关键词: 钉螺, 血吸虫病, 印度芽孢杆菌F19, 灭螺活性, 稳定性

Abstract: Objective　To assess the stability of the molluscicidal activity of the fermentation broth supernatant of Bacillus indicus F19, so as to provide insights into development of novel molluscicides derived from this isolate. Methods　Healthy Oncomelania hupensis snails free from Schistosoma japonicum infections and acclimated in the laboratory for 7 days were selected. The fermentation broth supernatant of B. indicus F19 was prepared. For each treatment condition, four experimental groups and one control group were established, with 10 mL of the fermentation broth supernatant each group. O. hupensis snails were exposed to different pH values (3, 6, 9 and 12), temperatures (40, 60, 80 ℃ and 100 ℃), and ultraviolet (UV) exposure durations (1, 2, 3 h and 4 h). Then, the supernatant was diluted with dechlorinated water to concentrations of 50%, 10%, and 5% (v/v), and 10 mL of each dilution solution was added to three separate tubes, with an equal volume of dechlorinated water as a blank control. Twenty O. hupensis snails were transferred to each tube, immersed for 48 h and 72 h, and collected for viability assessment using the shell⁃tapping method. The mortality of O. hupensis snails was calculated and compared among dilution groups and the blank control group to evaluate the molluscicidal activity of the fermentation broth supernatant of B. indicus F19.  Results　There was a significant difference in the mortality of O. hupensis snails immersed in 5% fermentation broth supernatants of B. indicus F19 for 48 h at various pH levels ([χ2] = 11.556, P < 0.05), and the mortality reduced to 13.33% (8/60) in the experimental groups at a pH value of 12, which was significantly lower than in the blank control group [33.33% (20/60)] ([χ2] = 6.708, P < 0.05). There was a significant difference in the mortality of O. hupensis snails immersed in 10% fermentation broth supernatants of B. indicus F19 for 72 h at various pH levels ([χ2] = 31.691, P < 0.001), and the mortality reduced to 71.67% (43/60) and 65.00% (39/60) in the experimental group at pH values of 9 and 12, which was significantly lower than in the blank control group [100.00% (60/60)] ([χ2] = 8.571 and 12.110, both P values < 0.01). The mortality of O. hupensis snails both reduced to 23.33% (14/60) following immersion in 5% fermentation broth supernatants of B. indicus F19 heated in a water bath at 80 ℃ and 100 ℃ for 48 h, which was lower than that of unheated O. hupensis snails [33.33% (20/60)] and snails heated in a water bath at 40 ℃[35.00% (21/60)] and 60 ℃ [33.33% (20/60)], and there was no significant difference in the mortality of O. hupensis snails among groups ([χ2] = 3.898, P > 0.05). The mortality of O. hupensis snails was 68.33% (41/60), 66.67% (40/60), 65.00% (39/60), and 68.33% (41/60) following immersion in fermentation broth supernatants heated in a water bath at 40, 60, 80 ℃ and 100 ℃ for 72 h, which was all lower than that of snails in the control group [71.70% (43/60)]; however, there was no significant difference seen in the snail mortality among groups ([χ2] = 0.674, P > 0.05). The mortality of O. hupensis snails was 73.33% (44/60), 71.67% (43/60), 71.67% (43/60), and 75.00% (45/60) following 48 h immersion in 10% fermentation broth supernatants of B. indicus F19 irradiated by UV for 1, 2, 3 h and 4 h, 33.33% (20/60), 31.67% (19/60), 35.00% (21/60), and 33.33% (20/60) following 48 h immersion in 10% fermentation broth supernatants of B. indicus F19 irradiated by UV for 1, 2, 3 h and 4 h, and 71.67% (43/60) and 33.33% (20/60) following 48 h immersion in 10% fermentation broth supernatants of B. indicus F19 without UV irradiation, and no significant differences were found among groups ([χ2] = 0.229 and 0.150, both P values > 0.05). In addition, the mortality of O. hupensis snails was 76.67% (46/60), 71.67% (43/60), 70.00% (42/60), and 76.67% (46/60) following 72 h immersion in 5% fermentation broth supernatants of B. indicus F19 irradiated by UV for 1, 2, 3 h and 4 h, and 71.67% (43/60) in the control group, and there was no significant difference in the snail mortality among groups ([χ2] = 1.193, P > 0.05). Conclusion　 The molluscicidal action of the fermentation broth supernatants of B. indicus F19 is tolerant to high temperature and UV irradiation; however, the activity is attenuated under alkaline conditions.

Key words: Oncomelania hupensis, Schistosomiasis, Bacillus indicus F19, Molluscicidal activity, Stability