Probiotics An Alternative Treatment For Bacterial Coldwater Disease

Specifically, probiotics are live microorganisms, which when consumed in adequate amounts provide a health benefit for the host.

Probiotics have been used to promote the health of humans and terrestrial livestock for a considerable period of time. Probiotics are available in foods and dietary supplements. Yogurt, fermented and unfermented milk, miso, and soy beverages are examples of foods that contain probiotics.

The use of probiotics in aquaculture is relatively new. The first application of probiotics in aquaculture occurred in 1986. Since then numerous studies have been published on probiotics, especially within the past decade.

Probiotic strains have been shown to inhibit pathogenic bacteria both in vitro and in vivo. Possible mechanisms that explain probiotic modes of action include the production of inhibitory compounds, competitive exclusion of other bacteria, enhancement of the immune response, improved water quality, source of nutrients, and beneficial effect on digestive processes.

A recent study* sought to identify bacteria as suitable probiotic candidates with the potential to control or reduce disease caused by Flavobacterium psychrophilum, the causative agent of bacterial coldwater disease. This study was supported by the University of Idaho/ Washington State University Aquaculture Research Initiative and the McNair Achievement Programme and Graduate Assistantship.

A total of 318 bacterial isolates were collected from the gastrointestinal tracts of rainbow trout. To be considered a suitable probiotic candidate bacterial isolates must meet several criteria. First, the bacterial isolate must be re-grown from frozen stock. Once it is established that the bacterial isolate can be re-grown, it is screened in vitro for its inhibitory activity against F. psychrophilum.

Then it must be shown that the bacterial isolate can colonise and survive in the gastrointestinal tract of the host. Finally, the bacterial isolate is tested to determine whether or not it is pathogenic in the host. Out of 318 bacterial isolates, this study identified 16 candidate probiotics which inhibit the growth of F. psychrophilum, can survive the GI tract of rainbow trout, and are not directly pathogenic to the host.

From the 16 isolates, ten of the most promising candidate probiotics were independently mixed with feed and individually administered to rainbow trout pre and post infection with F. psychrophilum. Of the ten evaluated, two strains (designated as C6-6 and C6-8) have shown a significant decrease in mortality compared to positive controls. Both are strains of Enterobacter species.

Results have been replicated in the lab and field trials conducted by the Utah Division of Wildlife Resources, have consistently shown a 40 to 50 per cent reduction in mortality compared to control groups.

Such results are exciting and clearly demonstrate commercial potential of these bacteria for use as probiotics in aquaculture. The University of Idaho is planning on moving forward to patent both of these bacterial strains.

*Principal Investigator: Dr. Kenneth Cain, University of Idaho; Collaborators: Dr. Scot LaPatra, Clear Springs Foods, Inc., and Gary Fornshell, University of Idaho; Support Personnel: David Burbank, Nicole Lindstrom, Timberly Maddox, and Kurt Eversman, University of Idaho.