Discovering, Characterizing and Applying Acyl Homoserine Lactone Quenching Enzymes to Mitigate Microbial-associated Problems in Saline Conditions

Quorum quenching (QQ) is proposed as a new strategy to mitigate microbial-associated problems (e.g. fouling, biocorrosion). However, most of the QQ agents reported till date have not been evaluated for their quenching efficacies in conditions representative for seawater desalination plants, cooling towers or marine aquaculture. In this study, bacterial strains were isolated from Saudi Arabian coastal environments and screened for acyl homoserine lactone (AHL)-quenching activities. Five AHL quenching bacterial isolates of genera Pseudoalteromonas, Pontibacillus and Altererythrobacter exhibited high AHL-quenching activity in salinity of 58 g/L and pH 7.8 at 50 °C. This demonstrates the potential of these QQ bacteria to mitigate microbial-associated problems in saline and alkaline conditions at high (>37 oC) temperature. Further characterization of the QQ efficacies revealed two bacterial isolates, namely Pseudoalteromonas sp. L11 and Altererythrobacter sp. S1-5 to possess enzymatic QQ activity. The genomic sequence of L11 and S1-5 plus homologous screening against reported AHL quenching genes suggest the existence of four possible QQ open reading frames in each strain. Specifically, two novel AHL enzymes, AiiAS1-5 and EstS1-5 from Altererythrobacter sp. S1-5 both contain signal peptide, and exhibit QQ activity over a broad range of pH, salinity and temperature. In particular, AiiAS1-5 demonstrated activity against a wide spectrum of AHL signals. When tested against three bacterial species, namely Aeromonas hydrophila, Pseudomonas aeruginosa and Vibrio alginolyticus, AiiAS1-5 was able to inhibit motility of all three species in saline conditions. Biofilm formation associated with P. aeruginosa was also significantly inhibited by AiiAS1-5. AiiAS1-5 also reduced the quorum sensing mediated virulence traits of A. hydrophila, P.aeruginosa and V. alginolyticus during mid and late exponential phase of cell growth. The enzyme did not impose detrimental effect on the cell growth, suggesting a lower potential for the target bacterium to develop resistance over long term exposure. Overall, this study suggested that selected enzymes that were obtained from bacterial hosts proliferating in saline and high temperatures demonstrate potential to mitigate microbial-associated problems that occurred during the use of seawater.