Fish larvae and juveniles are particularly vulnerable to pathogens, where vaccines are often not a viable option due to their immature immune systems. This situation has led to the search for sustainable alternatives to antibiotics, whose extensive use carries the risk of promoting antimicrobial resistance.
A new study by researchers from the Technical University of Denmark has shown that it is possible to develop a consortium of bacteria capable of inhibiting bacterial pathogens in this sector. This could potentially reduce the use of antibiotics in aquaculture and possibly in other applications. The study was published in Microbiology Spectrum, a journal of the American Society for Microbiology.
In this new study, researchers set out to find and develop non-antibiotic biological options for disease control and prevention in aquaculture.
The persistent challenge of diseases in larviculture
Infections by bacteria of the Vibrio genus, such as Vibrio anguillarum, are a significant problem in marine larviculture, affecting a wide range of fish species. Live feed, such as microalgae, although essential for many marine species in their larval stages, can also serve as a vector for the introduction of these pathogenic bacteria. The need for disease control measures that do not rely on antibiotics is therefore urgent.
Probiotics have emerged as a viable strategy. Using beneficial bacteria or probiotics to combat pathogens is a strategy that is becoming widespread in both animal husbandry and horticulture.
Traditionally, research and application of probiotics in aquaculture have focused on the use of pure cultures of isolated strains. However, in natural environments, protection against pathogens is rarely the work of a single microorganism; it is the concerted effort of complex microbiomes that keeps invaders at bay. This concept drives the hypothesis that microbial communities, with their intricate interactions, could offer synergistic advantages over single-strain probiotics. The scientists behind the recent study believed that the anti-pathogen effect would likely be stronger in a combination of beneficial bacteria.
Microalgae and their microbiomes to the rescue
The research authors of the study set out to evaluate the anti-pathogen efficacy of mixed microbiomes from two microalgae used as live feed: Tetraselmis suecica and Isochrysis galbana. The objective was to determine if these natural communities could inhibit the growth of Vibrio anguillarum, a major fish pathogen.
How did they measure inhibition?
To quantify the pathogen inhibition by these complex microbial communities, the research team developed an in vitro assay. “To test if the pathogen could be inhibited by a mixture of other bacteria, we needed a measure of the growth (and growth inhibition of the pathogen), so we tagged the fish pathogen with a green fluorescent protein (GFP),” explained the study’s corresponding author, Lone Gram, Ph.D., professor in the Department of Biotechnology and Biomedicine, Technical University of Denmark. “By measuring this—and the reduction in fluorescence—we could identify bacterial communities that inhibited the pathogen.” This method, using a GFP-tagged strain of Vibrio anguillarum, allowed for the measurement and quantification of inhibition as a reduction in fluorescence signal, offering a high-throughput way to assess anti-pathogenic effects.
The researchers prepared different fractions of the algal cultures:
- Full culture (FC): Containing both algal cells and their associated bacteria.
- Filtered microbiome (FM): Where algal cells were removed, primarily leaving the bacterial community.
- Axenic cultures (AX): Pure cultures of algae without associated bacteria, for control. These preparations were co-cultured with the GFP-tagged Vibrio anguillarum, and fluorescence and optical density were monitored over time to assess pathogen growth.
Strength is in the (microbial) union
The study results shed light on the potential of these natural communities:
- The Isochrysis galbana microbiome shows greater potency: It was observed that the microbiome associated with Isochrysis galbana was more inhibitory to V. anguillarum compared to the Tetraselmis suecica microbiome.
- It’s the microbes, not the algae per se: It is important to note that pathogen inhibition was not caused by the algal cells themselves, but by their associated microbiomes. Axenic cultures of the algae (without bacteria) showed no inhibition. Neither did the cell-free supernatants, indicating that the live microorganisms of the microbiome are responsible for the protective effect.
- Changes in the community during the “battle”: During co-culture with the pathogen, the bacterial density of the Isochrysis microbiomes increased, while diversity decreased, according to metataxonomic analyses. The bacteria that thrived and dominated in these enriched inhibitory microbiomes belonged mainly to the Alteromonadaceae and Rhodobacteraceae families, with a relative increase in Vibrionaceae (possibly native non-pathogenic or less virulent vibrios from the microbiome itself).
- The power of synergy: more than the sum of the parts: Researchers found that mixtures of bacteria could inhibit Vibrio anguillarum and subsequently isolated pure cultures of bacteria. They discovered that some of these bacteria only inhibited the fish pathogen when combined, not alone, demonstrating that some bacteria were “stronger together”. A notable example was the co-culture of Sulfitobacter pontiacus D3 and Vreelandella alkaliphila D2; neither of these strains was inhibitory as a monoculture, but together they showed a remarkable ability to curb the pathogen. This finding underscores that synergistic interactions within the microbial community can be crucial for probiotic efficacy. The bacteria isolated from the microbiomes that demonstrated complete inhibition belonged to families such as Alteromonadaceae, Halomonadaceae (later identified as Vreelandella), Rhodobacteraceae, Vibrionaceae, Flavobacteriaceae, and Erythrobacteraceae.
Implications for more sustainable aquaculture
This study convincingly demonstrates that microbial communities derived from the natural microbiomes of algae can have significant anti-pathogenic effects. Furthermore, it highlights that bacterial co-cultures may offer synergistic advantages over single-strain probiotics, opening new avenues for the development of health strategies in aquaculture.
“We have shown that it is possible in microbiomes (in our case, the microbiome of algae used as live feed in aquaculture) to find mixtures of bacteria that can inhibit the pathogen,” said Gram. “Thus paving the way for engineering microbiomes that can inhibit bacterial pathogens and reduce the need for use of antibiotics. We can then reduce the spread of antibiotic-resistant bacteria.”
The main points to highlight for the aquaculture sector are:
- New sources of probiotics: Live feed algae microbiomes represent a natural reservoir of potential probiotics.
- Co-culture strategies: The future of disease control could lie in the use of carefully selected mixtures of microorganisms that work together for more robust and stable protection.
- Reduction of antibiotics: The development of effective community-based probiotics can lessen the dependence on antibiotics, contributing to the fight against antimicrobial resistance.
- An efficient selection tool: The fluorescence-based screening methodology developed in this study can accelerate the identification of microbiomes and bacterial combinations with high anti-pathogenic potential.
Conclusion: A collaborative probiotic future
The research by Smahajcsik and colleagues reinforces the idea that “unity is strength,” also in the microbial world. By harnessing the natural and synergistic interactions within communities of microorganisms associated with algae, a promising avenue opens up for developing more effective and sustainable probiotic strategies for aquaculture. This approach not only addresses disease control but also promotes more environmentally friendly aquaculture production, reducing the need for chemical interventions and supporting the long-term health of farming systems. The study is a step forward towards harnessing the collective power of microbiomes for more resilient and productive aquaculture.
Contact
Lone Gram
Department of Biotechnology and Biomedicine, Technical University of Denmark
Lyngby, Denmark
Email: gram@bio.dtu.dk
Reference (open access)
Smahajcsik D, Roager L, Strube ML, Zhang S, Gram L.0.Stronger together: harnessing natural algal communities as potential probiotics for inhibition of aquaculture pathogens. Microbiol Spectrum:e00421-25. https://doi.org/10.1128/spectrum.00421-25
Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.
