In Vitro Testing of Potential Probiotic Bacteria against Vagococcus salmoninarum in Rainbow Trout (Oncorhynchus mykiss, Walbaum 1792)

In this study, probiotic properties of endogenous microbiota of rainbow trout against Vagococcus salmoninarum isolated during an outbreak of vagococcosis in a trout farm in the Mediterranean region were evaluated. The candidate probiotic bacteria were isolated from rainbow trout intestines. A total of 157 isolates were obtained and screened for antagonistic activity against V. salmoninarum via the Well Diffusion Agar method. Six isolates were determined for antagonistic activity against V. salmoninarum. Conventional microbiological tests and API 20 Strep tests (bioMe´rieux) were used for further phenotypic characterization of all six antagonistic isolates. For molecular identifications of isolates, L. garvieae specific PCR and 16S rRNA gene sequence analysis were used. Antagonistic strains were identified including TUB/2013/V47 (L. garvieae), TUB/2013/V27 (L.garvieae), TUB/2013/V10 (L. garvieae), TUB/2013/V2 (L. garvieae) TUB/2013/V1 (L. lactis) and TUB/2013/V4 (L. lactis). The strains were then tested for hydrophobicity, bile salts and acid tolerance and antimicrobial activity. All isolates were congo red-positive, indicating the presence of hydrophobic structures in their cell walls. It was determined that whole antagonistic strains were resistant to low pH conditions and 0.6-1.5% bile concentrations. Antimicrobial test results showed that most of the strains are susceptible in vitro to amoxicillin/clavulanic acid ampicillin, doxycycline, erythromycin and florfenicol, which are frequently used in aquaculture. As a result, it was found that strains have in vitro probiotic properties (hydrophobic, tolerant to bile salts and low pH conditions). Further study is needed to explore their in vivo probiotic effects against vagococcosi