Evaluation of Bacillus velezensis SM6 isolated from Solea solea: A score-heat map based approach for the selection of probiotic feed additives
This study identifies *Bacillus velezensis* SM6, isolated from common sole, as a superior probiotic candidate for aquaculture by employing a novel score-heatmap approach to evaluate its safety, broad-spectrum antimicrobial activity, and strong adhesion properties, which were subsequently validated through in vivo trials in European seabass.
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
Fish farming has become a cornerstone of global food production, filling the gap left by dwindling wild fish stocks. Yet, the crowded conditions of modern aquaculture create a perfect storm for disease. When fish are stressed, harmful bacteria can spread rapidly, leading to massive losses. For decades, the industry has relied on antibiotics to fight these infections, but this approach carries a heavy price. Overuse of these drugs breeds resistant bacteria that threaten both the environment and human health, while also disrupting the natural, beneficial microbes living inside the fish. In search of a safer path, scientists are turning to probiotics—live microorganisms that, when added to feed, can crowd out bad bacteria and boost the fish's own defenses. The challenge lies in finding the right kind of microbe: one that is tough enough to survive the harsh journey through a fish's digestive system, strong enough to fight off pathogens, and completely safe for the animal it is meant to protect.
Researchers from Tunisia and Spain set out to solve this puzzle by looking for answers in an unlikely place: the skin and guts of wild common sole, a flatfish found in the Mediterranean. Instead of guessing which bacteria might work, they collected samples from the fish and began a rigorous screening process. They isolated 87 different bacterial strains from the mucus and intestines of these wild fish. From this large group, they narrowed their focus to eight strains that showed a remarkable ability to fight off a wide range of dangerous fish pathogens. These eight candidates were then put through a gauntlet of tests designed to measure their usefulness and their safety. The scientists looked at how well the bacteria could stick to fish tissue, how effectively they produced digestive enzymes, and whether they could disrupt the chemical signals that bacteria use to coordinate attacks. Crucially, they also checked for any signs of danger, such as the ability to destroy red blood cells or carry genes that cause disease.
To make sense of this mountain of data, the team developed a scoring system that weighed every positive trait against every potential risk. They assigned points for beneficial behaviors, like strong adhesion to mucus or the ability to break down bacterial toxins, and subtracted points for any safety concerns. This method allowed them to visualize the performance of each strain on a single map, revealing a clear winner. One strain, identified as Bacillus velezensis SM6, stood out above all the others. It achieved the highest possible score by combining broad-spectrum protection against disease with an exceptional ability to cling to fish surfaces. In laboratory tests, this bacterium showed a hydrophobicity, or water-repelling nature, of 78.28 percent, which helps it stick to the fish's gut lining. It also demonstrated a 92 percent ability to clump together with itself, a trait that helps it colonize the gut and crowd out invaders.
The safety profile of this top candidate was just as impressive as its fighting ability. While other strains showed signs of potential harm, such as the ability to break down blood cells or carry virulence genes, Bacillus velezensis SM6 was clean. It showed no signs of toxicity, carried no known disease-causing genes, and remained sensitive to common antibiotics, meaning it would not contribute to the growing problem of drug resistance. To confirm these findings, the researchers took the final step: they injected the bacteria into European seabass. Over a ten-day period, the fish showed no signs of illness, no mortality, and no physical abnormalities. The bacteria did not harm the fish, proving that this strain is safe for use in a living animal.
The study concludes that Bacillus velezensis SM6 represents a highly promising candidate for the future of sustainable fish farming. By using a method that carefully balances functional performance with strict safety checks, the researchers identified a microbe that could replace harmful antibiotics. This bacterium, isolated from the wild sole, has the potential to protect farmed fish from disease while keeping the aquatic environment healthy. The work suggests that the best probiotics may not come from a lab, but from the very ecosystems where fish live, offering a natural solution to one of aquaculture's most persistent problems. While further studies are needed to test its effectiveness in large-scale farming operations, this discovery marks a significant step toward a safer and more sustainable way to feed the world's growing population.
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