| Sea cucumber (Apostichopus japonicus) belongs to Echinodemata, Holothuroidea, Apostichopus, which naturally distributed in the yellow sea and Bohai Bay of China, Japan, Korea, and the east coast of Russia. It is the most valuable species due to its nutritional and supposed medicinal properties. In the recent years, the mariculture of sea cucumber has been developed quickly in China, and obtained remarkable economic and social benefit. In 2007, the area of cultivation has reached 30,000 ha. in Shandong Province, the output has reached 80,000 tons valued about 10 billion RMB. However, rapid development and poor operations resulted in many problems such as the occurrence of various diseases and causing high mortality. The skin ulcer syndrome, which broke out cosmically during seed rearing and grow-out periods, has caused serious economic losses and became one of the limiting factors in the sustainable development of this industry.For many years, antibiotic were mainly used to prevent and treat diseases, but the massive use of antibiotic could cause drug resistance and drug residues. It can be seen that enhancing the immunity of sea cucumber themselves and strengthening their resistance to disease are the most important ways to solve the disease problems.Therefore, it's very important to use the extraneous source stimulant (immunostimulants, vaccine and so on) to enhance the animals' immunity in disease prevention. Until now, there were rear reports on immune effects of inactivated bacteria on sea cucumber.From June to July of year 2007, the skin ulcer syndrome of cultured sea cucumber Apostichopus japonicus was broke out in three sea cucumber farms in Yantai, its symptoms were anorexia, head shaking, mouth tumidity, eviscerating bowels, mouth ulceration and skin ulceration and caused massive mortality. Etiological study was carried out on the disease. One bacterial strain named HS-yt was isolated, and it was always dominant amongst the isolates. The bacterium was identified as Aeromonas veronii by the means of morphological, physiological and biochemical tests, and 16S rDNA sequence analysis. Artificial infection test proved that HS-yt was the causative pathogen associated with the disease.In this research, we selected 5 enzyme vigor indexes which were commonly used to appraise the immune effects of immunostimulants on invertebrate. The results indicated that lysozyme, SOD and AKP changed regularly and were considered as immunity index in further experiments.The bacteria were killed by formaldehyde and used as an inactivated bacterium to inject normal Apostichopus japonicus. The immune effects were shown as follows: The highest lysozyme activities were observed at 6h post-injection in 3.9×108CFU/mL group, and at 24h post-injection in 4.7×107CFU/mL group. The alkaline phosphatase (AKP) activities reached maximum at 48h post-injection in 3.9×108CFU/mL group, and at 96h post-injection in 4.7×107CFU/mL group. However, there was no significant difference in superoxide dismutase (SOD) between the experimental groups and the control group. In general, the inactivated bacteria could increase the immune effects on sea cucumber. In addition, as the concentration of the inactivated bacteria was higher, lysozyme and AKP activities could reach the maximum rapidly and significantly. But the increased activities could not last long.In this study, the inactivated bacteria Aeromonas veronii could increase the activities of AKP and lysozyme, which indicated that they could increase the immune effects on sea cucumber Apostichopus japonicus. This study provided a lot evidences to the immune study of sea cucumber, and also provided a reference to the prevention of the skin ulcer syndrome in the cultured Apostichopus japonicus.
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