Optimal methods for evaluating antimicrobial activities from plant extracts.

The search for antimicrobial agents from plants has been a growing interest in the last few decades. However, results generated from many of these studies cannot be directly compared due to the absence of standardization in particular antimicrobial methods employed. The need for established methods with consistent results for the evaluation of antimicrobial activities from plant extracts has been proposed by many researchers. Nevertheless, there are still many studies reported in the literature describing different methodologies. The aim of this study was to find optimal methods to give consistent quantitative antimicrobial results for studying plant extracts. Three different agar-based assays (pour plate disc diffusion (PPDD), streak plate disc diffusion (SPDD) and well-in agar (WA)) and one broth-based (turbidometric (TB)) assay were used in this study. Extracts from two plant species (Duabanga grandiflora and Acalypha wilkesiana) were tested on two bacterial species, namely Escherichia coli and Staphylococcus aureus. Amongst the agar-based assays, PPDD produced the most reproducible results. TB was able to show the inhibitory effects of the test samples on the growth kinetic of the bacteria including plant extracts with low polarity. We propose that both agar- (i.e PPDD) and broth-based assays should be employed when assessing the antimicrobial activity of plant crude extracts.