The intensification of aquaculture production has increased the need to control pathogens. Studies show that yeast derivatives can support the natural lines of defence of both freshwater and marine species.
At a time of growing concern about the use of antimicrobials in animals used for food, alternative − often preventive − strategies are being sought to promote health and performance in a sustainable way. Aquatic animals have several natural lines of defence against external threats. These include the gut, the skin, the gill physical barrier, the associated mucosal immune system and microbiota. All contribute to protecting the animal against an invasion or internal infection for which the animal’s systemic immunity provides the ultimate defence.
The trial results suggest that shrimp preventively fed the yeast derivative are better able to react to a pathogen challenge. Photo: Shutterstock
Yeast fractions can help balance intestinal microflora and stimulate natural defences in the host at both the mucosal and the animal level. Trials conducted in fish and shrimp indicate that an yeast derivative helps to support those natural lines of defences in both freshwater and marine species. Outcomes are very positive, both in terms of animal performance and economic benefits for the producer.
Pathogen binding ability
A research programme was carried out in order to identify and develop a new generation of yeast fractions with optimal characteristics that might help animals face various challenges. The results demonstrated that yeast cell wall properties are linked to the strain, with each strain exhibiting specific structure and functionality in terms of binding properties. Moreover, it was also shown that, for a given strain, binding properties can differ when applying varying production and inactivation processes. Hence, for every selected strain, it is essential to determine the optimal fermentation conditions, as well as the treatment of the live yeast used to obtain the yeast fractions (inactivation technique).
Based on these findings, a range of yeast strains were screened by scientists at Lallemand in order to select the best candidates for optimal binding of various bacteria in vitro, such as various Vibrio species which are of particular importance in aquaculture. The best strains were selected for their complementary properties. Transposed to in vivo settings, such properties indicate a potential to agglutinate a larger spectrum of undesirable bacteria inside the gut lumen prior to the potential colonization of the gut epithelium. Further mechanistic studies also indicated that, when combined, the different yeast fractions exert a synergistic activity on the immune system in vitro. A patent has been filed to cover this synergistic immune effect. The selected yeast fractions were combined into a unique formulated solution, which is called YANG (Yeast Association New Generation).
This year a trial was carried out using white shrimp juveniles (Litopenaeus vannamei) to assess the effect of yeast derivative supplementation on shrimp immunity. After 4 weeks of continuous supplementation, a numerically higher Total Hemocyte Count (THC; + 28%) was measured in the haemolymph of shrimp fed the yeast derivative compared to the non-supplemented control group. Furthermore, the THC profile revealed a significantly higher proportion of large-
granular cells with the supplement (x 3.4 increase); this was in line with the higher respiratory burst, phenoloxidase (PO) and lysozyme activities (Figure 1) in the hemolymph. All of these parameters are described biomarkers of innate immune components in shrimp. These results suggest that shrimp preventively fed the yeast derivative are better able to react to a pathogen challenge, with a larger quantity of immune cells and enhanced immune cell activity.
Figure 1 – Proportion of large-granular cell; b) Phenoloxidase activity and c) Lysozyme activity in the hemolyph of white shrimp fed a commercial diet non-supplemented (Control, displayed in GREEN) or supplemented with the yeast derivative (YANG, displayed in ORANGE) for 4 weeks period. Mean±S.D.
Another trial was conducted with the new yeast derivative on juvenile white shrimp challenged with EMS/AHPND (Vibrio parahaemolyticus). The yeast derivative supplementation for 3 weeks prior to the challenge was associated with a highly significant 4.6-fold improvement in shrimp survival (56% survival vs. 12% for control). Similar findings were repeated in a subsequent trial under similar conditions, where the yeast derivative supplementation was associated with a significantly higher survival, reaching 49.6 % compared to 16.5 % in the control group. Such consistent results at the animal level confirm the in vitro agglutination results and (potentially) positively modulated immunity under non-challenging conditions. They clearly show that specific yeast fractions can be used as an efficient tool to support shrimp health in the context of one of the most damaging pathologies in aquaculture.
Reinforcing the first line of defence
An 8-week trial was conducted in rainbow trout (Oncorhynchus mykiss) to evaluate the effect of the yeast derivative on skin mucus production. In this trial, skin mucus secretion increased over time in the control, indicating on-going recovery toward normal mucus levels following transfer into the rearing system. Compared to the control group, a positive effect of supplementation was observed within 4 weeks, with a 65% increase in skin mucus level. After 2 months (week 8), skin mucus level was 27% higher in the yeast supplemented group than in the control group. By supporting rapid recovery (in under 4 weeks) and high levels of skin mucus, the yeast derivative was shown to support a lower risk of secondary infection and physiological disruption following challenging conditions (Figure 2). Results related to skin mucus level were in agreement with the up-regulation of a molecular skin biomarker which is indicative of the immune function and regenerative capacity (wound healing) of the skin mucosal layer. These results have been demonstrated in several species, including seabass, seabream, trout and salmon. By supporting the skin mucosal barrier, yeast fractions contribute to resilience and recovery from challenging conditions including parasitic challenges, chemical treatment and mechanical disruptions resulting from handling and transfers.
Figure 2 – Effect of the yeast derivative supplementation on skin mucus quantity and quality in sea bream. Control is displayed in GREEN, the yeast derivative (YANG) is displayed in ORANGE.
In the same trial, yeast derivative supplementation was also shown to enhance the microvilli density of intestinal epithelial cells, the second line of defence. A higher surface area of the intestinal brush-border is associated with enhanced gut health, protective barrier function and nutrient absorption. Similar outcomes were observed in juvenile seabass in conditions of digestive stress (incorporation of high level plant- based raw materials in the diet – 40% soybean meal). Following 10 weeks of yeast derivative supplementation, the density of microvilli of the intestinal cells was increased, and significant differences in the length of microvilli and gut perimeter ratio were also observed. These were associated with improved feed utilisation and growth performance. Weight gain (21.6 vs. 18.5 g/fish in control), daily growth rate (1.39% vs. 1.25% in control) and feed conversion ratio (1.45 vs. 1.66 in control) were improved (with a statistically significant difference for all parameters) in fish fed with YANG. Finally, in the same trial, a strong correlation between dietary yeast derivative supplementation and the intestinal expression of genes involved in the immune response was demonstrated, indicating a positive modulation of the immune system.
Authors: Eric Leclercq, François Cellier, and Stéphane Ralite, Lallemand Animal Nutrition