Synergistic interactions of nisin in combination with cinnamaldehyde against Staphylococcus aureus in pasteurized milk

Staphylococcus aureus (S. aureus) is the leading cause of a broad range of human infections and associated with food poisoning worldwide. Nisin can be considered as a natural preservative and is widely used by the food industry. However, resistance of nisin has been reported in several species of bacteria. Identification of antibacterial agents that are synergistic with nisin is a method of minimising resistance to nisin. Cinnamaldehyde (CA), a well-studied essential oil isolated from cinnamon bark, has been reported to exhibit a broad spectrum of antimicrobial activity across a wide range of microorganisms. Previous studies have demonstrated that CA could be used in the food industry due to its active antibacterial activity and its safety. In this study, the synergistic antibacterial activity in vitro of nisin and CA against 13 food-borne isolates of S. aureus and S. aureus ATCC 29213 were evaluated by a checkerboard assay. The FICI values of nisin combined with CA ranged from 0.1875 to 0.375, which demonstrated a synergistic effect. The positive interactions were verified using the challenge tests in pasteurized milk and agar diffusion assays. The LIVE/DEAD BacLight experiment and scanning electron microscope (SEM) assay were also performed to distinguish the intact and damaged both of cell walls and membranes with the compounds, the combination group displayed stronger damage both of cell wall and cell membrane than when nisin or CA was used alone. These findings indicated that the combination of nisin and CA could not only be used as a promising naturally sourced food preservative but also to reduce the problem of bacterial resistance.