To explore the antibacterial mechanism of nisin Q13 against Lactobacillus bulgaricus LMG-1, the purity and molecular mass of nisin Q13, produced by Lactococcus lactis Q13 and purified by ammonium sulfate precipitation, ultrafiltration and dextran gel column chromatography, were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and the effects of nisin Q13 on the cell membrane permeability, intracellular enzyme activity and cell morphology of L. bulgaricus were evaluated. The results showed that the optimal saturation level of ammonium sulfate for nisin Q13 precipitation was 60%. After ultrafiltration, only one fraction, whose molecular mass was greater than 10 kDa, had an antibacterial effect. The fraction was further purified by dextran gel column chromatography, yielding two peaks with antibacterial activity. The polyacrylamide gel electrophoretogram showed only one protein band with a molecular mass of about 17.5 kDa, which may be a polymer of nisin. The antibacterial mechanism of nisin Q13 may be related to the fact that it can cause the formation of holes in the cell membrane of the target cells, increase the permeability of the cell membrane and destroy the integrity of the cell membrane, leading to the leakage of cellular contents, reducing the contents of key enzymes such as Na⁺/K⁺-ATPase and alkaline phosphatase (AKP), affecting the normal energy metabolism of cells, and eventually resulting in cell death.

A method for detecting Staphylococcus aureus in foods was established by insulated isothermal polymerase chain reaction (iiPCR). In this study, specific primers and probes were designed based on the nuc gene, and the extraction of template DNA from S. aureus in food samples was explored. After optimizing the amount of primers, probes and templates, the specificity, sensitivity and stability of iiPCR were evaluated, and the application of iiPCR and traditional PCR was compared to detect S. aureus in artificially contaminated samples and actual food samples. The results showed that the water bath method was suitable for rapid extraction of template DNA due to its simple operation, short time consumption, and low instrument requirements. The iiPCR method had strong specificity, high sensitivity and good stability without cross-reaction with other bacteria. It took six hours to detect S. aureus in artificially contaminated pork samples and milk samples by the iiPCR method with a detection limit of 10³ and 10² CFU/mL, respectively, while the traditional PCR method took eight hours with a detection limit of 10⁵ CFU/mL. For actual food samples, the verification experiments showed that the results of the iiPCR method at 6 h were consistent with those of traditional PCR at 12 h and the traditional culture method at 16 h. It was proved that the iiPCR method could detect S. aureus in foods more quickly and accurately.

A virulent phage P65, one of whose hosts is methicillin-resistant Staphylococcus aureus (MRSA), was isolated from the sewage of dairy cow farm. The transmission electron microscopic (TEM) observation showed that the head of the phage was polyhedral, belonging to the long-tailed Siphoviridae family. Its optimal multiplicity of infection (MOI) was 0.01. The one-step growth curve showed that the latent period of the phage was 10 min, the lysis period was 90 min, and the burst size was 35.46 PFU/cell. Its lytic activity remained stable at pH 4–10 and 30–45 ℃. After 70 min of UV irradiation, the titer of the phage decreased from 1.91 × 10⁸ to 2.65 × 10⁴ PFU/mL. After 48 hours of phage treatment, the biofilm clearance rates of strains MRSA 2 and MRSA 24 were 91.3% and 92.2%, respectively. At 25 ℃, P65 had an antibacterial effect in milk and beef contamination models. The results showed that phage P65 has a wide host spectrum and stable lytic activity, and thus has potential for the removal of biofilm and food preservation.