Quorum sensing (QS) regulates Phaeobacter gallaeciensis antagonism in broth systems; however, we demonstrate here that QS is not important for antagonism in algal cultures. QS mutants reduced Vibrio anguillarum to the same extent as the wild type. Consequently, a combination of probiotic Phaeobacter and QS inhibitors is a feasible strategy for aquaculture disease control.
Bacteria predominantly exist in mixed populations, and the behavior of the complete population is often affected by highly structured and sophisticated ways of communication.Many bacteria secrete small pheromone-like substances that allow the complete population to sense its density and subsequently alter, in a coordinated manner, gene expression. This type of communication between bacteria is known as quorum sensing (QS). QS systems play an important role in the regulation of genes related to virulence and, also, in genes involved in the production of bioactive compounds, such as antibiotics and enzymes (1-4).Roseobacter clade bacteria are some of the most common prokaryotes in oceanic and aquaculture environments (5-7). Some Roseobacter clade species, such as Phaeobacter gallaeciensis, Phaeobacter inhibens, and Ruegeria mobilis, produce a broad-spectrum antibacterial compound known as tropodithietic acid (TDA) (7-9). TDA does not induce resistance in the target organisms (10) and is the key compound for antagonism in laboratory broth cultures (11), in algae and rotifers, and in fish larval systems (12, 13). The ecological role of TDA is not known, but as TDA-producing bacteria can inhibit fish and shellfish pathogens, they have interesting prospects for application as probiotics in aquaculture (13,14).QS compounds of the acylated homoserine lactone (AHL) class are produced by some Roseobacter clade species (15-17), and TDA production is influenced by AHLs in Phaeobacter gallaeciensis. Genes homologous to the classical luxI (pgaI) and luxR (pgaR) genes have been identified, and a 24-h culture supernatant of a pgaI-negative mutant that is unable to produce the AHL compound did not antagonize other bacteria as the wild type did (18). Ruegeria mobilis does not produce AHLs, but in this bacterium, TDA itself can act as an autoinducer, controlling the expression of key genes for TDA production (19).Roseobacter bacteria are frequently associated with algae, both in oceanic waters and aquaculture systems (5, 7). In aquaculture, algae are used as a direct addition to fish larva-rearing tanks and to enrich live prey organisms, such as rotifers and Artemia, with fatty acids. Both algae and live feed have been suggested as vehicles for the introduction of probiotics into rearing systems (20,21), and a strategy of adding Phaeobacter strains to reduce pathogenic Vibrio in live feed and larval cultures has been proposed (12). As P. gallaeciensis antagonism is mainly caused by TDA and since TDA production is regulated by QS, we hypothesized that P. gallaeciensis antagonism in aquaculture settings (e.g., algal cultures) would also be affected by QS and, hence...