Influence of Cu2+-loaded silicate on the growth performance and microflora of crucian carp Carassius auratus.

We investigated the effects of Cu2+-loaded silicate (CLS) on the growth performance, microflora of skin, gill and intestine, and intestinal morphology of crucian carp Carassius auratus. A total of 225 native wild crucian carp, with an average initial body weight of 20 g, were randomly divided into 5 treatment groups using 3 replicate tanks of 15 fish per group. The dietary treatments were (1) basal diet, (2) basal diet + CuSO4, (3) basal diet + silicate, (4) basal diet + 0.5% CLS and (5) basal diet + 50 mg kg(-1) chlortetracycline (CTC, purity 98.8%). The trial lasted for 60 d. We found that body weight increased slightly while feed conversion ratio decreased in the CLS-treated group compared with the control groups. The total number of aerobic bacteria counted in the intestine of carp fed the diet supplemented with the CLS (i.e. Vibrio sp. and E. coli), was significantly lower (p = 0.05) compared with the control groups and the CTC-treated fish, while lactobacillus counts were significantly higher (p = 0.05). Lactobacilli counts of the intestine increased significantly (p = 0.05). However, the microflora of the skin and gill was not affected by the addition of CuSO4, silicate, CLS or CTC. The height of the villi in the proximal, mid and distal intestine mucosa of the silicate- and CLS-treated groups was found to be longer (p = 0.05) compared with the villi of the control or the CTC-treated fish. Supplementation with CuSO4 had no effect on the microflora and the intestinal morphology (p = 0.05). These results indicate that CLS had an antibacterial activity in vivo, which may help protect the intestinal mucosa from invasion of pathogenic bacteria and their toxins.