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Probiotics, which are regarded as micro-organisms administered orally leading to health benefits, are used extensively in aquaculture for disease control, notably against bacterial diseases. In contrast to use with terrestrial animals where lactic-acid producing bacteria dominate, a diverse range of micro-organisms including Gram-negative and Gram-positive bacteria have been considered in aquaculture. The source of these organisms is often the digestive tract of the host animal. The mode of action includes competitive exclusion and immunomodulation. Probiotics may also improve appetite and lead to enhanced growth and better feed conversion.

The effects of dietary probiotics on growth performance, health status of the digestive system and on the blood parameters of juvenile rainbow trout ( Oncorhynchus mykiss , Walbaum) were investigated. Compared to the controls, diets supplemented with Kocuria SM1 and Rhodococcus SM2 for 14 days caused no significant (P > 0.05) change on the growth of fish, although SM2 led to an increased specific growth rate. SM1 or SM2 started to colonize in the gut after 7 days of administration reaching a maximum, i.e. ~ 90–100% of total bacterial population, at 14 days. Yet, the probiotics disappeared from the gut once administration ceased. Structural differences were not noted in the pyloric caeca and foregut, but the overall condition of the midgut and hindgut improved in fish receiving probiotics. The profile of digestive enzymes, as determined by the use of API ZYM, and stress indicators, i.e. serum hemoglobin, urea, creatinine and glucose, was not affected by any of the experimental diets (P > 0.05).

A strain of Carnobacterium sp., isolated from the intestine of Atlantic salmon, was evaluated for potential use as a probiotic for salmonids. In vitro studies demonstrated antagonism against Aeromonas hydrophila, A. salmonicida, Flavobacterium psychrophilum, Photobacterium damselae subsp. piscicida, Streptococcus milleri, Vibrio anguillarum and V. ordalii but not towards Debaryomyces hansenii, Janthinobacterium lividum, V. alginolyticus, V. harveyi or Yersinia ruckeri. Feeding salmonids with diets containing the probiotic revealed that the isolate remained viable in the gastrointestinal tract. After reverting to feeding with control diets, the isolate was re-isolated from the intestine up to 4 and 10 days in fingerlings and fry, respectively. After feeding with the probiotic for 14 days, challenge by cohabitation indicated effectiveness at reducing disease caused by A. salmonicida, V. ordalii and Y. ruckeri but not V. anguillarum.

A commercially obtained spray-dried preparation of Tetraselmis suecica, as used for mollusc and prawn diets, was observed to rapidly inhibit prawn pathogenic strains of Vibrio spp., V. alginolyticus, V. anguillarum, V. parahaemolyticus and V. vulnificus.

From a comparison of 121 antimicrobial agents, the usefulness of substituted quinolines, in particular halquinol, has been indicated for the control of vibriosis in marine fish. From in vitro and in vivo experiments, it was deduced that halquinol inactivated rapidly the bacterial isolates, and controlled the disease manifestation in turbot ( Scophthalmus maximus L.).

From a study of the phenotypic and serological characters of representative strains of Yersinia ruckeri and other Enterobacteriaceae taxa, it was determined that the causal agent of enteric redmouth ( Y. ruckeri ) in salmonid fish was related to the type strain of Salmonella arizonae (NCTC 8397). This similarity suggested that the organisms were not sufficiently distinct to warrant their inclusion in two genera.

From a comparison of 103 compounds, the usefulness of substituted quinolines, in particular 5,7-dichloro-8-hydroxyquinoline, 5,7-dichloro-8-quinolyl-N-phenylcarbamate, Halquinol and oxolinic acid, have been indicated for the control of vibriosis in turbot (Scophthalmus maximus). From in vitro and in vivo experiments, it was deduced that these chemicals inactivated rapidly the bacterial isolates, and controlled the disease manifestation in fish.

The numbers of aerobic, heterotrophic bacteria, associated with cysts and nauplii of brine shrimp ( Artemia ), were estimated using a spread plate technique. Bacterial numbers in dried cysts were relatively low compared with counts obtained during the hatching processes when there was a rapid multiplication within 24 h of bacteria of some taxa to 1.2 × 10 7 micro-organisms/ml in the water. However, there was no evidence of intimate colonization of the nauplii by bacteria, and the loosely attached surface flora was removed readily by washing in sterile sea water. The internal structures remained comparatively free of bacteria. Representative isolates were examined and found to comprise Bacillus, Erwinia, Micrococcus, Staphylococcus and Vibrio spp.

From a comparison of 50 antimicrobial agents, the usefulness of oxolinic acid was indicated for the control of furunculosis in brown and rainbow trout. From in vitro and in vivo experiments, it was deduced that oxolinic acid rapidly inactivated the bacterial cells, and controlled the disease manifestation in fish.