Possible Mechanisms of Action of Two Pseudomonas fluorescens Isolates as Probiotics on Saprolegniosis Control in Rainbow Trout (Oncorhynchus mykiss Walbaum)

SIMPLE SUMMARY: In aquaculture, as in other production systems, diseases caused by microorganisms can produce great economic losses and it is essential to have preventive or curative treatments for their control. However, there is increasing concern and restrictions on the use of chemical products for the treatment of such diseases. In this context the use of probiotics could be a good alternative to chemical treatment. Previous studies of our research group have shown the usefulness of two Pseudomonas fluorescens isolates for the biocontrol of saprolegniosis in rainbow trout—a disease caused by a pseudo-fungus that affects freshwater fish—under experimental conditions when they are added to the tank water. In the present work we investigated some of the possible mechanisms of action of these two isolates, such as stimulation of the immune response, competition for binding sites or sources of nutrients, and the production of bioactive substances inhibitory to pathogenic agents. To sum up, the two P. fluorescens isolates used in the present study might be usable for biocontrol of saprolegniosis and the mode of action of these bacteria is likely to be related to the production of siderophores (small compounds with high affinity binding for iron). ABSTRACT: Probiotics have been proposed as one of the alternatives to the chemical treatments currently used in aquaculture. Recently, the possible usefulness of certain microorganisms, mainly bacteria, has been highlighted as a potential biocontrol for saprolegniosis. In the present work we investigated the possible mechanisms of action of two isolates of Pseudomonas fluorescens (LE89 and LE141) with proven ability to reduce Saprolegnia parasitica infection in rainbow trout under experimental conditions when they are added to the tank water. The stimulation of the innate immune response and the production of siderophores and bioactive substances inhibiting S. parasitica present in cells and supernatants of LE89 and LE141 were studied. Regarding the immune response the only noteworthy points were the increase in the phagocytic activity of macrophages and the concentration of serum proteins when LE141 was administered. Both bacteria produced siderophores. When analyzing the protein substances present in supernatants, it was observed that in both isolates the proteins with inhibitory activity present might be siderophores. In LE141, besides siderophores, a protein of 66 kDa was identified in the fraction responsible for inhibition. To sum up, the two P. fluorescens isolates might be usable for biocontrol of saprolegniosis and that the mode of action of these bacteria is likely to be related to the production of siderophores.