Bacillus species are widely used as probiotics in aquaculture due to their reported benefits, including growth promotion, immune enhancement, and water quality improvement. However, Bacillus cereus, a naturally occurring and potentially toxigenic microorganism, has been identified as a fish pathogen in several studies. In the present study, a mass mortality outbreak occurred at a rainbow trout farm in Isparta, Türkiye, where a daily mortality rate of 2% was recorded. Ten moribund fish (mean body weight: 15 g) were sampled for analysis. Bacterial identification was performed based on morphological characteristics, biochemical tests, and analyses of the 16S rRNA and gyrB genes. In addition, histopathological examination was carried out to assess the tissue alterations induced by the pathogen. Based on the antibiogram test using 12 antibiotic discs (Oxoid, Hampshire, UK), the isolate exhibited the highest susceptibility to florfenicol, erythromycin, and ofloxacin, while showing resistance to cefuroxime, amoxicillin, cefotaxime, ampicillin, and tetracycline. Histopathological examination revealed severe B. cereus-induced lesions, including necrosis and tissue damage in the spleen, kidney, liver, muscle, and skin. Melanomacrophage infiltration was observed in the kidney and spleen, while inflammatory cell infiltration was evident in the intestine and liver. This study concluded that the B. cereus strain isolated from a rainbow trout farm is pathogenic to fish. The combined use of molecular, microbiological, and histopathological methods improves diagnostic reliability and supports appropriate antibiotic selection for controlling B. cereus infections in aquaculture. Overall, the findings provide a valuable foundation for improving disease treatment and health management strategies in fish farming systems.

To Cite This Article: Uluköy G, Kubilay A, Kan S, Atsatan K, Baba E and Özmen O, 2026. Molecular and histopathological characterization of Bacillus cereus infection in rainbow trout (Oncorhynchus mykiss). Pak Vet J, 46(9): 2242-2249. http://dx.doi.org/10.29261/pakvetj/2026.207