The use of probiotics in aquaculture

Abstract

The research of probiotics for aquatic animals is increasing with the demand for environment-friendly aquaculture. The probiotics were defined as live microbial feed supplements that improve health of man and terrestrial livestock. The gastrointestinal microbiota of fish and shellfish are peculiarly dependent on the external environment, due to the water flow passing through the digestive tract. Most bacterial cells are transient in the gut, with continuous intrusion of microbes coming from water and food. Some commercial products are referred to as probiotics, though they were designed to treat the rearing medium, not to supplement the diet. This extension of the probiotic concept is pertinent when the administered microbes survive in the gastrointestinal tract. Otherwise, more general terms are suggested, like biocontrol when the treatment is antagonistic to pathogens, or bioremediation when water quality is improved. However, the first probiotics tested in fish were commercial preparations devised for land animals. Though some effects were observed with such preparations, the survival of these bacteria was uncertain in aquatic environment. Most attempts to propose probiotics have been undertaken by isolating and selecting strains from aquatic environment. These microbes were Vibrionaceae, pseudomonads, lactic acid bacteria, Bacillus spp. and yeasts. Three main characteristics have been searched in microbes as candidates to improve the health of their host. (1) The antagonism to pathogens was shown in vitro in most cases. (2) The colonization potential of some candidate probionts was also studied. (3) Challenge tests confirmed that some strains could increase the resistance to disease of their host. Many other beneficial effects may be expected from probiotics, e.g., competition with pathogens for nutrients or for adhesion sites, and stimulation of the immune system. The most promising prospects are sketched out, but considerable efforts of research will be necessary to develop the applications to aquaculture.

Introduction

Long before their discovery, microbes have been unawarely used to preserve food, and these empirical methods contributed to improve human health (Bengmark, 1998, Metchnikoff, 1908). Early in the century, Metchnikoff (1907), Metchnikoff (1908)(cited by Tannock, 1997) proposed to implant lactic acid bacteria into the human intestine, with a view to suppressing the detrimental activity of other microbes. The modern concept of probiotics was formulated only 25 years ago (Parker, 1974), then its pertinence was challenged for many years among the scientific community. Several definitions of probiotics were successively proposed. Parker (1974)originally referred to “organisms and substances which contribute to intestinal microbial balance” as probiotics. The definition was then restricted to “a live microbial feed supplement which beneficially affects the host animal by improving its intestinal microbial balance” (Fuller, 1989). Tannock (1997)noted that the effect on the “intestinal microbial balance” has not been demonstrated in most cases, and he proposed to speak of “living microbial cells administered as dietary supplements with the aim of improving health”.

To my knowledge, the first application of probiotics in aquaculture seems relatively recent (Kozasa, 1986), but the interest in such environment-friendly treatments is increasing rapidly. Scientific evaluation corroborated seldom the first empirical trials, and the information was mainly spread by “grey literature”. However, a growing number of scientific papers have dealt explicitly with probiotics, and it is now possible to survey the state of the art, from the empirical use to the scientific approach. A previous review was devoted to lactic acid bacteria in finfish (Ringø and Gatesoupe, 1998), but many other microbes have been tested as probiotics for various aquatic animals, appealing for a general overview.

It is essential to remind some definitions of ecological concepts to clear away the treatments termed improperly “probiotic”. The goals of this paper are (1) to examine the pertinence of such terminology applied to the aquatic environment, (2) to draw the different trends of applications, and (3) to point out needs for further research.