Probiotics, the natural, beneficial bacteria are now well accepted and widely used in shrimp aquaculture. Potentially, they may have one or more beneficial functions for aquaculture producers:
- Water and pond bottom sediment quality is improved, leading to less stress on shrimp and thus improved health.
- Effluent water is cleaner, thus environmental impact is low.
- Pathogenic bacteria and their virulence can be controlled, and the overall microbial ecosystem can be managed.
- Antibiotics are not used. This stops the increase in virulence and pathogenicity in aquatic bacterial pathogens due to antibiotics. It will also minimise the risk of multiple antibiotic resistance.
- Stimulation of the shrimp immune system.
- Improved gut flora and hence lower disease incidence and increased food assimilation
Hence, productivity and profits are increased when selected probiotics are used.
Source: D. J. W. Moriarty, O. Decamp and P. Lavens
The definition of probionts
Elie Metchnikoff’s work at the beginning of this century is regarded as the first research conducted on probiotics (Fuller, 1992). He described them as “microbes ingested with the aim of promoting good health”. This same definition was modified to “organisms and substances which contribute to intestinal microbial balance” (Parker, 1974), and later by Fuller (1989) to “a live microbial feed supplement which beneficially affects the host animal by improving its intestinal microbial balance”. These definitions originally applied to farm animals (ruminants, poultry and pigs) or humans, since the first studies were carried out in these species.
Probiotics are now also being used in aquaculture and therefore, the definition may have to be modified. In aquatic animals, not only the digestive tract is important but also the surrounding water. Gatesoupe (1999) defines probiotics as “microbial cells that are administered in such a way as to enter the gastrointestinal tract and to be kept alive, with the aim of improving health”. Gram et al. (1999) broadened the definition by removing the restriction to the improvement to the intestine: “a live microbial supplement which beneficially affects the host animal by improving its microbial balance”.
Biological control has been described as the utilisation of natural enemies to reduce the damage caused by noxious organisms to tolerable levels (Debach and Rosen, 1991) or more precisely, the control or regulation of pest populations by natural enemies (Smith, 1919). Strictly speaking, a probiotic ought not to be classified as a biological control agent, since a probiotic microorganism does not necessarily attack the noxious agent (pathogen). It need not necessarily be a natural enemy of the pathogen, but it merely prevents damage to the host caused by the pathogen, usually through competition but at most, it may produce substances that inhibit the growth or attachment of the harmful microorganism. Neither should probiotics be classified as growth promoters since their action is not confined to improved growth but is associated with a general improvement in health.
The microbial community inside the gut of some animals confers some degree of resistance to or protection against disease (Fox, 1988). In natural populations of aquatic animals, the microflora of the gut might reflect that of the aquatic environment. However, in massive artificial larval cultures, the balance can be altered by the use of disinfected water, microalgae, Artemia nauplii, rotifers, and antibacterials. As a result, a protective microbial community may not develop either in the environment or the digestive system of the larvae. The postlarvae reared in a relatively sterile environment of a hatchery do not grow well and show poor survival when they are exposed to the complex microbial populations of the nursery or grow-out ponds. They are particularly susceptible to diseases when exposed to environmental stress and potentially pathogenic bacteria.
Source: The use and selection of probiotic bacteria for use in the culture of larval aquatic organisms
Aquaculture 191 2000 259–270 www.elsevier.nlrlocateraqua-online