Use of Probiotics in Aquaculture
The growth of aquaculture as an industry has accelerated over the past decades; this has resulted in environmental damages and low productivity of various crops. The need for increased disease resistance, growth of aquatic organisms, and feed efficiency has brought about the use of probiotics in aquaculture practices. The first application of probiotics occurred in 1986, to test their ability to increase growth of hydrobionts (organisms that live in water). Later, probiotics were used to improve water quality and control of bacterial infections. Nowadays, there is documented evidence that probiotics can improve the digestibility of nutrients, increase tolerance to stress, and encourage reproduction. Currently, there are commercial probiotic products prepared from various bacterial species such as Bacillus sp., Lactobacillus sp., Enterococcus sp., Carnobacterium sp., and the yeast Saccharomyces cerevisiae among others, and their use is regulated by careful management recommendations. The present paper shows the current knowledge of the use of probiotics in aquaculture, its antecedents, and safety measures to be carried out and discusses the prospects for study in this field.
Aquaculture is the farming of aquatic organisms by intervention in the rearing process to enhance production and private ownership of the stock being cultivated. Compared to fishing, this activity allows a selective increase in the production of species used for human consumption, industry or sport fishing. Due to overfishing of wild populations, aquaculture has become an economic activity of great importance around the world. Aquaculture’s contribution to world food production, raw materials for industrial and pharmaceutical use, and aquatic organisms for stocking or ornamental trade has increased dramatically in recent decades. The report World Aquaculture 2012 found that global production of fish from aquaculture grew more than 30 percent between 2006 and 2011, from 47.3 million tons to 63.6 million tons. It also forecasts that by 2012 more than 50 percent of the world’s food fish consumption will come from aquaculture, so it is expected to overtaking capture fisheries as a source of edible fish. This growth rate is due to several factors: (1) many fisheries have reached their maximum sustainable exploitation, (2) consumer concerns about security and safety of their food, (3) the market demand for high-quality, healthy, low-calorie, and high-protein aquatic products, and (4) aquatic breeding makes only a minimum contribution to carbon dioxide emission [1, 2].
Aquaculture has a long history, originating at least in the year 475 B.C. in China , but became important in the late nineteen-forties, since the methods of aquaculture could be used to restock the waters as a complement to natural spawning. Nowadays, aquaculture is a lucrative industry [2, 4]. However, the intensification of aquaculture practices requires cultivation at high densities, which has caused significant damage to the environment due to discharges of concentrated organic wastes, that deplete dissolved oxygen in ponds, giving rise to toxic metabolites (such as hydrogen sulfide, methane, ammonia, and nitrites), that often are responsible for mortality. Additionally, aquaculture has appropriated of water bodies used for recreational purpose, and sometimes makes a water’s waste because this natural resource is not reused in extensive aquaculture systems [5, 6]. Moreover, under these conditions of intensive production, aquatic species are subjected to high-stress conditions, increasing the incidence of diseases and causing a decrease in productivity .
Outbreaks of viral, bacterial, and fungal infections have caused devastating economic losses worldwide, that is, China reported disease-associated losses of $750 million in 1993, while India reported $210 million losses from 1995 to 1996. Added to this, significant stock mortality has been reported due to poor environmental conditions on farms, unbalanced nutrition, generation of toxins, and genetic factors . In recent decades, prevention and control of animal diseases have focused on the use of chemical additives and veterinary medicines, especially antibiotics, which generate significant risks to public health by promoting the selection, propagation, and persistence of bacterial-resistant strains [9–11].
Resistance to antibiotics is acquired in two ways: chromosomal mutation or acquisition of plasmids. Chromosomal mutations are not transferred laterally to other bacteria but resistance plasmids can be transferred very rapidly, producing a high percentage of pathogenic bacteria that develop mediated plasmid-resistance in a short period of time . This can be an important public health issue as well. For example, transfer of multidrug resistance occurred in Ecuador during the cholera epizootic happened between 1991 and 1994, which originated from shrimp farm workers. Although the original epidemic strain Vibrio cholerae 01 was susceptible to 12 antimicrobial agents tested, on the coast of Ecuador it acquired multidrug resistance due to transfer of resistance genes from other Vibrio species that were pathogenic to shrimp .
