부산대학교 도서관

Simple Summary: Due to the increase in antibiotic resistance observed in pathogenic bacteria, new alternatives are being explored to reduce the use of these drugs. Probiotics, a group of bacteria that includes lactic acid bacteria (LAB), are capable of stimulating and strengthening the immune system and have showed antagonistic effects against various pathogenic bacteria. The aim of this study was to identify and test potentially probiotic LAB, isolated from Brown trout and Rainbow trout, to select those with the least antibiotic resistance, the best antagonistic effect against fish pathogens, and minimal negative effects during their application on embryonated Brown trout eggs. As a result, various bacterial strains were obtained that proved to be safe in terms of antibiotic resistance and effective against several of the pathogens to which they were exposed. Nevertheless, only a limited number of these strains were found to be safe for use in Brown trout eggs. However, some findings suggest the need to investigate the possibility that certain bacterial strains considered probiotics might have adverse effects on fish health. This research investigates the potential of lactic acid bacteria (LAB) from freshwater salmonids as prospective probiotics for application in aquaculture. LAB and pathogenic bacteria were obtained from mucus and tissues of Oncorhynchus mykiss and Salmo trutta from fish farms in northeast Spain that had not used antibiotics for the six months preceding the study. Isolates were identified using Gram staining and sequencing of 16S rRNA and ITS-1. To assess the safety of the LAB, antibiotic susceptibility tests (ASTs) against 23 antimicrobials were performed. In vitro antagonism assays were conducted to evaluate the inhibitory effects of living LAB using the agar diffusion test method and their metabolites using the agar well diffusion method. The assays targeted six specific pathogens: Aeromonas salmonicida subsp. salmonicida, Carnobacterium maltaromaticum, Vagococcus salmoninarum, Yersinia ruckeri, Lactococcus garvieae, and the marine pathogen Vibrio jasicida. Additionally, a toxicity assay was conducted on embryonic eggs of S. trutta. The ASTs on probiotic LAB candidates revealed varied responses to antimicrobials, but no resistance to oxytetracycline or florfenicol, which are two antibiotics commonly used in aquaculture, was detected. The in vitro assays indicate that LAB exhibit antagonistic effects against pathogens, primarily when directly stimulated by their presence. In applications involving embryonic eggs or larvae, certain live strains of LAB were found to have adverse effects, with some isolates resulting in higher mortality rates compared to the control group or other isolates. Furthermore, the potential pathogenicity of certain LAB strains, typically considered safe in salmonids, warrants deeper investigation. [ABSTRACT FROM AUTHOR]