by Dr Eric Leclercq, Global Aquaculture R&D and Fish Application Manager, Lallemand Animal Nutrition
In the current context, the need to secure the targeted production volumes and quality while carefully controlling production cost is more than ever essential. Maintaining fish health while minimising the use of antimicrobials has become a major target across the aquaculture sector. Prevention through nutrition thanks to functional ingredients has been recognised as a valuable approach. Among these functional ingredients, certain specific yeast products such as yeast cell wall (YCW) can participate to reinforcing fish robustness and resilience to external insults and diseases and thus contribute to fish farming performance.
Multifactorial approach to fish challenges, welfare and sustainable farming
All along their rearing-cycle, fish are constantly exposed to an array of external factors threatening the integrity and proper functioning of the gut and skin mucosa. Such factors include sudden variations in water quality and temperature, blooms of pathogens and non-infectious harmful agents as well as exposure to necessary management practices such as crowding, handling and transportation. These insults are typically cumulative resulting in complex and progressively aggravating conditions at gut and skin level. When these essential protective barriers are jeopardised, normal physiological functions cannot be maintained, immunity is breached, health and performance is compromised. These complex mucosal challenges are a major treat to demanding production schedule, cost and quality targets, compromising the sustainability of modern fish farming.
Against such multifactorial conditions, strategies focussing on prevention are essential and these should include measures minimizing stress and promoting fish robustness, resilience to stressors and immune capacity ahead any actual challenges. When arising, interventions measures boosting immune response and disease resilience, tissue repair and feed intake will help recover immediate and longer-term biological performance.
The good news is that, according to recent research publications in this area, specific yeast products can participate to this overall strategy.
Not all yeast are equal
When it comes to functionality, not all yeast, nor yeast products are equivalent: both the strain genetic makeup and its production process can influence its properties and activity in-vivo. One distinguishes the live active yeast used as probiotics, from the whole inactive yeast, which can come under several forms. The yeast extracts, which represents the soluble fraction of the inactive yeast, and can be used as rich source of nutrients; and the yeast cell wall, the cell external layer (YCW), which is the insoluble fraction of the inactivated yeast. YCW is a source of mannan oligosaccharides (MOS) and beta glucans.
Whole YCW as a source of MOS help increase gut and skin robustness though several demonstrated mechanisms, for example by increasing gut maturity, skin mucus excretion and quality. Continuous dietary supplementation with properly selected, well documented whole YCW benefits is an essential preventive health strategy to help secure health and performance.
Lallemand has established a laboratory dedicated to the characterisation of yeast cells and YCW in particular. In collaboration with aquaculture scientists, the team recently looked into the relationship between YCWs structure and immune benefits in a model fish, the zebrafish. In this published study (Rawlings et al., 2023), four different YCWs issued from different yeast strains were tested in aquafeed at similar inclusion rates.
Beyond the composition of the YCW (% MOS and % glucans), the study evidenced a relationship between certain YCW structural characteristics (e.g. MOS and beta-glucans chain-length) and the interaction with host-cells receptors (Fig 1). Each YCW elicited a distinct immune response documenting their specific properties with regard to particular infectious or non-infectious pathologies.
This new study provides a perfect illustration of the statement that not all yeasts, nor YCW are equal when it comes to their biological activity; paving the way for precision functional nutrition in aquaculture.
Figure 1: YCW fractions are characterised by differences in cells surface structures and composition. (A) Scanning electron microscopy images of 4 different YCWs, (B) YCW biochemical composition (% MOS and β-glucans), (C) Binding capacity of MOS or β-1,3-glucan of each of the YCW tested. (Adapted from Rawlings et al., 2023).
A versatile and cost-effective functional ingredient for bass and bream
A specific YCW product (OPTIWALL) has been developed by Lallemand Animal Nutrition as an all-round and consistent source of MOS and beta-glucans bringing practicality, efficacy and affordability. It has shown strong benefits at reinforcing gut barrier functions and integrity, i.e. robustness, bringing growth and feed performance benefits when fishes are chronically exposed to demanding conditions.
This new product is recommended for the on-growing of seabass and seabream during high density-high temperature periods to help secure production schedule and biological cost.
A recently published study (Leclercq et al., 2023) evaluate the inclusion of this YCW in the diet of European seabass juveniles in a dose-response manner. Results showed:
- The YCW can boost performance in a dose-response manner: Growth and feed conversion (FCR) were both improved by up to 6.7percent and 8.3percent respectively.
- Mucosal barriers were also improved. Among others, gut robustness, skin mucus excretion and antimicrobial defenses were increased.
- Specifically, the YCW helped better maintain FCR and enhanced gut and skin protection barrier when exposed to a chronic challenge (repetitive handling and high rearing density) at the highest inclusion level tested (4.5 kg/T feed) (Fig. 2).
Figure 2: Effect of different doses of the dietary YCW OPTIWALL on juveniles feed conversion rate (from Leclercq et al., EAS 2023)
The authors concluded OPTIWALL has the potential to protect the baseline performance and robustness of European seabass and that the feed supplementation strategy can be tailored to the level of challenge and of benefits targeted.
Addressing the cycle challenges
Management of fish transfer and handling are critical for the farm performance. They can generate significant stress, physical and physiological damages with long-lasting hang-over impacts. Physical wounds, both macro- and microscopic lesions, are evidently essential to minimize including to prevent secondary infections to which handled-fish are particularly sensitive.
YANG is a new generation yeast products developed by Lallemand using cutting-edge technologies. It combines inactivated yeast fractions from selected strains each produced using dedicated processes. By combining potent and contrasted yeast-microbial patterns, a broad panel of immune receptors are co-activated, including pro- and anti-inflammatory pathways, augmenting overall immune agility and responsiveness at gut but also skin and animal level without risk of immune fatigue.
Recently, the product was shown to speed-up skin wound-healing in a model of fish species (Edirisinghe et al., 2023). It was shown in particular to promote the resolution of inflammation, facilitating the subsequent synthesis of novel tissue at the wound base (Fig 3), and wound-closure.
Figure 3: (A) Transversal view of the skin wound showing delimitation of the newly synthesized granulation tissue and, (B) Surface area of the newly synthesized tissue at the wound base.
Conclusion
Preserving fish health and welfare though nutrition effectively contributes to preserving biological performance, production cost and volume. Specific yeast cell wall products have shown effective to enhance fish robustness and protect their health, especially during challenging situations.
Published in the April 2024 issue of International Aquafeed.
