Biotechnology and Bioinformatics (1+1)

6.4.2.1. Rationale for the use of probiotics in aquaculture

In order to understand the function and potential contribution of probiotics towards health and well-being, in-depth knowledge of the digestive tract as an ecosystem is required. The studies conducted in recent years confirm the importance of microbials in digestive tract. Indeed, a beneficial association of ‘‘lactic acid producing’’ microorganisms, particularly lactic acid bacteria (LAB), with the human host has been suggested more than 100 years ago.

The digestive tract of hydrobionts is an open system constantly contacting with the surrounding environment-water. Compared to water, digestive tract is an ecosystem far richer in nutrients and therefore more favourable for the growth of the majority of bacteria. Gastrointestinal bacteria (GIT) take part in the decomposition of nutrients, provide the macroorganisms with physiologically active materials, such as enzymes , amino acids , and vitamins.

The study conducted on the aerobic bacteria associated with the GIT of freshwater fish species revealed that selected bacterial strain s produced digestive enzymes, thus facilitating feed utilization and digestion. The same beneficial characters of probiotics added inthediet have also been reported.

It has also been proved that micro flora of the digestive tract of partial hydrobionts play an important role in the resistance to infectious diseases since they produce antibacterial materials preventing pathogenic bacteria from getting into an organism.

The initial major purpose of using probiotics is to maintain or re-establish a favorable relationship between friendly and pathogenic microorganisms that constitute the flora of digestive tract or skin mucus of hydrobionts.

Some bacteria used as candidate probiotics also have antiviral effects. Although the exact mechanism by which these bacteria do this is not known. Laboratory tests indicate that the inactivation of viruses can occur by chemical and biological substances, such as extracts from marine algae and extracellular agents of bacteria.

In addition, adhesion and colonization of the mucosal surfaces are possible protective mechanisms against pathogens through competition for binding sites and nutrients, or immune modulation.

Multiple ways exist in which probiotics could be beneficial and these could act either singly or in combination for a single probiotic. These include :

• inhibition of a pathogen via production of antagonistic compounds,
• competition for attachment sites,
• competition for nutrients,
• alteration of enzymatic activity of pathogens,
• immunostimulatory functions, and
• nutritional benefits such as improving feed digestibility and feed utilization.

It is often reported that a probiotic must be

• adherent and colonize within the gastro intestinal tract,
• it must replicate to high numbers,
• it must produce antimicrobial substances, and
• it must withstand the acidic environment of the gastro intestinal tract.

The effects of ecological factors on microbial growth and activity are of gene ral importance and also apply to the aquatic animals digestive tract flora and feed.

Moreover, microbial food webs are an integral part of all aquaculture ponds and have a direct impact on productivity, even where intensive, artificial feeding is practiced. Therefore, the probiotics prepared with microorganisms have important roles in pond culture, particularly with respect to productivity, the nutrition of the cultured animals, disease control, water quality, and environmental impact of the effluent.