Susceptibility testing of Listeria monocytogenes using the disk method and micro titre plate (micro broth dilution) method

Twelve South African medicinal plants which are used traditionally to treat symptoms associated with Listeria monocytogenes infections were screened for activity against the pathogen. Different plant parts were extracted separately with ethyl acetate and chloroform. All the extracts were first screened against the bacteria using disk method diffusion method. Zones of inhibition observed in the presence of chloroform extracts of Eucomis autumnalis and ethyl acetate extracts of Acacia karroo (50mg/ml) were 12 and 14mm, respectively. Active extracts were further tested against the bacteria for minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) determination using microtitre dilution method. Ethyl acetate extract of A. karroo and chloroform extract of E. autumnalis exhibited MIC of 3.1mg/ml and 12.5mg/ml respectively. Four samples namely Acacia karroo (ethyl acetate extract), Senecio inornatus ethyl acetate extract, S. inornatus (chloroform extract) and Aloe arborescens (ethyl acetate extract) showed good minimum bactericidal activity against L. monocytogenes MBC ranging from 3.1 to 12.5mg/ml. These extracts were further tested for cytotoxicity against Vero cells (XTT) using the Cell Proliferation Kit II. Fifty percent inhibition concentration (IC₅₀) of the extracts were as follows: A. karroo 45.5±7.9µg/ml, Senecio inonartus (ethyl acetate extract), 108.4±0995µg/ml, S. inonartus (chloroform extract) 99.9±4.2µg/ml and A. arborescens (ethyl acetate extract) >400µg/ml.

Acknowledgements: University of Pretoria, University of South Africa, Tshwane University of Technology, Reginald Mayekiso, Clement Letsoela, Ms Audrey Dhlamini.