14-12-2012, 06:19 PM
PROBIOTIC
Aquaculture.doc (Size: 40.5 KB / Downloads: 26)
INTRODUCTION:
Aquaculture is a very old industrial sector that is rooted about 2000 years ago in china (1) has become one of the important, fast-developing growth sectors in the world. Asia contributes about 90% of the total seafood production whereas India with 6%. The intensification of aquaculture and globalization of the seafood trade have led to remarkable developments in the aquaculture industry. However, the economics of most modern aquaculture operations require that hydrobionts be cultured at high densities and a consequence is the increased probability of exposure of the hydrobionts to elevated stressful conditions, problems related to diseases and deterioration of environmental conditions often occur and result in serious economic losses. Disease outbreaks are being increasingly recognized as a significant constraint on aquaculture production and trade, affecting the economic development in many countries. Prevention and control of diseases have led during recent decades to a substantial increase in the use of veterinary medicines. In the United States, 18,000 t of antibiotics produced each year for medical and agricultural purposes, 12,600 t are used for the non-therapeutic treatments of livestock in order to promote growth (2). These amounts of antibiotics have exerted a very strong selection pressure towards resistance among bacteria, which have adapted to this situation, mainly by a horizontal and promiscuous flow of resistance genes (2). So far, conventional approaches, such as the use of disinfectants and antimicrobial drugs, are limited success in the prevention or cure of aquatic disease (4). This is mainly due to evolution of antimicrobial resistance among pathogenic bacteria (3).
Resistance mechanisms can arise in two ways: Chromosomal mutation or Acquisition of plasmids. Chromosomal mutations cannot be transferred to other bacteria but plasmids can transfer resistance rapidly (5). Several bacterial pathogens can develop plasmid-mediated resistance. Plasmids carrying genes for resistance to antibiotics have been found in marine Vibrio species and they could be laterally exchanged. Such resistance can be readily transferred to other strains, either following alterations to the existing genome or by transfer of genetic material between cells through plasmids or bacteriophages (6). Hence, abuse of antimicrobials can result in the development of resistant strains in bacteria (7). This can affect both economic development of the country and socio-economic status of the local people in many countries of the particular affected region.
Therefore, there is an increasing interest within the industry in the control or elimination of antimicrobial use. One such method that is gaining acceptance within the industry is the use of probiotic bacteria to control potential pathogens. The use of probiotics or beneficial bacteria, which control pathogens through a variety of mechanisms, is increasingly viewed as an alternative to antibiotic treatment. The use of probiotics in human and animal nutrition is well documented and recently, they have begun to be applied in aquaculture (17).
DEFINITION OF A PROBIOTIC/PROBIONT:
The word “probiotic” is derived from the Latin words Pro and Bios where Pro: Favor and Bios: Life. Probiotics are useful microbes to compete with the harmful ones, thus suppressing their growth. These include certain bacteria and yeasts that are not harmful on continued use for a long time and Parker named it as “Probiotics” in 1920 when the production of bacterial feed supplements began on a commercial scale (8). The term probiotics is generally used to denote bacteria that promote the health of other organisms Lilley and Stillwell (1965) described them as substances secreted by one microorganism, which stimulated the growth of another (14). An expert with the Joint Food and Agriculture Organization of the United Nations/World Health Organization (15), stated that probiotics are live microorganisms, which when consumed in adequate amounts, confer a health
benefit for the host (15).A widely accepted definition is taken from Fuller (9), who considered that a probiotic is a cultured product or live microbial feed supplement, which beneficially affects the host by improving its intestinal (microbial) balance. Furthermore, Salminen et al [10] considered a probiotic as any microbial (but not necessarily living) preparation or the components of microbial cells with a beneficial effect on the health of the host. Here, the need for live cells in association with feed has been ignored. Moriarty [11] suggested that the definition of a probiotic in aquaculture should include the addition of live naturally occurring bacteria to tanks and ponds in which animals live, i.e. the concept of biological control as discussed by Maeda et al [12]. As a compromise, it would appear that a probiotic is an entire or component(s) of a microorganism that is beneficial to the health of the host. This all-embracing concept could impinge on other areas of disease control, particularly vaccinology. Many researchers have already investigated the relationship of the intestinal microbiota to the aquatic habitat or food. Cahill (13) summarized the results of these investigations on fishes, giving evidence that the bacteria present in the aquatic environment influence the composition of the gut microbiota and vice versa. The genera present in the intestinal tract generally seem to be those from the environment or the diet that can survive and multiply in the intestinal tract (13).
Generally, probiotic strains have been isolated from indigenous and exogenous microbiota of aquatic animals. Gram-negative facultative anaerobic bacteria such as Vibrio and Pseudomonas constitute the predominant indigenous microbiota of a variety of species of marine fish (16).