Susceptibility testing of Listeria monocytogenes to extracts of selected South African medicinal plants

In South Africa, the antimicrobial activity of various indigenous plants has been studied extensively. Most of the work however, has focused on their activity against planktonic bacteria with less attention given to biofilms. Many organisms, including the opportunistic pathogen Listeria monocytogenes which is associated with severe infections in humans, occurs more frequently as biofilms. The aim of this study was to identify and select the plants that exhibit the best antilisterial activity, isolate the bioactive compounds and determine their effect on the architecture of listerial biofilms. Ethyl acetate or chloroform extracts of thirteen plants were investigated for antilisterial activity. The ethyl acetate extract of Acacia karroo and Plectranthus ecklonii showed the best antilisterial activity (among the plants tested) exhibiting a minimum inhibitory concentration (MIC) activity of 3.1 mg/ml and 0.5 mg/ml respectively and were therefore further selected for the identification of bioactive compounds. Column chromatographic purification of the ethyl acetate extracts of the leaves of A. karroo led to the isolation of three known pure compounds namely β-sitosterol, epigallocatechin and epicatechin. The confocal scanning laser microscopy (CSLM) showed that the biomass of the listerial biofilm was reduced when the isolated compounds were added. The aggregation of cells which were exposed to β-sitosterol and epigallocatechin was reduced from 25μm as observed in untreated cells to < 10 μm in diameter.