Agriculture Business Week

Despite the doubts about probiotics – Due to unrealistic claims, poor quality products or mode of delivery – appropriate probiotics do actually work in aquaculture production.

Olivier Decamp and David Moriarty explain
Aquaculture is developing and intensifying in most regions of the world in response to the increasing demand for aquatic food products (FAO, 2006). This intensity has led to an increased use and misuse of drugs and chemicals in aquaculture, resulting in food safety concerns. Several alternative strategies to the prophylactic use of antibiotics in disease control have been proposed: installment of biosecurity management, effective vaccination, stimulation of the non-specific defense mechanisms of the host (alone or in combination with vaccines), as well as microorganisms (probiotics). Defining probiotics is a challenge – even more so for aquaculture applications. Historically, probiotics were defined according to their expected benefit or improvement to the host’s intestinal balance. Being concerned with humans and terrestrial animals, probiotics were generally Gram-positive obligate or facultative anaerobes, mostly lactic bacteria.

Fish are different
Aquatic animals differ from terrestrial animals in the level of interaction between the intestinal microbiota and the surrounding environment. The bacteria present in the aquatic environment influence the composition of the gut microflora and vice versa. This environmental influence is much greater for shrimp and other invertebrates than for fish. The bacterial community composition of the intestinal tract of aquatic animals is different from that found in terrestrial animals. Gram-negative facultative anaerobes generally prevail in the digestive tract of fish and shellfish; Gram-positive obligate or facultative anaerobes dominate that of humans and terrestrial animals (Gatesoupe, 1999). Aquatic animals are poikilothermic and their associated microbiota may vary with temperature changes; salinity changes in the rearing environment will also affect the microbiota. An important consequence is that the most efficient probiotics used for aquaculture will differ from those for terrestrial species.

Probiotics used in Aquaculture
Moriarty (1998) and Verschueren et al (2000) defined aquatic probiotics as live microorganisms that have a beneficial effect on the host by modifying the microbial community associated with the host, by ensuring improved use of the feed or enhancing its nutritional value, by enhancing the host response towards disease, or by improving the quality of its ambient environment. This implies a much wider range of microorganisms being used as probiotics for aquaculture animals than for terrestrial animals.

The development of suitable probiotics is not a simple task. It requires empirical and fundamental research, full-scale trials and the development of appropriate monitoring tools and production under stringent quality control. Probiotics currently used in aquaculture include a wide range of taxa, from lactic bacteria (Lactobacccillus, Lactococcus, Bifidobacterium, Pediococcus, Carnobacterium), to Bacillales (Bacillus, Paenibacillus, Brevibacillus), genera (Flavobacterium, Cytophaga, Pseudomonas, Alteromonas, Roseobacter, Aeromonas, Nitrosomonas, Nitrobacter, Vibrio ) and yeasts (Debaryomyces, Saccharomyces). The list is not exhaustive. The reasoning behind these microbial products varies:
• Availability of strains that had been originally selected for terrestrial animals or humans.
• Availability of very cheap microbial products. In this case, the performance of the product is less of an issue than the cost of such dream products.
• Availability of strains that had been used for waste water treatment. This is of interest for water quality control, biofilter start-up, etc.
• Research from universities and private companies leading to the selection of specific strains for aquaculture applications. Within this category, only strains that can be produced in large volumes and in a cost-efficient manner with the right quality control and safety assessments can reach the market.

Commercially available products include pure strains, defined mixtures of specific strains, but also consortia of tens of strains (as stated on label) and undefined mixtures. Products are supplied as liquids, frozen product or powder. Some of them require preparation (such as on-site fermentation, “brewing” for 1, 2 or even 3 days prior to application), whereas others are supplied at high concentration and do not require any step prior to delivery. Products that are supplied as “ready to use” powders have additional benefits, such as safety, expected consistency in performance and longer shelf life. The strains are supplied in a defined ratio (maintained until delivery) and the risk of contamination is eliminated with the absence of manipulations, such as on-site fermentation.

Benefits differ
The major benefits expected from these probiotics will differ with the species (freshwater, brackish or marine, fish or crustacean), the culture stage (larvae, juvenile, broodstock) and the rearing system (flow-through or recirculation; tanks, ponds or cages). The mode of delivery and the management of the facilities (appropriate biosecurity measures, water renewal, chemicals, etc.) will affect the performance, but also the survival or residence of the microorganisms in the rearing environment and/or the host.

Claims
Despite the doubts about probiotics – due to unrealistic claims, poor quality products or mode of delivery – appropriate probiotics do actually work in aquaculture production, as can be seen from the literature. Benefits have been reported in the literature; however, they are often restricted to academic studies. They provide very useful information on the mode of action of strains and on the microbial ecology of these man-made environments. Claims that have been documented include direct inhibition of shrimp (Vaseeharan and Ramasamy, 2003; Jayaprakash et al., 2005) and fish pathogens (Nikoskelainen et al., 2001; Decamp et al., 2006;), faster growth (Ziaei-Nejad et al., 2005), stimulation of the immune system of shrimp (Rengpipat et al., 2000; Gullian et al., 2004) and fish (Nikoskelainen et al., 2004; Brunt and Austin, 2005; Taoka et al., 2006), improved water quality and, more particularly, ammonia in fish (Taoka et al., 2006), shrimp (Rengpipat, 1999) or live food production (Rombaut et al., 2003). The ability of probiotic strains to affect the bacteria flora of live food, and hence the larvae microlfora as it gets established, has also been documented (Gatesoupe, 1991; Harzevili et al., 1998; Rombaut et al., 1999; Makridis et al., 2000). The performance of these products under commercial conditions is seldom reported. Reasons include strains from academic studies that cannot be produced in sufficient quantity to demonstrate the value at commercial scales repeatedly, or the difficulty in proving performance at the farm level. Only a few companies have taken the necessary steps to develop products specifically for aquaculture that can be exploited commercially at the same time. One of the companies that conducted extensive work in this field is INVE Aquaculture with its Sanolife line of products.