During the BIOMIN World Nutrition Forum 2016 in Vancouver, Canada, last week, Dr. Merrifield spoke about the role of probiotics and how they have the capacity to enhance gut performance in fish.
[Feedinfo News Service] Dr. Merrifield, our knowledge of fish microbiomes remains primitive in comparison to that of humans and other mammals. How advanced is our understanding today?
[Daniel Merrifield] I would say we probably need another 5-10 years of further research to get to the current level of understanding we have in humans and other mammals. There are a number of complicating factors that we have to consider when studying the microbiomes of fish. Fish are poikilothermic animals and thus their metabolism, and the metabolism of their microbiota, is heavily dependent on environmental conditions. The microbiomes of fish therefore can vary quite substantially depending on biogeography and seasonality. Further, many important fish species also have complex life histories. For example, the Atlantic salmon is an anadromous species; in salmon farming, the production cycle encompasses all life stages. During the first year, the eggs are fertilised and the fish are grown in controlled freshwater systems. Thereafter, they are transported to seawater cages (at ca. 100 g) and grown to harvest size over a period of 14-24 months. Salmon are therefore exposed to varied environmental conditions during their lifetime, and this heavily influences the composition and activities of their microbiomes at different life stages. These are just a few examples of the scientific challenges we face when trying to better understand the microbiomes of fishes. From a more practical perspective, which can impede our ability to “catch up” with our mammalian counterparts, we have: 1] less research funding available for fish and aquaculture, 2] a smaller pool of active researchers working with fish species, and 3] a greater diversity of relevant species to investigate (there are >100 important aquaculture fish species). It’s safe to say that research in this field will keep people like me busy for the next few decades.
[Feedinfo News Service] Why is current scientific literature somewhat mixed when it comes to probiotic feeding efficiency in fish species? Is the behavior of probiotics in the fish gut really that variable?
[Daniel Merrifield] The variability in fish microbiomes as a result of biogeography, life stage, seasonality and different diets goes a long way to explaining this. The probiotic concept is based on improving the gut microbiome by transplanting a population of beneficial microbes. Inter- and intra- species resident host gut microbiota variations therefore present a different baseline microbial community for the probiotic to modulate. There is also a host selective pressure which helps to shape the composition of the gut microbiome, and this selection pressure will also act upon the probiotic. When you combine these factors, along with the varying conditions in the gut (e.g. temperature, nutrient composition, salinity, pH, etc.), it becomes quite obvious that we can expect some variability in probiotic efficacy in different species of fish, and indeed, in individuals of the same species reared under different conditions.
[Feedinfo News Service] Healthy feeding is important for fish and shellfish hatcheries, especially at early stages for larvae or fry. But what are the challenges and benefits of feeding probiotics at early life stages?
[Daniel Merrifield] Firstly, we do not yet know enough about the microbiomes of larval species. Early indications suggest that there is a process of microbiome maturation across the early days and weeks after hatching. Gaining a better understanding of the microbiome at this life stage will help to improve the efficacy of probiotic intervention in live feeds and starter feeds. The provision of probiotics in live feeds has resulted in a number of reported benefits – including the exclusion of pathogens from the larvae and/or rearing water, improved stress tolerance, increased survivability and elevated growth performance. What is perhaps even more interesting is the revelation from several studies that feeding probiotics to broodstock can improve fecundity, and that eggs derived from probiotic fed broodstock can mature more rapidly and have higher survivability.
[Feedinfo News Service] What should the focus be in terms of developing a better understanding of the microbiomes of fish?
[Daniel Merrifield] Future research efforts must focus on four main themes. Firstly, gaining a better understanding of the normal microbiomes of fish, taking into account variables such as biogeography, life history, host genotypes etc. Secondly, a better understanding of the functional attributes of the microbiome is required. It is not enough to ascertain which microbes are present, we also need to know what they are doing. Thirdly, we need to dedicate more effort to researching the non-gut microbiome components, such as the microbiota on the skin and gills. Lastly, we need to gain a better understanding of the localized host responses to the microbiome, and how such responses may change when the microbiome is modulated or manipulated.
[Feedinfo News Service] What further opportunities have you identified in terms of probiotic usage in aquafeeds?
[Daniel Merrifield] There are many further opportunities to be exploited – expanding the usage of probiotics to emerging species, understanding the benefits of using probiotics in modern diets that contain lower levels of marine ingredients, and improved technologies for easier inclusion of viable probiotics into aquafeeds, to name but a few. In addition, it is well documented that diet can modulate fish skin mucus quantity and quality - I wonder then, to what extent dietary provision of probiotics may be able modulate epidermal mucus properties? Fortunately, we shall soon be able to provide some answers to this question as we progress through the Caligus-Salmon-microbiome project. The Caligus-Salmon-microbiome project is a multinational project funded by the BBSRC (UK), Newton Fund (UK) and Conicyt (Chile) which includes investigations of epidermal transcriptomes and microbiomes, mucus proteomes, and sea lice resistance of probiotic fed Atlantic salmon.
This interview was first published on www.feedinfo.com.