 and PP2-663) that were able to degrade all five synthetic AHLs, thus showing a wide spectrum of quorum quenching (QQ) activity. We subsequently confirmed and measured the QQ activity of the four strains by high-performance liquid chromatography plus mass-spectrometry analysis (HPLC-MS). One of the strains which showed the highest AHL-degrading activity, PP2-459, identified as being a member of the genus Thalassomonas was chosen for further study. Finally, using thin-layer chromatography (TLC), we went on to confirm this strain's capacity to degrade the AHLs produced by other non-pathogenic and pathogenic bacteria not taxonomically related.

Quorum-sensing inhibitory compounds from extremophilic microorganisms isolated from a hypersaline cyanobacterial mat

In this study, extremely halophilic and moderately thermophilic microorganisms from a hypersaline microbial mat were screened for their ability to produce antibacterial, antidiatom, antialgal, and quorum-sensing (QS) inhibitory compounds. Five bacterial strains belonging to the genera Marinobacter and Halomonas and one archaeal strain belonging to the genus Haloterrigena were isolated from a microbial mat. The strains were able to grow at a maximum salinity of 22-25 % and a maximum temperature of 45-60 °C. Hexanes, dichloromethane, and butanol extracts from the strains inhibited the growth of at least one out of nine human pathogens. Only butanol extracts of supernatants of Halomonas sp. SK-1 inhibited growth of the microalga Dunaliella salina. Most extracts from isolates inhibited QS of the acyl homoserine lactone producer and reporter Chromobacterium violaceum CV017. Purification of QS inhibitory dichloromethane extracts of Marinobacter sp. SK-3 resulted in isolation of four related diketopiperazines (DKPs): cyclo(L-Pro-L-Phe), cyclo(L-Pro-L-Leu), cyclo(L-Pro-L-isoLeu), and cyclo(L-Pro-D-Phe). QS inhibitory properties of these DKPs were tested using C. violaceum CV017 and Escherichia coli-based QS reporters (pSB401 and pSB1075) deficient in AHL production. Cyclo(L-Pro-L-Phe) and cyclo(L-Pro-L-isoLeu) inhibited QS-dependent production of violacein by C. violaceum CV017. Cyclo(L-Pro-L-Phe), cyclo(L-Pro-L-Leu), and cyclo(L-Pro-L-isoLeu) reduced QS-dependent luminescence of the reporter E. coli pSB401 induced by 3-oxo-C6-HSL. Our study demonstrated the ability of halophilic and moderately thermophilic strains from a hypersaline microbial mat to produce biotechnologically relevant compounds that could be used as antifouling agents.

Quorum sensing in some representative species of halomonadaceae

Cell-to-cell communication, or quorum-sensing (QS), systems are employed by bacteria for promoting collective behaviour within a population. An analysis to detect QS signal molecules in 43 species of the Halomonadaceae family revealed that they produced N-acyl homoserine lactones (AHLs), which suggests that the QS system is widespread throughout this group of bacteria. Thin-layer chromatography (TLC) analysis of crude AHL extracts, using Agrobacterium tumefaciens NTL4 (pZLR4) as biosensor strain, resulted in different profiles, which were not related to the various habitats of the species in question. To confirm AHL production in the Halomonadaceae species, PCR and DNA sequencing approaches were used to study the distribution of the luxI-type synthase gene. Phylogenetic analysis using sequence data revealed that 29 of the species studied contained a LuxI homolog. Phylogenetic analysis showed that sequences from Halomonadaceae species grouped together and were distinct from other members of the Gammaproteobacteria and also from species belonging to the Alphaproteobacteria and Betaproteobacteria.

