Screening of Antimicrobial Compounds in California Native Plants

Plants contain antimicrobial compounds that can be used for treatment of bacterial infections. Plants have a major potential for drug discovery.  Although 25 to 50% of the current pharmaceuticals contain plant derivatives, few plants are used as antimicrobials. The emergence of antibiotic-resistant bacteria is a leading global health concern that must be addressed. As the need for innovative antibacterial agents is increasing, it is crucial to consider antimicrobials from plant extracts that are effective against bacterial infections. Many California native plants have been used as herbal medicines for many years, yet their antimicrobial properties remain to be studied. The objective of this study is to screen California native plants for the presence of antimicrobial compounds. One hundred plants were collected in Irvine, Southern California. Using the bioautograph assay, plants were screened for antimicrobial activity against the Gram positive Staphylococcus aureus (1199B) and the Gram negative Escherichia coli. The disk diffusion method at concentrations of 0, 4, and 5 µg/mL of Ciprofloxacin was conducted to determine synergy between this antibiotic and the plant extracts. Ten solvent systems were tested to find the best separation of compounds in the bioautographs. Compounds in plant extracts were separated by thin layer chromatography and visualized by their derivatization with vanillin/sulfuric acid.  Twelve California native plants from five families were found to have the highest antimicrobial properties against S. aureus; these species are Platanus racemosa, Calocedrus decurrens, Salix lasiolepis, Isocoma menziesii, Baccharis salicifolia, Salix gooddingii, Artemisia californica, Baccharis pilularis, Heteromeles arbutifolia, Artemisia douglasiana, Rosa californica, and Populus sp. The results show that these plants inhibit the growth of S. aureus. The native plants species that show the greatest synergy with Ciprofloxacin by measuring inhibition zones included P. racemosa, B. salicifolia, B. pilularis, A. douglasiana, and Populus sp. Our results support the use of California native plant species in the search for new antimicrobial agents that can help combat infectious diseases and resistant bacteria. This is the first report of antimicrobial activity for the California native plants P. racemosa, B. salicifolia, and B. pilularis. This research was supported by the NIH-MBRS-IMSD Grant GM-55246.