JS25噬菌体生物特性及对食品中致病菌的生物防治

该试验以JS25噬菌体为研究对象,在分析稳定性的基础上,确定其对宿主细胞的侵染时间及最佳感染复数,最后通过在即食食品中的抑菌效应确定对即食食品的生物防治效果。结果表明,JS25噬菌体最佳感染复数为0.1、侵染时间为20 min、最适温度及p H值分别为4~60℃和6~9;另外,JS25噬菌体的潜伏期为20 min,60 min后平均裂解量为24.55 PFU/cell;试验结果表明,JS25噬菌体在即食食品中有较高的稳定性,且随噬菌体初始效价的增加,对即食食品中金黄色葡萄球菌的抑制作用也逐渐增加;但是在不同即食食品中的生物防治效果存在差异,主要表现为当效价为108及1010 PFU/g时,能分别有效的抑制鲜牛奶和圆白菜在贮藏过程中的金黄色葡萄球菌数量,当效价大于109 PFU/g时,对肉干及火腿中的致病菌有良好的防治效果。

Biological properties of bacteriophages and biocontrol of staphylococcus aureus in foods

Staphylococcus aureus, as one of the most common food-borne pathogens, is related to the health, safety in the food industry and fields of medicine and it can be transmitted to humans and animals through the food. Although many foods can serve as vehicles,Staphylococcus aureus is often isolated from ready-to-eat (RTE) foods, such as milk ,ham and cabbage. RTE foods have been implicated in most of the major listeriosis outbreaks in the last 30 years. Of particular concern is the fact that they are consumed directly, without a final bactericidal processing step. Since the preservation methods applicable to minimally processed RTE foods often seem to be insufficient to preventStaphylococcus aureus contamination and growth, new strategies to combat this opportunistic pathogen in foods are needed. Bacteriophages are natural enemies of bacterias and are suitable candidates for the environmentally friendly biocontrol of these pathogens. Using bacteriophages as a tool to the control of pathogens is a complementary to antibiotic therapy. We have isolated a lytic bacteriophage, broad-host-range phages, designated JS25 (Morphologic analysis of JS25 revealed that it was closely related to other Myoviridae phages infecting Staphylococcus species.), from sewage effluent on a dairy farm in Jiangsu, using as a biocontrol agent againstStaphylococcus aureus infections. In a comprehensive set of experiments, we evaluated thermal and pH stability,studied phage adsorption and determined the optimal multiplicity of infection. Then, we determined the latent period and burst size of JS25 bacteriophages by one-step growth. Last, we evaluated the virulent, broad-host-range phages JS25 bacteriophages for control of Staphylococcus aureus in different RTE foods (ham, dried meat, fresh milk, cabbage) known to frequently carry the pathogens and analyzed its stablity during storage. Food samples were spiked with bacteria (1×103CFU/g) respectively, with phage added thereafter (1×108 PFU/g), and then samples were stored at 4 and 20℃ separately for 12 days. The results showed that its latent period and average burst size during stable period were approximately 20 min and 24.55 PFU/cell, respectively. Optimum pH range for JS25 phage was between 6 and 9. However, a treatment of 80℃ for 40 min completely inactivated the phage. Thus, JS25 was stable below 60℃. The adsorption rate of JS25 showed at 20 min post-infection, approximately 100% of the JS25 was adsorbed to the host cells. The bacteriolytic activity of JS25 at a multiplicity infection (MOI) 0.1 indicated its optimum efficiency for reducing bacterial growth. Also, we can see phage JS25 was of high stability in ham, dried and fresh milk during storage at 4 and 20℃. JS25 titer dropped 0.48 logs and 0.98 logs only in cabbage storage at 4 and 20℃, respectively. Therefore, the added phages retained most of their infectivity during storage in foods storage at 4 and 20℃. For RTE foods, such as dried meat, fresh milk and cabbage, biological control effect of phage againststaphylococcus aureus was higher with the increasing of phage initial titer, and was different among different RTE foods. For fresh milk and cabbage, the optimum phage titer was 108 and 1010PFU/g, respectively, while for the ham and dried meat, whose phage titer was 109 PFU/g.