Extracts from plants have been shown to be a good source of a number of potent antimicrobial agents. Significant effort has been invested in looking at the activity of extracts from tropical regions of Australia, with less interest in more temperate environments. We sought to redress this imbalance by examining antimicrobial activities of extracts from native plants in the Ballarat area.
Thirteen plants from around Victoria were sampled taking predominantly leaves. Two plants from tropical regions were also included to compared and contrast their activities with the Victorian plants. Plant material was desiccated and ground before extracted with methanol at 4°C. Methanol extracts were then dissolved in water, filtered and freeze dried.Aqueous extracts were tested for antimicrobial efficacy by broth dilution using a variety of Gram positive and Gram negative bacteria. Approximately 2mg/ml of each extract was used. Activity was assessed by measuring absorbance of the broths at 600nm relative to a broth culture with no extract.
In general, the extracts had greater activity against Gram positive bacteria, with Micrococcus luteus being the most sensitive organism tested (every extract had some effect on M. luteus). The most potent extracts being native mint, kangaroo apple fruit and Banksia leaf, showing 95%, 97% and 87% reduction in growth of M. luteus, respectively. Native mint was the most active extract against Gram negative bacteria.
The tropical plant Rosella had the greatest activity of all the plant extracts tested, showing 97%, 89%, 99% and 100% reduction in growth against E. coli, Salmonella, S. aureus and M. luteus, respectively. This suggests that while tropical plants remain an important source of bioactive compounds, there remains considerable potential in plants from temperate climates.