Aquaculture is a fast-growing sector and one of the main challenges is represented by disease outbreaks like Vibriosis, that is considered a significant problem for several aquatic species. In the context of an increasing demand for responsible and sustainable aquaculture, the study of compounds with antimicrobial properties alternative to antibiotics, such as organic acids (OA), is receiving a growing attention. The aim of this study was to screen in vitro the antimicrobial activity of 10 organic acids (OA) against 2 Vibrio species, V. harveyi and V. anguillarum. The OA tested were short-chain fatty acids (formic [For], propionic [Pro], butyric [But] acids), medium-chain fatty acids (hexanoic [Hex], octanoic [Oct], decanoic [Dec] and dodecanoic [Dod] acid), and others like sorbic (Sor), benzoic (Ben) and citric (Cit) acid.
V. harveyi, isolated from farmed gilthead seabream, and V. anguillarum (ATCC 19264) were cultured at 25°C in tryptic soy broth (TSB) supplemented with 1.5% NaCl and used as inoculums. The antimicrobial activity was evaluated after 24h incubation with microdilution method in TSB in which the substances were dissolved at the following concentrations: 7,5, 3.75, 1.88, 0.94, 0.47, 0.23, 0.12 mM for Oct, Dec and Dod and 100, 50, 25, 12.5, 6.25, 3.13 mM for the others. Control strains were incubated with TSB only. The analysis was performed on 96-wells microtiter plates with 105 cfu/mL and the growth inhibition was evaluated by absorbance measurement at 620 nm after incubation.
Dod showed the lowest MIC against V. harveyi (0.94 mM) and V. anguillarum (0.94 mM) followed by Oct and Dec (0.94 mM against V. harveyi and 1.88 mM against V. anguillarum). Cit decreased the growth of V. harveyi at 100 mM whereas no inhibition was observed for V. anguillarum at the tested concentrations. Pro, But and For were not effective even at the highest dose tested. Hex, Sor and Ben showed a MIC value of 50 mM against V. harveyi whereas they inhibited V. anguillarum at 12.5 mM, 25 mM and 50 mM respectively.
All the results are reported in Fig. 1. In conclusion, these findings showed the high antimicrobial power of Sor, Ben and medium-chain fatty acids against 2 Vibrio spp., highlighting the interest of their use in aquafeeds.