The use of Ocimum americanum essential oil against the pathogens Aeromonas hydrophila and Gyrodactylus sp. in silver catfish (Rhamdia quelen).

Abstract

The bactericidal activity (minimum inhibitory concentration (MIC)-test) of Ocimum americanum (inflorescences) essential oil (OAEO) against Aeromonas hydrophila was determined in this study. Also investigated was the potential of OAEO and the main compound found in the oil (linalool) at subinhibitory concentrations to be inhibitors of haemolysis caused by Aer.hydrophila in fish erythrocytes. An invivo experiment was conducted to evaluate the survival of fish (Rhamdia quelen) experimentally infected with Aer.hydrophila and exposed to OAEO. A second experiment was conducted to evaluate the invitro and invivo activity of OAEO (mix from inflorescences and leaves) against the parasite Gyrodactylus sp. The OAEO showed weak invitro activity against Aer.hydrophila (6400μgml(-1) ). Subinhibitory concentrations of OAEO (100μgml(-1) ) inhibited haemolysis (90%) caused by Aer.hydrophila in fish erythrocytes, however, linalool did not inhibit haemolysis activity. At the low concentrations (10 and 20mgl(-1) ) added to the water, OAEO promoted the survival of fish experimentally infected with Aer.hydrophila. Lastly, the OAEO mix (50mgl(-1) ) was effective against Gyrodactylus sp., significantly reducing (60%) the number of parasites in the fish. Phytochemicals, such as essential oils (EOs) are a great source of new molecules and have shown potential to be used in aquaculture systems. However, additional studies focused on the invivo efficacy, mode of action and identification of the active compounds are needed. This study determined the potential of Ocimum americanum EO for use against two important fish pathogens, Aeromonas hydrophila and Gyrodactylus sp., as well as providing preliminary information about the role of the main EO compound (linalool) against Aer.hydrophila virulence.