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A few reports indicate the beneficial effects of organic acids and their salts on anti-infective immunity in fish species. In the role of immunostimulants, inorganic acids may prove to be a much cheaper alternative to their organic equivalents. However, no report has described the effect of using inorganic acid as a feed additive on fish immunity. This study is the first attempt to evaluate the effect of hydrochloric acid (HCl) as a fish feed supplement on cellular immunity in the stomachless fish, cypriniform vimba bream, Vimba vimba (L.). Two groups, three replicates each, of juvenile vimba were fed a commercial dry fish diet or its variant containing a 1.5% additive of HCl for 55 days. The experiment was conducted in a recirculating aquaculture system at 25°C. Mortality, growth, condition factor, feed conversion ratio, respiratory burst activity (RBA), potential killing activity (PKA), and the proliferative response of head kidney lymphocytes were determined for both groups at the end of the experiment and compared. The PKA was higher in the group fed the diet with HCl. For all other parameters studied, there were no significant differences between the experimental groups. The results of the current study prove that the dietary additive of inorganic hydrochloric acid can substantially improve immune response to bacterial infections in juvenile vimba bream.

Since 2007 in Poland, skin lesions have been observed in salmon (Salmo salar L.) and sea trout (Salmo trutta L.) entering Pomeranian rivers to spawn. The clinical picture of affected fish resembled ulcerative dermal necrosis (UDN), a disease of unknown etiology affecting mainly the scalps of wild salmonids. The aim of the 2009-2012 study was to determine with microbiological tests the etiological agent of the skin lesions observed in salmonids entering the Pomeranian rivers. During the study, a total of 13 species of Gram-negative bacteria belonging mainly to the Yersiniaceae, Enterobacteriaceae, and Pseudomonadaceae families were isolated and identified from the skin and kidneys of diseased fish. The Gram-positive bacteria isolated were aerobic, hemolytic granules of the genus Streptococcus. Mycological examinations on damaged fish body surfaces revealed yeasts of the genus Rhodotorula and fungi of the family Saprolegniaceae. The results of our study did not permit us to identify unequivocally the direct cause of the fish diseases analyzed since all the bacteria isolated during the study are normally found in aquatic environments. Some of the isolated bacteria identified were assumed to be potentially pathogenic to fish. The fungal infections observed were probably secondary and only exacerbated ongoing disease processes.

The presence of lactic acid bacteria (LAB) favors the stabilization of intestinal flora, facilitates digestion, improves the assimilability of fodder, and has an immunomodulatory effect on the immune system. According to current research, the application of LAB following antibiotic treatment prevents the development of opportunistic bacteria inhabiting the digestive tract. In the study the potential probiotic properties of Lactobacillus plantarum strains, which can be administered as an alternative to antibiotic treatment in aquaculture, were investigated under in vitro conditions. The strains of L. plantarum were characterized for important properties such as the ability to grow in the presence of 10% fish bile, a tolerance of low pH, and antagonism to pathogens dangerous for fish such as Aeromonas salmonicida and Pseudomonas fluorescens; therefore, they meeting the criteria for strains with probiotic properties. In view of currently increasing resistance to antibiotics and a decrease of their efficiency, probiotic bacteria can serve to support immunity to infections in the future.