Antimicrobial screening of Plectranthus madagascariensis and P. neochilus extracts

Natural products are widely used as traditional medicines and are a common source of bioactive molecules for the treatment of bacterial infections. In particular, some plants of the genus Plectranthus (Lamiaceae) have demonstrated several applications, including the treatment of various infections. In this work, aqueous, acetonic and methanolic extracts of P. madagascariensis and P. neochilus were prepared using several extraction methods (infusion, decoction, maceration, microwave- and ultrasoundassisted and supercritical fluids). All extracts were screened for their antimicrobial activity against Gram positive bacteria (Enterococcus faecalis, Staphylococcus aureus, Bacillus subtilis and Mycobacterium smegmatis) and Gram negative bacteria (Pseudomonas aeruginosa, Klebsiella pneumoniae and Escherichia coli) and two yeast (Candida albicans and Saccharomyces cerevisiae) strains. The P. madagascariensis acetonic extracts obtained using ultrasound and maceration methods and P. neochilus acetonic extract obtained by ultrasound technique showed activity against the five tested Gram positive bacteria (5‒24 mm of inhibition zone using the well diffusion test). The antimicrobial activity was further evaluated by the microdilution method (MIC values were between 250‒0.49 μg/mL) and the bioautography assay against S. aureus. The P. madagascariensis ultrasound acetonic extract was the most active extract against all the tested Gram positive bacteria (MIC values ranged between 31.25 - 0.49 μg/mL). It was also active against resistant MRSA and VRE strains (MIC values ranged between 31.25-0.98 μg/mL). The S. aureus bioautography assay showed that the more polar compounds were the responsible for the antimicrobial activity.