Bacterial interactions in early life stages of marine cold water fish

The intensive rearing of various fish species in aquaculture has revealed i ntimate relationships between fish and bacteria that eventually may affect establishment of a "normal" mucosal microflora or result in disease epizoot ics. Interactions between bacteria and mucosal surfaces play important role s both at the egg and larval stages of marine fish. Bacterial adhesion and colonization of the egg surface occur within hours after fertilization. The diverse flora which eventually develops on the egg appears to reflect the bacterial composition and load of the ambient water, but species-specific a dhesion at the egg surface may also play a role in development of the egg e piflora. Proteolytic enzymes produced by members of the adherent epiflora m ay cause serious damage to the developing egg and may also affect further a dhesion of the epiflora. Ingestion of bacteria at the yolk sac stage results in establishment of a p rimary intestinal microflora which seems to persist beyond first feeding. E stablishment of a gut microflora is likely to undergo several stages, resul ting in an "adult" microflora weeks to months after first feeding. Ingested bacteria may serve as an exogenous supply of nutrients or essential factor s at an early life stage. Early exposure to high bacterial densities is pro bably important for immune tolerance, and thus for the establishment of a p rotective intestinal microflora. Successful rearing of early life stages of several marine fish species depe nds on knowledge of the complex interactions among the cultured organisms a nd the bacterial communities which develop at the mucosal surfaces and in t he ambient water and rearing systems. The routine use of antibiotics during rearing of fish larvae is not advisable, since it may increase the risk of promoting antibiotic resistance and adversely affect the indigenous microf lora of the larvae. The use of probiotics has proven advantageous in domest ic animal production, and the search for effective probiotics may have a gr eat potential in aquaculture of marine organisms. Bacteria with antagonisti c effects against fish pathogens have been successfully administered to sev eral fish species, resulting in decreased mortality or increased growth rat e.