Five essential oils from aromatic plants of Cameroon: their antibacterial activity and ability to permeabilize the cytoplasmic membrane of Listeria innocua examined by flow cytometry

Aims: To investigate the antibacterial effect of five essential oils (EO) extracted from aromatic plants (Cymbopogon citratus, Ocimumbasilicum, Ocimum gratissimum, Thymus vulgaris and Zingiber officinale) of Cameroon against strains of Listeria monocytogenes, L. innocua and Staphylococcus aureus. The ability of selected EO to permeabilize the cytoplasmic membrane of L. innocua was also examined. Methods and Results: The antibacterial activity of the EO determined by the agar diffusion method showed that T. vulgaris had the highest activity followed by O. gratissimum and C. citratus. Lowest activity was recorded from Z. officinale and O. basilicum. Significant differences in sensitivity between strains of Listeria and S. aureus were observed. Flow cytometry of L. innocua stained with carboxy-fluorescein diacetate showed that the fluorescence intensity of cells exposed to EO decreased faster than nonexposed cells, indicating that EO permeabilized the cytoplasmic membrane with the leakage of carboxy-fluorescein. Conclusions: Almost all the EO tested showed antibacterial activity to a different extent. The antibacterial effect was due to permeabilization of the cytoplasmic membrane. Significance and Impact of the Study: This study has identified the preservative potential of the EO examined. The use of sensitive method, such as flow cytometry, is advantageous for quick generation of data on the antibacterial effect of EO.