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 ten hydrophilic plant extracts against two strains of C. perfringens facing various food-relevant influencing factors. Due to the high opacity of PEs, resazurin was used as an indicator for bacterial growth instead of pellet formation. The highest value of the minimum inhibitory concentration (MIC) of the replications of each PE was defined as effective plant extract concentration (EPC), whereas the next concentration beneath the lowest MIC value was defined as the ineffective plant extract concentration (IEPC). The EPC of seven PEs: allspice, cardamom, cinnamon, clove, coriander, ginger and mace were between 0.625 - 10 g/kg, whereas extracts of caravey, 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 and sodium nitrite, displayed either synergistic/additive effects or no interaction with most PEs. By combination with PEs at its ineffective plant concentration (IEPC, 0.08 – 1.25 g/kg), MIC of NaCl and NaNO2 decreased from 25 – 50 g/kg to 6 – 25 g/kg and > 200 mg/kg to 0.2 – 100 mg/kg respectively. On the contrary, lipid (sun flower 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.
