No abstract

Quorum quenching (QQ), the enzymatic degradation of N-acyl homoserine lactones (AHLs), has been suggested as a promising strategy to control bacterial diseases. In this study, 10 AHL-degrading bacteria isolated from the intestine of barramundi were identified by 16S rDNA sequencing. They were able to degrade both short and long-chain AHLs associated with several pathogenic Vibrio species (spp.) in fish, including N-[(RS)-3-Hydroxybutyryl]-l-homoserine lactone (3-oh-C4-HSL), N-Hexanoyl-l-homoserine lactone (C6-HSL), N-(β-Ketocaproyl)-l-homoserine lactone (3-oxo-C6-HSL), N-(3-Oxodecanoyl)-l-homoserine lactone (3-oxo-C10-HSL), N-(3-Oxotetradecanoyl)-l-homoserine lactone (3-oxo-C14-HSL). Five QQ isolates (QQIs) belonging to the Bacillus and Shewanella genera, showed high capacity to degrade both synthetic AHLs as well as natural AHLs produced by Vibrio harveyi and Vibrio alginolyticus using the well-diffusion method and thin-layer chromatography (TLC). The genes responsible for QQ activity, including aiiA, ytnP, and aaC were also detected. Analysis of the amino acid sequences from the predicted lactonases revealed the presence of the conserved motif HxHxDH. The selected isolates were further characterized in terms of their probiotic potentials in vitro. Based on our scoring system, Bacillus thuringiensis QQ1 and Bacillus cereus QQ2 exhibited suitable probiotic characteristics, including the production of spore and exoenzymes, resistance to bile salts and pH, high potential to adhere on mucus, appropriate growth abilities, safety to barramundi, and sensitivity to antibiotics. These isolates, therefore, constitute new QQ probiotics that could be used to control vibriosis in Lates calcalifer.

Several soil bacteria were found to degrade N-Acylhomoserine lactones (NAHLs), thereby interfering with the bacterial quorum sensing system. In this research, fifteen strains of NAHL degrading rhizobacteria were isolated from potato rhizosphere. Based on phenotypic characteristics and 16S rDNA sequence analyses, the strains were identified as members of genera Bacillus, Streptomyces, Arthrobacter, Pseudomonas and Mesorhizobium. All tested isolates were capable to degrade both synthetic and natural NAHL produced by Pectobacterium carotovorum subsp. carotovorum (Pcc) strain EMPCC. In quorum quenching experiments selected isolates, especially Mesorhizobium sp., were markedly reduced the pathogenicity of Pcc strain EMPCC in potato tubers and totally suppressed tissue maceration on potato tubers. These led to consider the latter as a useful biocontrol agent against Pectobacterium spp.Keywords: Mesorhizobium, NAHLs, Pectobacterium carotovorum, Quorum sensing Many bacterial species employ complex communication systems that link cell density and gene expression to regulate a broad range of biological functions (Fuqua et al., 2001;Miller and Bassler, 2001). Such cell-to-cell communication, termed quorum sensing (QS), depends on the production, diffusion, and recognition of small signal molecules. Bacteria recognize the changes in their population density by sensing the concentrations of signal molecules, which are accumulated as bacterial cells proliferate. NAcyl-homoserine lactones (NAHLs), also known as autoinducers, are widely conserved signal molecules that are present in the quorum-sensing systems of many gramnegative bacteria. Bacteria release, detect, and respond to accumulation of these signal molecules resulting in synchronizing expression of diverse biological functions including bioluminescence, antibiotic synthesis, biofilm formation, swarming, plasmid conjugation, siderophore production and production of virulence determinants (Whitehead et al., 2001). Since pathogenicity or pathogenicity-associated functions are controlled by QS in some major plant and animal pathogens, it has been proposed that the QS system could be an appropriate novel target for biological control agents or drugs in order to attenuate and limit virulence (Faure and Dessaux, 2007). Anti-QS mechanisms have been employed by inhibiting the synthesis or perception of the NAHL signal, as well as its enzymatic degradation by wild-type bacteria and eukaryotes, and genetically modified organisms (Molina et al., 2003;Rasmussen and Givskov, 2006).The NAHL-degrading enzymatic activities were initially discovered in a few bacterial species (Dong et al., 2000;Leadbetter and Greenberg, 2000). However, they have now been reported in Proteobacteria belonging to the Agrobacterium, Bosea, Commamonas, Delftia, Ochrobactrum, Pseudomonas, Ralstonia, Sphingopyxis and Variovorax genera (d'Angelo-Picard et al., 2005;Flagan et al., 2003;Huang et al., 2003;Jafra et al., 2006;Leadbetter and Greenberg, 2000;Lin et al., 2003;Park et al., 2003;Uroz et al., 2003). In...

No abstract

No abstract