Viability of probiotic C6-6 in fish feed and ability to inhibit columnaris-causing bacteria in rainbow trout Oncorhynchus mykiss.

Probiotics are increasingly recognized as sustainable alternatives to antibiotics in aquaculture. This study evaluated the viability and disease-mitigating potential of the candidate probiotic Lelliotia amnigena strain C6-6 in rainbow trout Oncorhynchus mykiss. Viability was assessed under varying storage temperatures (-15, 4, and 17°C), time intervals (up to 24 wk), 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. The viability of C6-6 over time was enhanced in feeds prepared without oil. In vitro assays demonstrated that C6-6 inhibited the growth of 3 species of columnaris-causing bacteria, namely 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 gastrointestinal 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 for coldwater aquaculture.