Fermented products of Bacillus velezensis improve growth, intestinal health and immune functions of crucian carp (Carassius auratus) by regulating its intestinal core microbiota
This study demonstrates that fermented products of *Bacillus velezensis* LK1, particularly its culture, serve as a sustainable antibiotic alternative that significantly enhances the growth, immunity, and intestinal health of crucian carp by regulating core gut microbiota, specifically promoting beneficial *Cetobacterium* species.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Inside the belly of every fish lies a bustling, invisible city. This is the gut microbiome, a complex community of bacteria that acts as a second immune system and a primary engine for digestion. In a healthy animal, this community is a balanced ecosystem where helpful bacteria work alongside neutral ones, keeping harmful invaders at bay. However, when this balance is disturbed—by stress, poor diet, or the overuse of medicine—the city can fall into chaos, leading to disease and stunted growth. For decades, farmers have relied on antibiotics to prevent these outbreaks and keep livestock growing fast. But the heavy use of these drugs has created a dangerous side effect: bacteria are learning to resist them, and traces of the drugs are finding their way into the food we eat. As governments move to ban antibiotics as routine growth promoters, the search is on for a natural replacement that can keep animals healthy without the risks.
Researchers in China have turned their attention to a specific type of bacteria called Bacillus velezensis. This microbe is a natural survivor, capable of forming tough, dormant spores that allow it to withstand harsh conditions, and it produces substances that fight off other harmful bacteria. While scientists have long known that adding live Bacillus bacteria to feed can help, a new study suggests that the real power lies not in the living bacteria themselves, but in the fermented liquid they produce. By feeding crucian carp, a popular freshwater fish in Asia, a diet enriched with the fermented culture of Bacillus velezensis, the researchers discovered a way to boost growth and immunity more effectively than using the live bacteria alone or even traditional antibiotics.
The study began with a simple question: which form of this beneficial microbe works best? The team set up an experiment with hundreds of young crucian carp, dividing them into four groups. One group received a standard diet as a control. A second group was fed the live Bacillus bacteria. A third group received the fermented culture, which contains the bacteria along with all the metabolic byproducts they created during fermentation, such as enzymes and antimicrobial compounds. The final group was fed a diet containing standard antibiotics. Over the course of nearly two months, the researchers watched how the fish grew, how well they digested food, and how their immune systems responded.
The results were striking. The fish fed the fermented culture grew faster and converted their food into body mass more efficiently than any other group, including those on antibiotics. They also showed stronger immune defenses and better antioxidant capabilities, meaning their bodies were better equipped to handle stress and damage. Surprisingly, the group fed only the live bacteria did not show the same significant improvements in growth or immunity. This suggests that the magic of the treatment was not in the bacteria colonizing the fish's gut, but in the cocktail of beneficial substances they produced while fermenting. The fermented culture acted like a powerful prebiotic, reshaping the internal environment to favor the fish's health.
To understand why this happened, the scientists looked inside the fish's intestines. They found that the fermented culture dramatically changed the composition of the gut bacteria. It reduced the diversity of the overall bacterial community, which might sound negative, but in this case, it meant the gut was becoming more focused and efficient. The treatment specifically boosted the numbers of a beneficial bacterium called Cetobacterium, which is known to produce vitamin B12 and help maintain a healthy gut environment. At the same time, it suppressed harmful bacteria that often cause disease in fish. The researchers also observed that the intestinal walls of the fish fed the fermented culture were healthier, with taller finger-like projections called villi that absorb nutrients and more mucus-producing cells that protect the gut lining. These physical changes explained why the fish could digest food better and grow faster.
The study also revealed how these internal changes linked to the fish's overall health. The rise in Cetobacterium was directly connected to better growth rates and stronger immune activity. The fermented culture essentially acted as a switch, turning on the fish's natural defenses and metabolic processes while turning off the pathways that lead to disease. Unlike antibiotics, which often wipe out a broad range of bacteria indiscriminately, the fermented culture seemed to guide the gut ecosystem toward a more stable and beneficial state. It improved the fish's ability to fight off infections and reduced the buildup of fats in their livers and muscles, leading to a healthier, leaner fish.
This research offers a promising path forward for sustainable aquaculture. It demonstrates that a fermented product derived from a single strain of bacteria can outperform both the live bacteria and traditional antibiotics in promoting animal health. The key finding is that the byproducts of fermentation, rather than the live microbes themselves, are the primary drivers of these benefits. By using this fermented culture, farmers could potentially replace antibiotics with a natural, multifunctional feed additive that supports growth, strengthens immunity, and stabilizes the gut microbiome. This approach not only addresses the urgent need to reduce antibiotic use but also provides a safer, more effective way to raise fish, ensuring that the food on our plates is produced in harmony with the natural biology of the animal.
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