Bacterial protein gave a positive immune response against furunculosis
Researchers have tested what replacing fishmeal with microbes will mean for factors such as growth and immune response in rainbow trout fry
In a scientific paper, researchers write that the fish feed industry has made significant progress over the past 20 years in improving the efficiency of the use of marine resources for feed formulation.
“This has been achieved through a combination of improving the feed factor (FCR), reduction in the use of fishmeal (FM) and fish oil (FO), and increased use of FM from offal.”
However, they point out that the volume of wild-caught forage fish needed to support the increase in aquafeed production (estimated at 87.1 million tonnes in 2025) is unattainable based on current feed formulations.
Therefore, the levels of fishmeal and fish oil in feed must be reduced.
Fishmeal alternatives
Alternatives to fishmeal range from plant ingredients (e.g. soybeans, corn gluten and rapeseed meal) to animal by-products (e.g. meat, bone and poultry meal, and offal from fish processing from both wild-caught and aquaculture sources). But also, single cell proteins (SCP) have received special attention in recent years.
“Regardless of their origin (microalgae, yeast, fungus, or bacteria), SCP have important advantages over conventional protein ingredients since they have a good nutritional profile, require shorter production time, less land use, production is not affected by seasonal and climatic variations, and they can be produced from a wide range of free substrates,” write the researchers.
“Among different sources of SCP, bacterial SCP (BSCP) is one of the most popular and sustainable, as they are characterised by a high protein level (up to 80% on a dry weight basis) compared to other sources of SCP, good amino acid profile similar to FM, as well as for the content of vitamins, phospholipids and other functional compounds.”
Many substrates
In addition, they point out that bacteria can be produced using a wide range of substrates and nutrients, from industrial and agricultural residues to bio-industrial by-products, thereby allowing value addition to waste streams and reducing the downstream costs associated with industrial waste disposal.
“It is better to sell residual biomass as feed than to sell it as fertiliser,” write the researchers.
However, BSCP also has some disadvantages. One of the most important is taste, which in some studies has been reported to reduce feed intake (FI).
“However, this must be assessed from time to time, since different sources of BSCP can vary in nutritional properties (i.e., chemical composition from amino acids, nucleic acids, minerals) and thus taste.”
The study
The aim of the study was to evaluate the suitability of BSCP from methanotrophic bacteria in composite diets for rainbow trout fry as an alternative ingredient to FM to test the hypothesis that this can be used as a safe and valuable alternative protein source for fish feed.
For this purpose, the researchers took a holistic approach to the use of this alternative protein ingredient based on the evaluation of classic key performance indicators (KPIs) related to growth and feed efficiency. In addition, the effect of this ingredient on the nutritional quality of the fillet, the histological organisation of the liver and intestine, and the intestinal microbiota were looked at.
The research also tested whether replacing FM with BSCP could compromise the immune response of fish when they were exposed to the bacteria responsible for furunculosis in salmonids.
In the experiment, rainbow trout juveniles of approximately 11.3 grams were given different levels of FM replacement (0, 25, 50, 75 and 100%) with a BSCP (0, 3.75, 7.5, 11.25 and 15% of Uniprotein® Aqua).
The experiment was divided into two different parts. First, there was a nutritional study that lasted 83 days where the effect of experimental diets was evaluated in relation to key performance indicators (KPIs) related to fish growth and feed utilisation. Then an infection experiment was carried out with Aeromonas salmonicida subsp. salmonicida to evaluate the immunological competence of fish fed experimental diets (15 days).
Results
Weight (BW) after the feeding trial for fish fed 100% FM replacement with BSCP was 183.4 ± 4.5 g, while the rest of the diet groups varied between 191.3–201.2 g.
Feed intake was not affected by the level of FM replacement with BSCP, while feed conversion ratio (FCR) increased slightly with increasing levels of BSCP (P < 0.05), although the lowest FCR values were found in fish fed 50% FM replacement with BSCP (0.79 ± 0.01) compared to the control group (0.81 ± 0.01).
Using quadratic linear regression, the optimal dietary FM replacement with BSCP in terms of BW values was estimated to be 41.7%.
The nutritional quality of the fillet was not altered with experimental diets; all diet groups showed similar amino acid levels as well as their contents of HUFA n-3 and n-6 that remained unchanged ( P > 0.05) and lipid nutritional indices (P > 0.05).
No negative effects were observed in the histological examination of the liver or intestine, neither in the level of lipid peroxidation nor the activity of antioxidative stress enzymes in livers from fish fed with increased inclusion levels of BSCP (P < 0.05).
The replacement of FM with tested BSCP obtained from methanotrophic bacteria did not cause imbalance or dysbiosis in the gut microbiota of rainbow trout regardless of the level of FM replaced.
The replacement of FM with BSCP by 50% improved disease resistance in rainbow trout exposed to furunculosis compared with the control diet (cumulative mortality = 83.3 ± 7.2% vs. 49.2 ± 15.6%; P < 0.05), suggesting that this ingredient may also have some immunomodulatory properties that would protect fish against infections, although their mode of action deserves further attention.
Conclusion
“Current results indicated that the tested BSCP from methanotrophic bacteria is a safe, functional and sustainable alternative protein source for aquatic feed,” write the researchers.
The researchers A. Ruiza, I. Sanahuja(a), NW Thorringer(b), J. Lynegaard(b), E. Ntokou(b), MD Furones(a), E. Gisbert(a) from respectively; (a) IRTA, Center de la Ràpita, Aquaculture Program in Spain and (b) Unibio A/S, Roskilde in Denmark, are behind the experiment published in the journal Aquaculture.
