Do you know that the organs responsible for reproduction, the ovaries and testes, host their own unique communities of microbes? Just like the gut or skin, the gonads contain bacteria that may play crucial roles in function. In a new study published in Microbiome, researchers focus on the hermaphroditic swamp eel, and explore how these gonadal microbes change during natural sex reversal.

This study, led by Prof. LUO Daji's team from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences, revealed that the swamp eel’s gonads contain microbial communities as rich as those found in the gut, skin, and blood, and more than half of these gonadal bacteria originate from the gut and bloodstream, suggesting an intriguing internal migration of microbes.

The swamp eel (Monopterus albus) is an animal which begins life as a female and later transforms into a male. This natural sex reversal provides a rare window into how gonad-associated bacteria respond to, and possibly influence, reproductive biology.

By comparing ovarian and testicular microbiomes, researchers identified ovarian and testicular dominant bacteria. They identified a bacterium called Bacillus which dominated in ovaries. After injecting Bacillus into the fish, they observed that the ovarian microbial balance was disrupted, and metabolic activity shifted, particularly in processes related to amino acid and lipid metabolism. These changes could be vital for maintaining normal ovarian function.

In male models, researchers found that the same Bacillus triggered a different response. They observed that Bacillus stimulated inflammation in the testes, and led to reduced sperm motility, which suggested that the same microbe could have distinct, sex-dependent effects.

“Our findings show that gonadal microbiomes are not just passive residents, they interact with host physiology in ways that may be essential for reproduction and even for facilitating natural sex reversal processes,” explained Dr. MENG Kaifeng from IHB, the first author of this study.

The study opens a new frontier in reproductive biology and microbiology. If bacteria in gonads can influence metabolism and immune responses, they might affect fertility, development, and health across species. Besides, the findings of this study provide new insights into aquaculture where strategically managing microbial communities could enhance breeding efficiency and reproductive health.