Antimicrobial Effects of Herb Extracts Against Foodborne Pathogen Listeria monocytogenes in Vitro

Herb extract is a type of well-known natural antimicrobial from plants. Food Drug Administration recognized that most herb extracts as Generally Recognized as Safe for human consumption. The objective of this study is to conduct three experiments and to evaluate the inhibitory and bactericidal effects of nine herb extracts against five representative strains of Listeria monocytogenes in vitro. In the experiment 1, each of herb extracts 2, 4, 5, 8 exhibited inhibitory effects against five strains of L. monocytogenes individually at 37 °C in Mueller-Hinton broth (MHB). The MIC values of those four herb extracts ranged between 5 - 50 mg/mL. In experiment 2, herb extract 4, which showed the lowest MIC value (5 mg/mL), reduced populations of L. monocytogenes in a range of 0.38 - 0.91 log CFU/mL after 30 min treatment at 37oC in MHB, indicating that herb extract 4 may not expected to be used as an antimicrobial agent for the purpose of reducing L. monocytogenes within a short period of time. In experiment 3, at concentrations of 1.56 and 0.78 mg/mL, herb extracts 2, 4, 5, 8 inhibited the growth of a five-strain L. monocytogenes cocktail individually at the abused refrigerator temperature of 12 oC, except herb extract 8 at the concentration of 0.78 mg/mL. At a concentration of 3.13 mg/mL, those four herb extracts reduce cell populations in a range of 2.2 to 1.6 mg/mL at 11 days. Herb extracts 2, 4, 5 and 8 could be potentially developed into food preservatives for controlling foodborne L. monocytogenes.

Keywords: Antimicrobial effect; Herb extracts; Listeria monocytogenes

Abbreviations: Herb extract (HE); Colony forming unit (CFU); Minimum inhibitory concentration (MIC); Mueller-Hinton broth (MHB); Ready-to-eat (RTE); Transfers in tryptic soy broth (TSB); Tryptic soy agar (TSA); Buffered peptone water (BPW); Phosphate-buffered saline (PBS)