Bacterial communication system known as quorum sensing (QS) is a pivotal system for bacterial survival, adaptation and pathogenesis. Members in the multicellular community may synthesize or acquire a signaling molecule in order to elicit downstream cellular processes. Roles of indole and derivatives, a new class of quorum-sensing signal molecules, in various bacterial physiologies and virulence have been reported recently. Indole is normally found in mammal gastrointestinal tract as a metabolite of tryptophan metabolism by microbiota. Therefore, interspecies connection via indole signaling among commensal bacteria and enteric pathogens could be anticipated. Effects of indole exposure on the virulence of Listeria monocytogenes were investigated by phenotypic and molecular approaches. Results demonstrated that synthetic indole and indole-rich conditioned medium significantly diminished biofilm formation and related virulence of L. monocytogenes including motility, cell aggregation and exopolysaccharide production. Transcript levels of virulence-associated (pssE, dltA, flaA, fliI, motB, agrA and hly) and regulatory genes (codY, sigB, prfA and gmaR) were substantially downregulated in indole-treated cells. Only mogR gene encoding for a repressor of motility genes was upregulated after indole exposure. Our findings raise the possibility that L. monocytogenes may acquire indole signaling from gut microbiota for resource-effective adaptation upon transition to new environment.

Acute hepatopancreatic necrosis disease, a severe disease of shrimp, is caused by Vibrio parahaemolyticus (AHPND Vp), a halophilic bacterium harboring a plasmid that contains toxin genes homologous to Photorhabdus insect-related toxins. We obtained 9 isolates of Bdellovibrio and like organisms (BALOs) from water and sediment samples in Thailand. Using 16S rRNA sequencing, all of the organisms were identified as Bacteriovorax spp. and were able to attack all tested AHPND Vp isolates. In addition, their various susceptible hosts, including Gram-positive and Gram-negative bacteria, were observed. The optimal ratio for interaction between the Bacteriovorax isolate BV-A and AHPND Vp was determined to be 1:10. The suitable conditions applied for co-culture between BV-A and AHPND Vp were 30°C, 2% NaCl, and pH 7.6. The capability of BV-A to reduce numbers of AHPND Vp in vitro was observed in co-culture after incubation for 2 d and continued until the end of the incubation period. In vivo, BV-A was able to reduce mortality of shrimp post-larvae infected with AHPND Vp. In addition, BV-A significantly decreased the formation of biofilm by AHPND Vp. These findings provide evidence for using Bacteriovorax as a biocontrol of AHPND Vp in shrimp aquaculture.

The bacterium, Vibrio parahaemolyticus was isolated from 776 patients at Hat Yai Hospital in Southern Thailand from 2006 to 2010. 51.3-73.6% of the isolates were tdh (+) trh (-) and Group-specific PCR positive pandemic strains. A comparison of the number of V. parahaemolyticus isolates in this study and that from the same hospital in 2000-2005 indicates that this region of Thailandis endemic for V. parahaemolyticus.

Thailand is one of the leading exporting countries of rambutan and rambutan peels are considered as a biological waste. In this study, rambutan (Nephelium lappaceum L. cv. Rong Rian) peel extracts (RPE) obtained by water extraction were analyzed for their phytochemical composition, antibacterial and antioxidant activities, and cytotoxicity. The bioactive compounds in RPE identified by GC-MS were mome inositol (35.99 mg/g), catechol (29.37 mg/g), 5-hydroxymethylfurfural (5.69 mg/g), 2-pentenal, (E)-(5.22 mg/g), acetic acid (3.69 mg/g), 1,2,3-propanetriol (3.67 mg/g), 2-furan-carboxaldehyde (2.66 mg/g), and other compounds. FT-IR analysis confirmed the presence of alcohol and phenol in the extract. Antibacterial activities of RPE against food pathogenic and spoilage bacteria showed that RPE could inhibited Bacillus subtilis, B. cereus, Staphylococcus aureus, Vibrio cholerae, V. parahaemolyticus, Pseudomonas aeruginosa, and P. fluorescens, with MIC values ranging between 1024 and 8192 µg/mL. The extract also showed antioxidant properties, as determined by DPPH and ABTS assays. The cytotoxicity analysis after 72 h of treatment showed the IC₅₀ values at 194.97 ± 4.87, 205.92 ± 2.55, and 94.11 ± 1.33 µg/mL for L929, Vero, and MCF-7 cell lines, respectively. Therefore, this study provided a basis of knowledge of rambutan peels as an excellent source of natural bioactive compounds for various applications.

Melanization due to the inactivation of the homogentisate-1,2-dioxygenase gene (hmgA) has been demonstrated to increase stress resistance, persistence, and virulence in some bacterial species but such pigmented mutants have not been observed in pathogenic members of the Vibrio Harveyi clade. In this study, we used Vibrio campbellii ATCC BAA-1116 as model organism to understand how melanization affected cellular phenotype, metabolism, and virulence. An in-frame deletion of the hmgA gene resulted in the overproduction of a pigment in cell culture supernatants and cellular membranes that was identified as pyomelanin. Unlike previous demonstrations in Vibrio cholerae, Burkholderia cepacia, and Pseudomonas aeruginosa, the pigmented V. campbellii mutant did not show increased UV resistance and was found to be ~2.7 times less virulent than the wild type strain in Penaeus monodon shrimp virulence assays. However, the extracted pyomelanin pigment did confer a higher resistance to oxidative stress when incubated with wild type cells. Microarray-based transcriptomic analyses revealed that the hmgA gene deletion and subsequent pyomelanin production negatively effected the expression of 129 genes primarily involved in energy production, amino acid, and lipid metabolism, and protein translation and turnover. This transcriptional response was mediated in part by an impairment of the quorum sensing regulon as transcripts of the quorum sensing high cell density master regulator LuxR and other operonic members of this regulon were significantly less abundant in the hmgA mutant. Taken together, the results suggest that the pyomelanization of V. campbellii sufficiently impairs the metabolic activities of this organism and renders it less fit and virulent than its isogenic wild type strain.