ABSTRACT

The application of plant extracts (PEs) could be a promising option to satisfy consumers' demand for natural additives to inhibit growth of variable pathogenic bacteria. Thus, the aim of this study was to develop a standardized microdilution method to examine the antimicrobial effects of 10 hydrophilic PEs against two strains of Clostridium perfringens facing various food-relevant influencing factors. Because of the high opacity of PEs, resazurin was used as an indicator for bacterial growth instead of pellet formation. The highest value of the MIC of the replications of each PE was defined as effective plant extract concentration (EPC), whereas the next concentration beneath the lowest MIC was defined as the ineffective plant extract concentration (IEPC). The EPCs of seven PEs, allspice, cardamom, cinnamon, clove, coriander, ginger, and mace, were between 0.625 and 10 g/kg, whereas extracts of caraway, nutmeg, and thyme showed no antimicrobial activity up to the maximum concentration tested (10 g/kg) against C. perfringens in vitro. Two intrinsic factors, sodium chloride (NaCl) and sodium nitrite (NaNO2), displayed either synergistic or additive effects or no interaction with most PEs. By combination with PEs at their IEPC (0.08 to 1.25 g/kg), MIC of NaCl and NaNO2 decreased from between 25 and 50 g/kg to between 6 and 25 g/kg and from more than 200 mg/kg to between 0.2 and 100 mg/kg, respectively. In contrast, lipid (sunflower oil) at a low concentration inhibited the antimicrobial effects of all tested PEs. For extrinsic factors, only allspice, ginger, and coriander could maintain their antimicrobial effects after being heated to 78°C for 30 min. The synergistic effect between PEs and pH values (5.0 and 5.5) was also found for all PEs. The established screening method with resazurin and defining EPC and IEPC values allows the verification of antimicrobial effects of PEs under various food-relevant influencing factors in a fast and reproducible way.

HIGHLIGHTS
  • Seven plant extracts (PEs) show antimicrobial effect against C. perfringens.

  • MIC of NaCl decreased from between 2.5 and 5 g/kg to between 6 and 25 g/kg when combined with IEPC of PEs.

  • MIC of NaNO2 decreased from >200 mg/kg to between 0.2 and 100 mg/kg when combined with IEPC of PEs.

  • pH 5.0 and 5.5 enhanced the efficacy of all PEs.

  • Lipid and heat treatment at 78°C for 30 min reduced the efficacy of several PEs.

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