ReviewThe current status and future focus of probiotic and prebiotic applications for salmonids
Introduction
Although there is now global recognition that aquaculture production is expanding to a wide diversity of cultured finfish, salmonids remain an important contributor to fish production in many countries. Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss W.) are reared principally in Norway, Scotland and Chile with some output arising from Canada, the USA, regions of Europe and to a lesser extent Australia and New Zealand. Total global production of salmonids was reported to exceed 2.2 million mt in 2007 (FAO, 2009). These species attract high market prices and are a stable source of high quality fish due to consumer acceptance for quality seafood. Salmon in particular are noteworthy for their characteristic pink-reddish flesh pigmentation (Davies, 2008) and versatility with respect to processing into a wide range of food products. Rainbow trout is also a popular salmonid fish species and has received great attention in terms of importance to aquaculture and remain a core of inland fish production in many countries throughout the world. It should also be recognised that native species of brown trout (Salmo trutta) are important especially in terms of their contribution to recreational fisheries and angling. Additionally, developments in rearing techniques for Arctic charr (Salvelinus alpinus) have increased interest in commercial farming and consequently global production has risen to over 2000 mt in 2007 (Jobling et al., 1993, FAO, 2009). Collectively, there has been an abundance of scientific literature underpinning the genetics, nutrition, health and disease issues concerning the development of the salmonid aquaculture industry.
Given the importance of nutrition in maintaining the health of fish, with respect to nutritional involvement on immuno-competence and disease resistance, as well as its role in stress mediation, there is a growing trend towards exploring dietary components of a non-nutritional nature to provide various functional attributes. This has been compounded by the constraints of employing antibiotics in the aquaculture industry, as reflected by the EU moratorium on the banning of antibiotic growth promoters in animal feeds, including fish (Regulation, EC No, 1831).
There have been numerous investigations on salmon and trout to evaluate the feasibility of supplementing diets with a range of potentially probiotic bacteria. Several general reviews have been published over the past decade summarising the latest available literature (Ringø and Gatesoupe, 1998, Gatesoupe, 1999, Ringø and Birkbeck, 1999, Vershuere et al., 2000, Irianto and Austin, 2002a, Ringø, 2004, Burr et al., 2005, Balcázar et al., 2006a, Gram and Ringø, 2005, Ringø et al., 2005, Gatesoupe, 2007, Kesarcodi-Watson et al., 2008, Wang et al., 2008a). Furthermore, specific reviews have focused on larvae (Gomez-Gil et al., 2000, Vine et al., 2006, Tinh et al., 2008), shrimp (Farzanfar, 2006, Ninawe and Selvin, 2009), shellfish (Balcázar et al., 2006b) along with reviews specific to applications for Indian (Panigrahi and Azad, 2007) and Chinese aquaculture (Qi et al., 2009). However, to the authors' knowledge, no reviews have been put forward to summarise the effects of probiotics on salmonids. Additionally, with the growing interest and assessment of prebiotic applications for fish, it is pertinent to review the present findings with regards to salmonid fish.
The aim of this review is to evaluate the literature currently available regarding the use of probiotics and prebiotics (collectively referred to as “biotics” hereafter) on salmonids. Specific emphasis is placed on highlighting application strategies on a practical basis and potential future research. In order to discuss these issues we must first examine the complex microbe–host interactions within the gut, which ultimately influence the health and development of the host.
Section snippets
Endogenous microbiota, mucosal tolerance and development
The immune system of teleost fish appears to be an efficient means by which the host protects itself upon pathogenic challenge. But not all microbes represent a pathogenic threat; resident commensal microbes help maintain efficient functioning of the gut by supporting gut mucosal barrier function: mounting efficient immune responses to pathogens that break through barrier defences or maintaining tolerance (i.e. immune non-responsiveness) to luminal contents which allow for nutrient absorption.
Probiotics
The word probiotic is constructed from the Latin word pro (for) and the Greek word bios (life) (Zivkovic, 1999). The definition of a probiotic differs greatly depending on the source, but the first generally accepted definition was proposed by Fuller (1989) as “…a live microbial feed supplement which beneficially affects the host animal by improving its microbial balance”. Given the nature of fish farming and the fact that water harbours microbial communities it is accepted that we must have a
Prebiotics
The use of probiotics in many cases, as discussed previously, may be difficult in commercial aquaculture because of the low viability of the bacteria after pelleting and during storage, leaching from the feed particle in rearing water, as well as problems related with feed handling and preparation. As an alternative (or also considered for use in tandem: synbiotics), prebiotics have been assessed in an attempt to overcome issues associated with probiotic applications. From an endothermic point
Synbiotics
Synbiotics, the combined application of probiotics and prebiotics, is based on the principle of providing a probiont with a competitive advantage (a fermentable energy source) over competing endogenous populations; Thus, effectively improving the survival and implantation of the live microbial dietary supplement in the gastrointestinal tract of the host (Gibson and Roberfroid, 1995). To the authors knowledge only one synbiotic study has been conducted in salmonids (Rodriguez-Estrada et al., 2009
Concluding remarks and future perspectives
Current research provides a foundation but applications within these studies are often impractical at industrial level farming that it is difficult to plan feeding strategies for commercial level applications. Future efforts must focus on implementing more practical applications as well as scientific studies designed to understand the mechanisms that underpin and mediate the observed host benefits. In this context, growth performance parameters and body composition analysis should be
Acknowledgements
The authors, as well as their respective institutions, would like to dedicate this article to their dear departed colleague and friend, Bruno Rochet (who died on 5 November 2009 at the age of 55 years). Bruno was the business development director for Lallemand Animal Nutrition, having joined Lallemand in 1998, after numerous years of working on the use of probiotics in Animal Nutrition. He was the founder and first president of the European Probiotic Association established in 1999. As one of
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