Salt and Seawater Science & Technology
Online ISSN : 2435-4619
Enhancement of Biodefense Activity and Improvement of Physiological Condition of Rainbow Trout, Oncorhynchus mykiss, by Oral Administration of a Preparation of Heat-killed Cells of Enterococcus faecalis FK-23
Izuru KAKUTAKiyomi TAKASE
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ジャーナル フリー

2023 年 4 巻 p. 1-8

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Heat-killed Enterococcus faecalis FK-23 strain (heat-killed FK-23) was administered orally to rainbow trout Oncorhynchus mykiss at 24,120, and 600 mg/kg body weight/day for up to 6 weeks to investigate its effects on the biological defense activity and the physiological indices of the fish. Administration of heat-killed FK-23 did not affect the appearance, swimming performance, feeding behavior, growth rate, number of blood cells or the lysozyme activity of skin mucus, but did increase significantly the phagocytic activity of granulocytes and monocytes. The NBT reduction activity of granulocytes was significantly higher at week 4 of the group whose dosage was 120 mg/kg body weight/day. Plasma protein, glucose and ALT levels did not change, but diminished plasma total lipids and enhanced antioxidant activity of plasma were found at the 600 mg/kg body weight/day dosage at week 4. Aerobic and facultative anaerobic bacteria increased in the FK-120 group. There was no difference in the composition of aerobic and facultative anaerobic bacteria in the intestine of fish at the same time. Bacteroides spp., an obligatory anaerobe, increased in the FK-600 group. These results indicate that 1) the oral administration of heat-killed FK-23 enhanced the innate immune activity of fish, mainly through activation of phagocytosis of granulocytes and monocytes, 2) the intake is about 120 mg/kg body weight/day, which is necessary to maintain high biological defense in rainbow trout, 3) the intake at 600 mg/kg body weight/day may be effective for improving lipid metabolism and enhancing antioxidant activity in the fish.

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