Disruption of Cell-to-Cell Signaling Does Not Abolish the Antagonism of Phaeobacter gallaeciensis toward the Fish Pathogen Vibrio anguillarum in Algal Systems

Abstract

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...

The marine bacterium Phaeobacter inhibens secures external ammonium by rapid buildup of intracellular nitrogen stocks

“…Berger et al. 2011; Prol García, D'Alvise and Gram 2013) could contribute to the higher maximal levels of TDA during growth with excess of nitrogen (Fig. S7, Supporting Information).…”

Acyl-Homoserine Lactone Quorum Sensing in the Roseobacter Clade

“…Initially, it was found that null mutants of pgaI and pgaR cannot synthesize tropodithietic acid (TDA), an antibiotic produced by several members in the Roseobacter clade, and also fail to produce a yellow-brown pigment, when grown in marine broth with shaking for 24 h [ 26 ]. However, recent studies show that different carbon sources determine the role of QS in regulating the production of TDA [ 28 ] and the production is only delayed in QS mutants in agitated culture in marine broth [ 27 ]. More importantly, QS is not important for the TDA production when P. inhibens DSM17395 is used as a probiotic treatment in aquaculture system [ 27 ].…”

Vibrio anguillarum Is Genetically and Phenotypically Unaffected by Long-Term Continuous Exposure to the Antibacterial Compound Tropodithietic Acid

“…Phaeobacter and Ruegeria strains have repeatedly been isolated from aquaculture units and are harmless to fish larvae and their live feed (12)(13)(14)(15)(16). The probiotic (antibacterial) effects of Phaeobacter and Ruegeria are predominantly caused by their production of the dual-sulfur tropone-derived com-pound tropodithietic acid (TDA) (16)(17)(18)(19). TDA-producing Phaeobacter and Ruegeria strains can prevent vibriosis in cod larvae (16) and reduce mortality rates for turbot larvae (15) and Artemia (20) challenged with V. anguillarum and V. harveyi, respectively.…”