Viability of Probiotic C6-6 in Fish Feed and Its Ability to Inhibit Columnaris-Bacteria and Prevent Columnaris Disease in Rainbow Trout (Oncorhynchus mykiss)

Authors: MAXWELL, RYAN - University Of Idaho; Ma, Jie; SWEARINGEN, SHELBY - University Of Idaho; Lafrentz, Benjamin; CAIN, KENNETH - National Oceanic & Atmospheric Administration (NOAA)

Submitted to: Journal of Fish Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2026
Publication Date: N/A
Citation: N/A

Interpretive Summary: The use of probiotics as alternative feed additives is gaining popularity in aquaculture due to their benefits for growth and survival. This study examines the potential of the probiotic strain C6-6 in improving salmonid aquaculture by addressing two aspects. The first involves understanding how temperature, storage time, and feed preparation affect the viability of C6-6 when incorporated into rainbow trout feed. The second aspect evaluates the ability of C6-6 to inhibit Flavobacterium columnare, a harmful fish pathogen, through laboratory and fish trial experiments. Key findings suggest that proper handling and storage conditions, particularly avoiding fish oil in feed formulations, can enhance C6-6 viability. Additionally, C6-6 effectively combats F. columnare infections, reducing fish mortality in experimental settings. These findings emphasize practical strategies to improve the application of C6-6 as a disease treatment, offering valuable insights for fish farmers and researchers in the aquaculture field.

Technical Abstract: Probiotics are increasingly recognized as sustainable alternatives to antibiotics in aquaculture. This study evaluated the viability and disease-mitigating potential of the candidate probiotic strain C6-6 in rainbow trout. Viability was assessed under varying storage temperatures (-15'°C, 4'°C, 17'°C), time intervals (up to 24 weeks), and feed preparation methods. Results showed that colder storage, particularly at -15'°C, preserved higher concentrations of C6-6, while viability rapidly declined at 17'°C. Feeds prepared without oil retained significantly more viable cells over time. In vitro assays demonstrated that C6-6 inhibited the growth of Flavobacterium columnare, F. covae, and F. davisii. In vivo, trout fed C6-6 showed significantly reduced mortality (14%) compared to controls (26%) following F. columnare challenge, and C6-6 was reisolated from the GI tract of treated fish. These findings highlight C6-6 as a promising probiotic with strong antagonistic activity and stable viability under proper storage, supporting its potential use in disease management strategies in coldwater aquaculture.