Reduction of viability, heat resistance, and biofilm formation and persistence of Bacillus cereus spores in beef stew broth by herb and spice extracts

This study investigated the inhibitory effects of ethanol extracts from 16 plants (8 herbs and 8 spices) on the viability, heat resistance, biofilm formation, and persistence of two different toxigenic Bacillus cereus spores in beef stew broth. Two B. cereus strains, namely, B. cereus CH3 (diarrheal type) and B. cereus JCM 17690 (emetic type), were tested. Based on the lowest minimum inhibitory concentration (MIC) both in vitro and in situ, the most effective herbs and spices were selected: rosemary (0.3125 mg/mL; identical in both strains and tests) and sage (0.3125 mg/mL) among herbs; mace (0.625 mg/mL) and turmeric (0.625 mg/mL) among spices. In addition, all extracts prepared from powdered forms of herbs and spices consistently exhibited equal or lower MICs than those prepared from dried forms. Meanwhile, all four selected extracts prepared from powdered forms significantly reduced spore viability (reduction rate of herbs: 50.46–90.91 %; spices: 52.36–86.01 %) and heat resistance (reduction rate of herbs: 16.64–48.63 %; spices: 9.69–45.41 %). All the extracts also completely inhibited the biofilm formation of B. cereus spores and effectively disrupted the preformed biofilms (i.e., reduction of biofilm persistence; reduction rate of herbs: 12.16–77.83 %; spices: 14.13–70.99 %). In addition, they significantly reduced the heat resistance (reduction rate of herbs: 9.45–35.67 %; spices: 8.90–28.27 %) of spores in biofilm. Such inhibitory effects of the extracts were shown in a concentration-dependent manner, and the intensity of inhibition appeared to be strain specific. These findings indicate that the application of natural plant extracts, in combination with conventional heat treatment, may provide a promising strategy for controlling B. cereus contamination in beef stew and other ready-to-eat meat products.