Abstract
ABSTRACT A microplate assay was modified for the detection of antimicrobial activity in plant extracts. The aim was to develop an in vitro assay that could rapidly screen plant extracts to provide quantitative data on inhibition of microbial growth. A spectrophotometric assay using a microplate with serial dilutions of the plant extract and the bacteria was developed. Two bacteria, Staphylococcus aureus and Escherichia coli, were used for this study. Essential oils, oregano (Origanum vulgare) and lemon myrtle (Backhousia citriodora), and three active components carvacrol, thymol and citral were evaluated. The reproducibility of the assay was high, with correlation coefficients (r2) of the replicates of lemon myrtle and oregano with S. aureus and E. coli between 0.9321 and 0.9816. Similarly, r2 values for carvacrol, thymol and citral were between 0.8455 and 0.9814. This assay could also be used to measure antimicrobial activity in plant extracts which vary in pH and color.PRACTICAL APPLICATIONSThis research could be used for the quantitative determination of antimicrobial activity in plant extracts. As inhibition of growth is expressed as a percentage from 0 to 100, this method could also be used to study synergies within plant extracts to enhance antimicrobial activity for future studies. When plant extracts are used in food as natural preservatives, the challenge is to use a concentration that does not interfere with the flavor of the food product. In general, the amount of plant extract required to extend the storage life of food is greater than that required to produce an acceptable flavor. The advantage of knowing the percent inhibition relates to the effect of synergies where the inhibitions less than 100% can be enhanced with other antimicrobial plant extracts, thereby reducing the amount of plant extract required.
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