Effect of selected essential oils and pure compounds on Saprolegnia parasitica

Saprolegnia parasitica Coker is the main agent of saprolegniosis, one of the most important causes of economic losses in the fish farming industry, affecting all developmental stages. In this work the activity of some essential oils are compared with the activity of their isolated chemical constituents (alcohols, aldehydes, ketones, phenols, and hydrocarbons) to evaluate molecular classes active against S. parasitica. This suggests potential new products to use against saprolegniosis. Sixteen commercial essential oils and 37 pure constituents were tested against a strain of Saprolegnia parasitica from cutaneous lesions of Perca fluviatilis Linnaeus, using a continuously agitated broth technique that allows both the Minimum Inhibitory Concentration (MIC) and the Minimum Lethal Concentration (MLC) to be evaluated. It was possible to evaluate the efficacy of each essential oil against S. parasitica in relation to its specific composition and the efficacy of its main constituents. Carvacrol (MIC of 50 ppm and a MLC of 100 ppm) and p-cymene (MIC and MLC = 200 ppm) are the main constituents of Origanum vulgare L. and Thymus vulgaris L. (MIC = 100 ppm, MLC = 200 ppm) and are the primarly responsible for the efficacy of these two essential oils against S. parasitica. The structure of the constituents was also evaluated in relation to their activities against S. parasitica. Besides monoterpenic aromatic compounds, the acyclic mono- and sesquiterpenic primary alcohols and aldehydes seem to be the most active compounds. The open chain alcohol 1-decanol [CH3-(CH2)8-CH2-OH] proved to be the most active agent with a MLC = 50 ppm.