56 DECEMBER • WORLD AQUACULTURE • WWW.WAS.ORG We began with a series of in vitro tests, applying standard microbiological techniques to measure how these OSCs affected the growth of the three most concerning seabass pathogens. The results were eye-opening: clear zones of inhibition surrounded the treated samples, and PTSO in particular showed potent bactericidal activity, especially against T. maritimum and V. anguillarum. These compounds weren’t just s lowing the bacteria down; they were actively eliminating them. Encouraged by these findings, we proceeded to in vivo trials. Juvenile seabass were allocated into groups and fed diets containing either 150 or 300 parts per million (ppm) of a blend of both compounds or a control diet without additives. After 70 days, the fish were challenged with V. anguillarum under controlled experimental conditions. What followed was a clear trend: fish fed the supplemented diets had significantly higher survival rates. While the control group ended the trial with a 48.75% survival probability, fish receiving 150 ppm and 300 ppm of the PTS/PTSO blend reached 62.5% and 61.25% respectively. Notably, the lower dosage performed slightly better, suggesting that when it comes to these natural compounds, more is not always better. These results add to a growing body of evidence supporting the use of phytogenics plant-based compounds with bioactive properties in aquaculture. OSCs from onion, in particular, appear to work by disrupting microbial cell membranes and interfering with bacterial metabolism. Unlike antibiotics, which can contribute to resistance and persist in the environment, these compounds degrade naturally and have shown no adverse effects on fish growth or water quality in our trials. From a practical standpoint, this approach offers real-world benefits. Seabass producers now have a natural tool that can be In Mediterranean aquaculture, the European seabass (Dicentrarchus labrax) reigns as one of the most valuable species. But this prized fish faces a relentless challenge: bacterial infections that not only threaten fish health and survival but also cut deeply into the profitability and sustainability of the industry. Pathogens such as Photobacterium damselae subsp. piscicida, Vibrio anguillarum or Tenacibaculum maritimum are well known to seabass farmers. When outbreaks strike, losses can be swift and severe, forcing many producers to rely heavily on veterinary drugs. However, growing concerns about environmental impacts and the long-term sustainability of intensive treatments have triggered a search for safer, more natural and effective solutions. That’s where onions come in In Allium cepa, better known as the common onion, the amino acid derivative propiin, structurally analogous to alliin in garlic, serves as the primary precursor of sulfur-containing bioactives. Upon tissue disruption, propiin undergoes enzymatic cleavage by alliinase, yielding volatile thiosulfinates such as propyl-propane-thiosulfinate (PTS) and propyl-propane-thiosulfonate (PTSO). These organosulfur compounds (OSCs) have been extensively reported to exhibit potent antimicrobial, antioxidant and immunomodulatory activities. Curious about the potential of these compounds in aquaculture, our team set out to test whether dietary supplementation with PTS and PTSO could help protect European seabass against bacterial infections, first in the lab, then in live fish. All trials were conducted at the Andalusian Aquaculture Technology Centre (CTAQUA, Spain), with the technical support of DOMCA S.A.U.’s R&D team, who collaborated in the development and provision of the experimental formulations. Onions to the Rescue: Natural Compounds From Allium Cepa Show Promise Against European Seabass Pathogens Jose F. Cabello-Gómez, Iria Folgueira López, Carmen García de Lomas del Cid, María del Mar Agraso Martínez and Alberto Baños Arjona FIGURE 1. The amino acid derivative propiin, found in onions and structurally analogous to alliin in garlic, serves as the primary precursor of sulfur-containing bioactives. What followed was a clear trend: fish fed the supplemented diets had significantly higher survival rates. While the control group ended the trial with a 48.75% survival probability, fish receiving 150 ppm and 300 ppm of the PTS/PTSO blend reached 62.5% and 61.25% respectively. Notably, the lower dosage performed slightly better, suggesting that when it comes to these natural compounds, more is not always better.
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