Oxygen tolerance of Cetobacterium somerae isolated from the gut of freshwater fish and their environments

Cetobacterium somerae is a predominant bacterium found in the gut of freshwater fish. However, being an anaerobic bacterium, its survival is believed to be prevented in oxygen-rich environments. Therefore, in this study, we investigated the oxygen tolerance of 83 C. somerae strains collected from the guts of freshwater fish, rearing water, and sediment of culture ponds and tanks. When placed in sterile bottles containing common carp- and goldfish-rearing water, C. somerae showed a 1-log decrease after 24 hours, suggesting that this organism faces challenges in growing in rearing water. Subsequently, we inoculated the bacterial strains onto agar plates and exposed them to air for 12 hours to measure the oxygen inhibition index (OII). The OII values ranged from 0.01 to 4.65 among different strains, indicating significant variation in oxygen tolerance within the bacterium. Furthermore, the OII values varied considerably depending on the isolation source, with sediment, rearing water, and gut samples showing increasing values in that order. This suggests that oxygen tolerance plays a substantial role in the ecological behavior of C. somerae.