Quorum sensing in extreme environments

Microbial communication, particularly that of quorum sensing, plays an important role in regulating gene expression in a range of organisms. Although this phenomenon has been well studied in relation to, for example, virulence gene regulation, the focus of this article is to review our understanding of the role of microbial communication in extreme environments. Cell signaling regulates many important microbial processes and may play a pivotal role in driving microbial functional diversity and ultimately ecosystem function in extreme environments. Several recent studies have characterized cell signaling in modern analogs to early Earth communities (microbial mats), and characterization of cell signaling systems in these communities may provide unique insights in understanding the microbial interactions involved in function and survival in extreme environments. Cell signaling is a fundamental process that may have co-evolved with communities and environmental conditions on the early Earth. Without cell signaling, evolutionary pressures may have even resulted in the extinction rather than evolution of certain microbial groups. One of the biggest challenges in extremophile biology is understanding how and why some microbial functional groups are located where logically they would not be expected to survive, and tightly regulated communication may be key. Finally, quorum sensing has been recently identified for the first time in archaea, and thus communication at multiple levels (potentially even inter-domain) may be fundamental in extreme environments.

The hanR/hanI quorum-sensing system of Halomonas anticariensis, a moderately halophilic bacterium

Quorum sensing is a cell density-dependent gene expression mechanism found in many Gram-negative bacteria which involves the production of signal molecules such as N-acylhomoserine lactones (AHLs). One significant group of micro-organisms in which quorum sensing has not been previously studied, however, are the moderate halophiles. We describe here the results of our studies of the quorum-sensing system in Halomonas anticariensis FP35T, which is composed of luxR/luxI homologues: hanR (the putative transcriptional regulator gene) and hanI (the autoinducer synthase gene). To understand how the hanR/hanI system is organized and regulated we conducted RT-PCR and quantitative real-time PCR assays. Transcriptional analysis indicated that the hanR and hanI genes are on the same transcript and that their transcription is growth phase-dependent. HanI seems to be the only autoinducer synthase responsible for the synthesis of AHLs by the bacterium, since the inactivation of hanI resulted in the complete loss of its AHLs. We also found that the hanI gene appears to be transcribed from its own promoter and that its expression does not depend upon HanR. This finding was supported by the fact that the FP35hanR mutant showed AHL-producing activity and hanI expression similar to that of the wild-type strain, the latter being measured by RT-PCR. Moreover, hanR is expressed from its own promoter and appears to be independent of the AHL signalling molecules produced by HanI.

Extraction, purification and identification of bacterial signal molecules based on N-acyl homoserine lactones

Bacteria possess an extraordinary repertoire for intercellular communication and social behaviour. This repertoire for bacterial communication, termed as quorum sensing (QS), depends on specific diffusible signal molecules. There are many different kinds of signal molecules in the bacterial community. Among those signal molecules, N‐acyl homoserine lactones (HSLs, in other publications also referred to as AHLs, acy‐HSLs etc.) are often employed as QS signal molecules for many Gram‐negative bacteria. Due to the specific structure and tiny amount of those HSL signal molecules, the characterization of HSLs has been the subject of extensive investigations in the last decades and has become a paradigm for bacteria intercellular signalling. In this article, different methods, including extraction, purification and characterization of HSLs, are reviewed. The review provides an insight into identification and characterization of new HSLs and other signal molecules for bacterial intercellular communication.

Growth phase-dependent biosynthesis of Nep, a halolysin-like protease secreted by the alkaliphilic haloarchaeon Natrialba magadii

AIMS

The alkaliphilic haloarchaeon Natrialba magadii secretes a halolysin-like protease (Nep) that is active and stable in high salt and in organic solvents, which represents a potential resource for biocatalysis in low water activity conditions. In this study, the effect of the growth stage on Nep biosynthesis was examined.

METHODS AND RESULTS

Nep mRNA and extracellular protease activity were measured by RT-PCR and azocaseinolytic activity determination, respectively. Increased abundance in Nep mRNA was observed in Nab. magadii cells with culture age, which correlated with accumulation of extracellular protease activity. Moreover, a 'stationary phase behavior' on synthesis of Nep was evidenced in low-density cultures incubated with stationary phase medium.

CONCLUSIONS

nep gene expression is up-regulated during the transition to the stationary phase in response to 'factors' (metabolite and/or regulatory molecule) occurring in high-density cultures of Nab. magadii. Although the identity of these molecules remains to be determined, preliminary evidence suggests that they are hydrophobic and stable in high salt and high pH values (3.5 mol l(-1) NaCl, pH 10).

