Purpose: This study was done to screen plant extracts for inhibitory activities against Listeria monocytogenes or Staphylococcus aureus, and to determine the combinations of plant extracts with synergistic lethal effects against those pathogens.
Methods: Plant extracts (625 types) were tested for their antimicrobial activities against L. monocytogenes or S. aureus using an agar well diffusion assay. For selected plant extracts, their minimal inhibitory concentrations (MICs) against L. monocytogenes or S. aureus were determined. Finally, synergistic lethal effects between plant extracts against those pathogens were evaluated using a checkerboard test.
Results: Morus alba, Rhus chinensis, Carex pumila, Sappan lignum, Coptis chinensis, Siegesbeckia glabrescens, and Dryopteris erythrosora extracts showed antimicrobial activities against L. monocytogenes or S. aureus. D. erythrosora extract showed the lowest MIC (0.0039 mg/ml) against L. monocytogenes followed by M. alba (0.0313 mg/ml) and C. chinensis (0.50 mg/ml). A combination of D. erythrosora and M. alba and a combination of D. erythrosora and C. chinensis showed synergistic lethal effects against L. monocytogenes. D. erythrosora extract showed the lowest MIC (0.0039 mg/ml) against S. aureus followed by M. alba (0.0156 mg/ml) and C. chinensis (0.25 mg/ml). A combination of C. chinensis and M. alba and a combination of C. chinensis and D. erythrosora showed the synergistic lethal effects against S. aureus.
Significance: Plant extracts with inhibitory activities against L. monocytogenes or S. aureus were screened, and their MIC values were evaluated. Combinations of plant extracts with synergistic lethal effects against L. monocytogenes or S. aureus were determined.