Lactobionic acid enhances the synergistic effect of nisin and thymol against Listeria monocytogenes Scott A in tryptic soy broth and milk

- The possibility of chelating lactobionic acid (LBA) enhancing activities of nisin and thymol was studied.
- Antimicrobials were studied alone or in combination in tryptic soy broth (TSB) and milk.
- LBA enhanced the synergistic effect between nisin and thymol against L. monocytogenes.
- Only the ternary combination inhibited the growth of L. monocytogenes in TSB at 32 °C for 24 h.
- The ternary combination also showed best anti-listerial activity in milk.

Listeria monocytogenes is a Gram-positive opportunistic human pathogen and it remains a significant cause of foodborne illnesses. A variety of natural and synthetic compounds have been studied to inhibit the growth of L. monocytogenes in foods. Antimicrobial combinations with synergistic antilisterial properties can reduce the dose of each antimicrobial, which can be further enhanced by chelating compounds. Therefore, the objective of this study was to determine antilisterial properties of binary or ternary combinations of lactobionic acid (LBA), nisin, and thymol in tryptic soy broth (TSB), 2% reduced-fat milk, and whole milk. The results showed that the minimum inhibitory concentration (MIC) of nisin, thymol and LBA was 125 IU/mL, 0.25 mg/mL, and 10 mg/mL, respectively. The ternary combination was the most effective in reducing MICs of antimicrobials, with the MIC of nisin, thymol, and LBA being 31.25 IU/mL, 0.0625 mg/mL, and 1.25 mg/mL, respectively. In TSB with 0.6% yeast extract, L. monocytogenes grew in individual or binary antimicrobial treatments of 31.25 IU/mL nisin, 0.0625 mg/mL thymol, and 1.25 mg/mL LBA within 24 h at 32 °C, while it was completely inhibited by the ternary combination. In 2% reduced-fat milk at 21 °C, the ternary combination of nisin, thymol, and LBA at respective concentrations of 250 IU/mL, 2 mg/mL, and 10 mg/mL completely inhibited the bacterium to below the detection limit in 72 h while > 2 log (CFU/mL) bacteria was still detected in all the binary combinations after 120 h. In whole milk, the combination of 500 IU/mL nisin, 2 mg/mL thymol, and 10 mg/mL LBA reduced bacteria to around 2 log (CFU/mL) in 4 h at 21 °C, and no bacterial recovery was observed after 5 d. This study suggested the potential of the ternary combination of nisin, thymol and LBA for food preservation.