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.

In the recent years, effort to interfere with the quorum sensing (QS) has taken researchers consideration into the discovery and identification of bioactive compounds secreted by prokaryotes and eukaryotes. In this research, the anti-QS activity of thirty one plant species was detected through the inhibition of the QS related behaviors, violacein pigmentation in Chromobacterium violaceum CV026 and plant tissue maceration in Pectobacterium carotovorum. The results revealed that the arial parts extract of seven plant species interfered quorum sensing phenotypes in bacteria which amoung them, the dill (Anethum graveolens L.) extract effectively inhibited violacein production in CV026 and singificantly reduced tissue maceration in test plants. The result substantiates that the dill extract has decreased (>13 mm) violacein production in biosensor besides possessing slight antimicrobial property (<7.5 mm). Both potato tubers and calla-lily slices inoculated with Pcc-extract showed significant reduction in tissue maceration, while the tubers and slices inoculated with Pcc and Pcc-DMSO were strongly macerated. GC-MS analysis of the A. graveolens extract indicated that the oil was consisted eight phytocompounds of which the Eugenol (49.62 %) as major component. This results suggest that plants have mimic quorum-sensing signals which could be serving as potential sources to disrupt quorum sensing in associated bacteria.

The fungitoxicity of Foeniculum vulgare herb essential oil was evaluated by mycelial growth inhibition assessment in medium culture. The radial growth of the Alternaria alternata was completely inhibited at the oil levels of 500 and 300 ppm in poisoned food and volatile activity assessments, respectively. Microscopic observations using Light Microscope showed morphogenic changes in the test fungus with 300 ppm of the oil. These changes included a significant loss of conidiation, decrease hyphal diameter and induce cytoplasm condensation. The treatment with the oil also resulted in a highly lobate and bulbous anomalous structure on the hyphal tips. Essential oil of F. vulgare at 500 ppm effectively reduced the percentage of rotted tomatoes in in vivo. Gas chromatography-mass spectrometry analysis of the Fennel oil revealed that it was considered of 25 monoterpenoid compounds of which trans-anethole, estragole, and fenchone as the major components. Findings of the current study suggest that essential oils have natural phytochemical and could be used as a substitute bioactive compounds for chemicals in control of plant pathogenic fungi on fruits or vegetables.

Quorum sensing is a regulatory mechanism of gene expression in bacteria which regulated many physiological processes such as production of virulence factors in Pectobacterium carotovorum. In this research, the plant expression vector, pBI121, harbouring the anti-quorum sensing gene, aiiA, was transformed into Potato (Solanum tuberosum cv. Agria) explants by Agrobacterium mediated transformation procedure and putative transformants containing pBI121/aiiA were obtained. The results of bioassay test revealed that the expression of aiiA gene in potato plant induces the resistance to the early stage of bacterium pathogenesis. The soft rot symptoms such as wilting, yellowing and tissue maceration were not observed until 48 hr after inoculation of transgenic potatoes with P. carotovorum. Although in non-transgenic plants, the disease symptoms were appeared after 24 hr of plants infection. Hence, this study proves that the heterologous expression of aiiA gene enhances the resistance against potato soft rot disease.