J. A. Olafsen The intensive rearing of marine organisms has revealed their intimate relationship with bacteria which may result in disease epizootics - but also allow the study of new models of host-microbe interactions that may serve to better understand microbial pathogenicity and host colonization in general. Various forms of such interactions between bacteria and biological surfaces occur at egg, larval and adult stages, and may result in the formation of an indigenous microflora - or be the first step in an infective process. Better understanding of host-microbe interactions and natural defenses is imperative for the successful mass production of marine larvae. In the aquatic environment hosts and micro-organisms share ecosystems, and bacteria may travel in a continuous medium between habitats and hosts. In aquaculture fish eggs are kept in incubators with a microflora that differ in numbers and characteristics from that in the sea, and become heavily over-grown with bacteria within hours after fertilization. Members of the adherent microflora of eggs may cause serious damage, whereas we do not know whether the natural egg epiflora may bestow some protection. The factors that may protect eggs from bacterial invasion or infection are not yet understood. Marine fish larvae ingest bacteria and are thus primed with antigens before active feeding commences. At present we know very little about the establishment, composition and role of the normal microflora of fish. Uptake and sequestering of intact bacterial antigens by newly hatched larvae may result in immune tolerance, but as yet we have no information about acquired tolerance of fish to the microflora. A number of marine bacteria adhere to fish mucus. Fish pathogenic vibrios (Vibrio anguillarum and Vibrio salmonicida) may colonize the gastrointestinal brush border of marine fish larvae and result in extensive damage to the microvilli, but little is known about adhesion, receptors or colonization factors. Moreover, for marine bacteria there is a conspicuous lack of information about important invasion strategies such as antigen shift or phase variation. Marine invertebrates may harbor bacteria that are pathogenic to other organisms, and thus may serve as vectors for fish-pathogenic bacteria. A better understanding of this system is essential to understand epidemiology in the aquatic environment. Successful aquaculture will depend on extensive knowledge of the complex interactions between the cultured organisms and the bacterial communities which develop on mucosal surfaces and in the rearing systems. The use of probiotics has proven advantageous in domestic animal production, and such modulations of the microflora may also have a potential in aquaculture.
(University of Troms, Norwegian College of Fishery Science, N-9037 Troms, Norway)