Probiotic is a relatively new term which is used to name microorganisms that are associated with the beneficial effects for the host. Kozasa made the first empirical application of probiotics in aquaculture , considering the benefits exerted by the use of probiotics on humans and poultry. He used spores of Bacillus toyoi as feed additive to increase the growth rate of yellow tail, Seriola quinqueradiata. In 1991, Porubcan [15, 16] documented the use of Bacillus spp, to test its ability to increase productivity of Penaeus monodon farming and to improve water quality by decreasing the concentrations of ammonia and nitrite. In order to avoid or reduce the use of certain antimicrobials, biological control was tested, described as the use of natural enemies to reduce the damage caused by harmful organisms. Strictly speaking, a probiotic should not be classified as a biological control agent because it is not necessarily a natural enemy of the pathogen . However, certain probiotics have the ability to inhibit the growth of pathogenic bacteria. Moriarty determined the ability of Bacillus spp. to decrease the proportion of Vibrio spp. in shrimp ponds, especially in sediments . Further studies have stressed probiotics ability to stimulate appetite, improve absorption of nutrients, and strengthen the host immune system [19, 20]. The aim of this paper is to present an updated concept of probiotics, current uses in aquaculture, commercial presentations, and a final approach to prospect applications in this area.
The term “probiotic” comes from Greek pro and bios meaning “prolife” , having different meanings over the years. In 1905, Dr. Elie Metchnikoff was the first to describe the positive role played by some bacteria among farmers who consumed pathogen-containing milk and that “reliance on gut microbes for food makes it possible to take steps to change the flora of our bodies and to replace harmful microbes by beneficial microbes” . However, the term probiotic was introduced until 1965 by Lilly and Stillwell  as a modification of the original word “probiotika.” It was used to describe substances produced by a microorganism that prolong the logarithmic growth phase in other species. It was described as an agent which has the opposite function of antibiotics. Later, Sperti  modified the concept of “tissue extracts that stimulate microbial growth.”
The first use of the term to describe a microbial feed/food supplement was by Parker in 1974 . He defined it as “organisms and substances that contribute to intestinal microbial balance.” Fuller  expanded the definition to “live microbial food supplement that benefits the host (human or animal) by improving the microbial balance of the body” and said that it would be effective in a range of extreme temperatures and salinity variations. Afterwards, it was suggested that probiotics were “monocultures or mixed cultures of microorganisms applied to animals or humans that benefit the host by improving properties of indigenous microflora” . In 1998, Guarner and Schaafsma assumed that probiotics are live microorganisms which, when consumed in adequate amounts, confer health benefits to the host . Gatesoupe in 1999, defined them as “microbial cells administered in a certain way, which reaches the gastrointestinal tract and remain alive with the aim of improving health” . In the same year, studies were carried out on the inhibition of pathogens using probiotics, this expanded the definition to “… live microbial supplement which benefits the host by improving its microbial balance” .
Knowledge of probiotics has increased, currently it is known that these microorganisms have an antimicrobial effect through modifying the intestinal microbiota, secreting antibacterial substances (bacteriocins and organic acids), competing with pathogens to prevent their adhesion to the intestine, competing for nutrients necessary for pathogen survival, and producing an antitoxin effect. Probiotics are also capable of modulating the immune system, regulating allergic response of the body, and reducing proliferation of cancer in mammals. Because of this, when provided at certain concentration and viability, probiotics favorably affect host health . In fact, terms such as “friendly bacteria,” “friendly,” or “healthy” are commonly used to describe probiotics .
For many years, studies focused on microorganisms characteristic from intestinal microbiota, and the term “probiotic” was mainly restricted to gram-positive lactic acid bacteria , particularly representative of the genera Bifidobacterium, Lactobacillus, and Streptococcus . In contrast to terrestrial animals, gastrointestinal microbiota of aquatic species is particularly dependent on the external environment due to the flow of water passing through the digestive tract. Thus, the majority of bacteria are transient in the intestine, due to constant intake of water and food, together with microorganisms present in them. Although in the gastrointestinal tract (GIT) of aquatic animals have been reported potentially pathogenic bacteria such as Salmonella, Listeria, and Escherichia coli, probiotic bacteria and other microorganisms have also been identified. These include gram-positive bacteria such as Bacillus, Carnobacterium, Enterococcus, and several species of Lactobacillus; gram-negative, facultative anaerobic such as Vibrio and Pseudomonas, as well as certain fungi, yeasts, and algae of the genera Debaryomyces, Saccharomyces, and Tetraselmis, respectively [20, 67, 68]. Due to the increasing interest of probiotics in aquaculture, Moriarty  proposed extending the definition of these to “living microbial additives that benefit the health of hydrobionts and therefore increase productivity.”
A more general and common concept of probiotic is “one or more microorganisms with beneficial effects for the host, able to persist in the digestive tract because of its tolerance to acid and bile salts” . Although the use of probiotics in aquaculture is relatively recent, interest in them has increased due to their potential in disease control ; however, gradually other applications have been proposed (summarized in Table 1), that will be addressed in detail later.
See more at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3671701/