Biological properties of bacteriophages and biocontrol of staphylococcus aureus in foods

Abstract: 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 prevent Staphylococcus 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 against Staphylococcus 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 (1×103 CFU/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 against staphylococcus 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 1010 PFU/g, respectively, while for the ham and dried meat, whose phage titer was 109 PFU/g.