Probiotics in aquaculture operations

Bioremediation to relieve challenging pond conditions in shrimp and fish farming is a promising technology.

These naturally occurring live microorganisms improve the growth and survival of fish and shrimp. Fish pathogens such as Vibrio spp. and Aeromonas spp. cause diseases, frequently affecting growth and mortalities. Live bacteria are used not only in the intestine of the animals, but also to improve the water quality of their environment.

Probiotics are “live microbial feed supplements which beneficially affect the host animal by improving the intestinal microbial balance” (Fuller, 1989).

Although probiotics have been a topic of much interest and research in the past 30 years, the extensive application of probiotics in aquaculture is relatively recent and widely becoming recognized as important for disease control (Irianto and Austin, 2002). However, the aquatic probiotics differ from the use of terrestrial based probiotics.

In the aquatic environment, hosts and microorganisms share the same ecosystem. Potential pathogens are able to maintain themselves in the external environment of the animal (water) and multiply independently of the host animal. Surrounding bacteria are constantly ingested by the animal through osmoregulation and feeding. (Verschuere et al., 2000)

Therefore the definition of an aquatic probiotic differs from that by Fuller (1989) as there is no longer the precondition for the probiotic to be acting in the gastro-intestinal tract of the host. Modes of action can also occur in the culture water and an aquatic probiotic can have additional effects, including change of the water quality and interaction with phytoplankton. (Verschuere et al., 2000)

A diverse range of beneficial bacteria is used as probiotics in aquaculture, in some cases as alternative to the use of antimicrobial compounds (Table 1).

Table 1: Probiotic use in aquaculture species

Lactic acid bacteria (LAB) are potential probiotic candidates in aquaculture and are also known to be present in the intestine of healthy fish (Balcázar et al., 2008). The most researched and used LAB are the lactobacilli and bifidobacteria (Ross et al., 2005; Senok et al., 2005).

Other commonly studied probiotics or bio-remediators include Saccharomyces, Pediococcus, Nitrosomonas, Nitrobacter, Paracoccus, Thiobacillus and the spore forming Bacillus spp.

Probiotics can be provided to the host or added to its aquatic environment in several ways: addition via live food, bathing, addition to culture water or addition to any commercial diet.

Mechanisms of action

Various ways exist in which probiotics could be beneficial. They can act either singly or in combination (Kesarcodi-Watson et al., 2008). Several studies have demonstrated certain mechanisms of action:

• Competitive exclusion of pathogenic bacteria: Adhesion and colonization of the mucosal surfaces are possible protective mechanisms against pathogens through competition for binding sites and nutrients (Westerdahl et al., 1991). Different lactobacilli have reduced the adhesion of A. salmonicida, C. piscicola and Yersinia ruckeri to intestinal mucus from rainbow trout (Balcázar et al., 2006).
• Production of inhibitory compounds: Recent studies by BIOMIN (2009) have demonstrated that various probiotic strains exhibited antibacterial activities against several common fish pathogens, including Enterococcus durans, Escherichia coli, Micrococcus luteus and Pseudomonas aeroginosa.
• Enhancement of the immune response against pathogenic microorganisms: The non-specific immune system can be stimulated by probiotics. Immunostimulants vary according to their mode of action and the way they are used. Certain derivates, such as polysaccharides, lipoproteins, nucleotides and ß-glucans, have the capability to increase phagocytic abilities by activating macrophages. Rengpipat et al. (2000) indicated that the use of Bacillus sp. provided disease protection by activating both cellular and humoral immune defenses in tiger shrimp (Penaeus monodon).

Benefits

Some possible benefits for fish and shrimp linked to the administering of probiotics have already been suggested:

B. subtilis and B. licheniformis fed fish displayed a significant improvement of feed conversion ratio (FCR), specific growth rate (SGR) and protein efficiency ratio (PER) (Merrifield et al., 2009). It has also been shown that survival rates of European eels (Anguilla Anguilla L.) fed with Enterococcus faecium were significantly higher than in the control groups after challenged with Edwardsiella tarda (Chang and Liu, 2002). Furthermore, Enterococcus faecium was also found along the shrimp digestive system when fed diets containing this probiotic strain (Supamattaya et al., 2006). It was concluded that this probiotic strain has a positive impact on shrimp gut bacterial ecology and can reduce the number of Vibrio spp. through competitive exclusion (Supamattaya et al., 2005).

A trial with Vibrio parahaemolyticus infected shrimp (P. vannamei) resulted in excellent shrimp performance with a 30 % increase in survival and better growth and FCR after application of a multi-strain probiotic product (AquaStar^®, BIOMIN GmbH, Austria) to improve water and gut health (Figure 1).

Figure 1: Performance of V. parahaemolyticus infected shrimp (survival, growth rate, FCR and productivity) under intensive culture conditions (300 shrimp/m²) when combining probiotics in feed (AquaStar^® Growout) and water (AquaStar^® Pond), p < 0.05. Adapted from Krummenauer et al., 2009.

Probiotics for the pond

A common procedure to improve water quality – and therefore the immediate environment of fish and shrimp - is the application of probiotics directly to the ponds. This type of biotechnology is equal to “bioremediation”, which involves manipulation of microorganisms in ponds to reduce pathogenic bacteria, enhanced mineralization of organic matter and removal of undesirable waste compounds.

The presence of high levels of ammonia or nitrites not only pollutes the water but also blocks the appetite of the fish well before causing fish mortalities (Guillaume et al., 1999). Removal is important for both reasons and can be carried out by addition of specialized nitrifying bacteria such as Nitrobacter and Nitrosomonas and denitrifying bacteria such as Thiobacillus and Paracoccus. (Beneficial pond) microorganisms present in the immediate environment of aquatic species also have a large impact on farmed fish welfare as well as on growth and health status. This is due to the fact that animals in an aquatic environment carry a bacterial flora, which is a reflection of the flora of their environment (Chandrasekaran, 1985). Recent research shows that the use of commercial probiotics in P. vannamei ponds can reduce concentrations of nitrogen and phosphorus and increase the shrimp yields (Wang et al., 2005).

Conclusion

The pond environment is a complex system of inter-linking processes. Maintaining the balance of critical parameters is a fundamental requirement for successful aquaculture. There is already experimental evidence that the prophylactic use of beneficial bacteria can improve health and performance of cultured aquatic species. The use of probiotics to control pathogens by Competitive Exclusion is gaining acceptance in aquaculture and considered as alternative to antibiotics. Probiotics play important roles as biological control agents in pond culture, particularly with respect to performance of fish and shrimp, disease control and water quality of the pond.

The application of probiotics for the improvement of aquatic environmental quality and for disease control in aquaculture seems promising. However the information is limited and sometimes contrasting. Due to these uncertain and incomplete results, there is no standardized protocol to test the beneficial effects of these products and their impact on farmed fish welfare, growth and health status (Ringo and Olsen, 2008).