SIGNIFICANCE AND IMPACT OF STUDY

This study contributes to gain insight into the regulation of haloarchaeal protease biosynthesis, facilitating the large-scale production of this extremozyme for basic studies or potential applications.

Quorum sensing and bacterial biofilms

This review describes the chemistry of the bacterial biofilms including the chemistry of their constituents and signalling compounds that mediate or inhibit the formation of biofilms. Systems are described with special emphasis, in which quorum sensing molecules (autoinducers) trigger the formation of biofilms. In the first instance, N-acyl-L-homoserine lactones (AHLs) are the focus of this review, whereas the inter-species signal known as furanosyl borate diester and peptide autoinducers used by Gram-positive bacteria are not discussed in detail. Since the first discovery of an AHL autoinducer from Vibrio fischeri a large and further increasing number of different AHL structures from Gram-negative bacteria have been identified. This review gives a summary of all known AHL autoinducers and producing bacterial species. A few systems are discussed, where biofilm formation is suppressed by enzymatic degradation of AHL molecules or interference of secondary metabolites from other species with the quorum sensing systems of communicating bacteria. Finally, the multi-channel quorum sensing system, the intracellular downstream processing of the signal, and the resulting response of whole populations including biofilm formation are discussed for the Vibrio genus that has been extensively investigated.

Identification of hydroxyl radical oxidation products of N-hexanoyl-homoserine lactone by reversed-phase high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry

A reversed-phase high-performance liquid chromatography/electrospray tandem mass spectrometry method was developed for the characterization of hydroxyl radical oxidation products of N-hexanoyl-homoserine lactone (C6-HSL), a member of the N-acylhomoserine lactone (AHL) class of microbial quorum-sensing signaling molecules identified in many Gram-negative strains of bacteria. Six products were identified: four with molecular weight (MW) of 213 and two with MW of 260. The characteristic product ions formed through collision-induced dissociation (CID) provided diagnostic structural information. One of the photolysis products was determined to be N-(3-oxohexanoyl)homoserine lactone (3OC6-HSL), a highly active quorum-sensing signal, by comparison with a reference standard. Three structural isomers with the same mass as 3OC6-HSL were identified as acyl side chain oxidized C6-HSL (keto/enol functionalized) by accurate mass measurement and the structures of these products were proposed from CID spectral interpretation. Two structural isomers formed from concurrent oxidation and nitration of C6-HSL were also observed and their structures were postulated based on CID spectra. In addition to the six hydroxyl radical oxidation products formed from the C6-HSL precursor, five additional compounds generated from combined oxidation and lactonolysis of C6-HSL were identified and structures were postulated.

Second acyl homoserine lactone production system in the extreme acidophile Acidithiobacillus ferrooxidans

The acidophilic proteobacterium Acidithiobacillus ferrooxidans is involved in the industrial biorecovery of copper. It is found in acidic environments in biofilms and is important in the biogeochemical cycling of metals and nutrients. Its genome contains a cluster of four genes, glyQ, glysS, gph, and act, that are predicted to encode the alpha and beta subunits of glycine tRNA synthetase, a phosphatase, and an acyltransferase, respectively (GenBank accession no. DQ149607). act, cloned and expressed in Escherichia coli, produces acyl homoserine lactones (AHLs) principally of chain length C14 according to gas chromatography and mass spectrometry measurements. The AHLs have biological activity as shown by in vivo studies using the reporter strain Sinorhizobium meliloti Rm41 SinI-. Reverse transcription-PCR (RT-PCR) experiments indicate that the four genes are expressed as a single transcript, demonstrating that they constitute an operon. According to semiquantitative RT-PCR results, act is expressed more highly when A. ferrooxidans is grown in medium containing iron than when it is grown in medium containing sulfur. Since AHLs are important intercellular signaling molecules used by many bacteria to monitor their population density in quorum-sensing control of gene expression, this result suggests that A. ferrooxidans has two quorum-sensing systems, one based on Act, as described herein, and the other based on a Lux-like quorum-sensing system, reported previously. The latter system was shown to be upregulated in A. ferrooxidans grown in sulfur medium, suggesting that the two quorum-sensing systems respond to different environmental signals that may be related to their abilities to colonize and use different solid sulfur- and iron-containing minerals.

