Synergistic antimicrobial effects of nisin and p-Anisaldehyde on Staphylococcus aureus in pasteurized milk

- Nisin and AS displayed synergism against S. aureus.
- Mechanisms were proposed for the synergetic effects.
- Nisin combined with AS may be used in food industry and reduce drug resistance problem.

Staphylococcus aureus (S. aureus) infections resistant to a variety of drugs have been increasingly documented in recent years. Drug resistance has appeared largely due to extensive use of antibiotics. Nisin has been widely used in food industry to prevent bacterial growth. However, nisin resistance has been found in several foodborne pathogenic bacteria. p-Anisaldehyde (AS), an extract from Pimpinella anisum seeds, is a very common digestive herb of north India. In this study, synergistic interactions of nisin combined with AS against S. aureus were observed. The synergy was confirmed by checkerboard microdilution method, with the fractional inhibitory concentration index values ranging from 0.25 to 0.375. The positive interactions were verified by the challenge tests in pasteurized milk and agar diffusion assays. In addition, when treatment with nisin or AS alone or in combination, an obvious loss of 260 nm absorbing materials was observed. And flow cytometry assay showed that nisin or AS alone or in combination caused bacterial membrane permeabilization. The LIVE/DEAD BacLight experiment, scanning electron microscope and transmission electron microscopy observation further confirmed that the compounds could kill bacteria via disrupting membranes. All results above verified that the combination of nisin and AS which showed antibacterial activity by a membrane damage mechanism could not only reduce drug resistance but also be used as a new promising naturally sourced food preservative.