A case study on chemical defense based on quorum sensing: Antibacterial activity of sponge-associated bacterium Pseudoalteromonas sp. NJ6-3-1 induced by quorum sensing mechanisms

A case study to investigate the relationship between antibacterial activity and quorum sensing mechanisms was carried out on a sponge-associated bacterium with remarkable biological activities: Pseudoalteromonas sp. NJ6-3-1. The dependence of active substance production on cell density was studied under various growth conditions. Bacteria NJ6-3-1 was found to start producing antibacterial compounds only when cell density reached the threshold value of OD ₆₃₀=0.4. To simulate the competitive real marine environment, NJ6-3-1 at low cell density (OD ₆₃₀ value below the required threshold value) was cocultured with the terrestrial bacterium Staphylococcus aureus. Antibacterial activity assays indicated the existence of some signal molecules in the metabolites of S. aureus that could induce NJ6-3-1 to produce antibacterial substances even at low cell density. Three diketopiperazines (DKPs) as metabolites and potential autoinducers of NJ6-3-1 were synthesized and co-cultured with low density NJ6-3-1. The antibacterial activity assay showed that one of these DKPs-cyclo-(L-Phe-L-Val)-was the autoinducer and could indeed induce NJ6-3-1 to produce antibacterial substances under low cell density. Our results thus provide preliminary support to the hypothesis that the antibacterial activity of NJ6-3-1 is controlled by the quorum sensing system in both an intra-species and an inter-species manner.