Beneficial Microorganisms for Aquaculture: Nutrition, Bacteriostatic Effects, and Immunoregulation

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Aquaculture, the farming of aquatic organisms, has surged in significance as a pivotal food production sector, propelled by escalating global seafood demand and technological advancements. However, this rapid expansion has unveiled critical challenges. The industry grapples with a dearth of sustainable protein sources for feed, heightened vulnerability to pathogens in high-density farming setups, and the deterioration of product quality during culture and storage. Traditional protein sources like fishmeal are not only economically prohibitive but also environmentally unsustainable. Moreover, the frequent outbreaks of diseases caused by bacterial, viral, and fungal pathogens lead to substantial economic losses.

Nutritional Benefits of Beneficial Bacteria

Bacteria as Alternative Feeds

Beneficial bacteria, particularly those rich in proteins, offer a viable alternative to conventional fishmeal. These bacteria can constitute up to 65% of the total dry weight in proteins and are replete with nucleotides, surpassing traditional fishmeal. Studies have demonstrated that incorporating bacterial biomass into diets can bolster the growth, immunity, and stress response of aquatic animals without compromising diet palatability. For instance, in a 10-week trial with juvenile Florida pompano, partially replacing fishmeal with dried fermented biomass did not significantly alter growth metrics such as final weight, survival rate, or food conversion ratio. Similarly, rainbow trout showed no adverse effects when fishmeal was partially substituted with bacterial single-cell protein. Methanotroph bacteria, which thrive on natural gas, have also shown promise as fishmeal substitutes. In Atlantic salmon culture, Methylococcus capsulatus was effectively used to replace a portion of the fish diet without inducing health issues or significantly altering growth rates.

Micronutrients Produced by Beneficial Bacteria

Beyond macronutrients, aquatic animals require micronutrients like vitamins, fatty acids, and essential amino acids for optimal growth and physiological function. Many beneficial bacteria produce these micronutrients, which can be crucial when traditional feeds are deficient. Vitamin B-12, for example, is synthesized by various probiotic strains and is essential for DNA synthesis and metabolism. Research on carp has shown that commensal bacteria can produce sufficient vitamin B-12 to support fish growth even without dietary supplementation. Additionally, probiotic strains can enhance the essential amino acid content of plant protein sources through fermentation, reducing anti-nutritional factors and improving overall feed quality.

Inhibition of Pathogens and Spoilage Microorganisms

• Anti-pathogenic Mechanisms
  The high-density nature of modern aquaculture systems makes them susceptible to disease outbreaks. Beneficial bacteria can mitigate this risk through various mechanisms. They can outcompete pathogens for essential nutrients such as iron, produce inhibitory compounds like bacteriocins, and disrupt quorum sensing, a communication system used by bacteria to coordinate behavior. For example, certain strains of Pseudomonas fluorescens can inhibit Aeromonas salmonicida by competing for free iron, thereby protecting fish from furunculosis. Lactococcus lactis TW34 produces nisin Z, a bacteriocin that effectively inhibits the fish pathogen Lactococcus garvieae. Moreover, some beneficial bacteria can degrade quorum-sensing molecules, thereby neutralizing the negative effects of pathogens like Vibrio harveyi in shrimp culture.
• Anti-spoilage Mechanisms
  Aquatic products are prone to spoilage due to their high water and protein content. Beneficial bacteria can inhibit spoilage bacteria through similar mechanisms used against pathogens. Fermentation, a traditional preservation method, leverages beneficial bacteria to produce organic acids, alcohol, and CO2, creating an environment that suppresses spoilage bacteria. For example, nisin Z-producing Lactococcus lactis KT2W2L has been used to extend the shelf life of tropical shrimp by inhibiting spoilage bacteria. Other bacteriocins like AMPNT-6 and Coagulin L1208 have also shown efficacy in preserving seafood quality.

Regulation of Host Immunity

Improving Product Quality

• Enhancing Growth and Flesh Quality
  Beneficial bacteria can enhance the growth rate and flesh quality of aquatic animals by improving nutrient utilization and modulating the immune system. For example, Lactobacillus casei in Porthole livebearer resulted in higher protein and lipid content in fillets, indicating improved product quality. Additionally, probiotics can prevent spoilage by inhibiting spoilage bacteria, thereby extending the shelf life of aquatic products. For instance, nisin treatment has been shown to enhance the quality and shelf life of seer fish steaks and large yellow croaker.
• Practical Applications and Future Directions
  Practical applications of beneficial bacteria in aquaculture include biofloc technology and the use of recombinant probiotics. Biofloc technology, which recycles waste nutrients into microbial biomass, has been successfully applied in various aquaculture systems, improving feed utilization efficiency and reducing water usage. Recombinant probiotics, such as Anabaena sp. PCC 7120 expressing VP28, have shown promise as oral vaccines against viral infections in shrimp. Future research should focus on optimizing the application of beneficial bacteria, exploring novel strains, and understanding their long-term effects on aquatic health and product quality.

Conclusion

The aquaculture industry stands at a critical juncture, facing significant challenges that threaten its sustainability and productivity. Beneficial bacteria offer a multifaceted solution, providing essential nutrients, inhibiting pathogens, and modulating host immunity. By enhancing growth performance, improving product quality, and reducing environmental impacts, beneficial bacteria can pave the way for a more sustainable and efficient aquaculture sector. Future research must delve deeper into the mechanisms underlying these benefits, optimize application methods, and explore the potential health benefits of consuming aquaculture products enriched with beneficial bacteria. As the industry continues to evolve, the strategic use of beneficial bacteria will be instrumental in meeting global seafood demand while safeguarding environmental and consumer health.

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This article is for research use only and cannot be used for any clinical purposes.