Identification of bacterial N-acylhomoserine lactones (AHLs) with a combination of ultra-performance liquid chromatography (UPLC), ultra-high-resolution mass spectrometry, and in-situ biosensors

N-Acylated homoserine lactones (AHLs) are produced by Gram-negative bacteria as communication signals and are frequently studied as mediators of the "quorum sensing" response of bacterial communities. Several reports have recently been published on the identification of AHLs from different species and attempts have been made to study their role in natural habitats, for example the surface of plant roots in the rhizosphere. In this article, different analytical methods, including bacterial biosensors and chromatographic techniques, are reviewed. A concept for assignment of the structures of AHLs is also presented. The retention behaviour of derivatives of AHLs containing beta-keto or hydroxyl groups and/or double bonds has been evaluated in relation to the separation behaviour of AHLs with saturated and unsubstituted alkanoyl chains. Samples have also been analysed by high resolution mass spectrometry (Fourier-transform ion-cyclotron-resonance mass spectrometry, FTICR-MS), nano liquid chromatography-electrospray ionization ion trap mass spectrometry (nano-LC-MS) and by the aid of a biosensor. The results obtained from ultra performance liquid chromatography (UPLC), FTICR-MS, nano-LC-MS, and bioassays have been compared to attempt structural characterisation of AHL without chemical synthesis of analytical standards. The method was used to identify the major AHL compound produced by the rhizosphere bacterium Acidovorax sp. N35 as N-(3-hydroxydecanoyl)homoserine lactone.

Exopolysaccharides produced by the recently described halophilic bacteria Halomonas ventosae and Halomonas anticariensis

We studied exopolysaccharides (EPSs) produced by Halomonas ventosae and Halomonas anticariensis, two novel species of halophilic bacteria. Under optimum environmental and nutritional conditions, H. ventosae strains Al12(T) and Al16 excreted 28.35 mg and 28.95 mg of EPS per 100 ml of culture medium (34.55 and 38.6 mg of EPS per gram of dry cell weight) respectively. The molecular masses of the polymers were about 50 kDa and their main components were glucose, mannose and galactose. They had high protein fractions and showed emulsifying activity on several hydrophobic substrates. Under optimum environmental and nutritional conditions, H. anticariensis strains FP35(T) and FP36 excreted about 29.65 and 49.95 mg of EPS per 100 ml of culture medium (43.6 and 50.95 mg of EPS per gram of dry cell weight) respectively. The molecular masses of the polymers were about 20 and 46 kDa respectively and were composed mainly of glucose, mannose and galacturonic acid. All EPSs produced solutions of low viscosity and pseudoplastic behaviour. They also had a high capacity for binding cations and incorporated considerable quantities of sulphates, which is highly unusual in bacterial polysaccharides. All strains assayed formed biofilms both in polystyrene wells and borosilicate test tubes.

Halomonas maura is a physiologically versatile bacterium of both ecological and biotechnological interest

Halomonas maura is a bacterium of great metabolic versatility. We summarise in this work some of the properties that make it a very interesting microorganism both from an ecological and biotechnological point of view. It plays an active role in the nitrogen cycle, is capable of anaerobic respiration in the presence of nitrate and has recently been identified as a diazotrophic bacterium. Of equal interest is mauran, the exopolysaccharide produced by H. maura, which contributes to the formation of biofilms and thus affords the bacterium advantages in the colonisation of its saline niches. Mauran is highly viscous, shows thixotropic and pseudoplastic behaviour, has the capacity to capture heavy metals and exerts a certain immunomodulator effect in medicine. All these attributes have prompted us to make further investigations into its molecular characteristics. To date we have described 15 open reading frames (ORF's) related to exopolysaccharide production, nitrogen fixation and nitrate reductase activity among others.