A bioassay-guided phytochemical study of extracts from Vernonia crotonoides active against multi-resistant hospital bacteria

In our continuous searching for new natural products active against resistant pathogens, the vegetal species Vernonia crotonoides, colected in Jurubatiba shoal, Rio de Janeiro, was selected for this study. The main goal of this investigation was to evaluate the antibacterial activity of extracts from V. crotonoides leaves and, to identify the compounds responsible for the activity. The crude ethanol extract was submited to a liquid-liquid partition furnishing the n-hexanes, dichloromethane (DCL), ethyl acetate, buthanol and aqueous fractions. For the antibacterial activity there were applied the agar dilution and the disk diffusion techniques, both standardized by the National Committee for Clinical Laboratory Standards. The ethanolic extract and the fractions were tested at 512, 256, 128 and 64 µg/mL against 16 oxacillin-resistant Staphylococcus aureus strains from nosocomial sources and 12 ATCC strains, including gram-positive and gram-negative bacteria. The results showed that at 512 µg/mL the DCL fraction inhibited 15 S. aureus oxacillin-resistant strains and 11 ATCC strains. At 256 µg/mL and 128 µg/mL this fraction inhibited 4 and 2 strains, respectively. The other fractions did not present any relevant activity. By the disk diffusion method the DCL promoted great inhibition zones that varied from 12 mm to 23 mm against S. aureus oxacillin-resistant strains. The antibiotics vancomicine and oxacillin were used as control (d = 13 mm). Based on the promissing results, the DCL was fractioned by silica gel column chromatography producing about 500 fractions that were assembled based on their similarity profile by TLC. The fractions were analyzed by GC-MS, which showed the presence of sesquiterpenes. The pure substances will be analyzed by the NMR technique and also tested for their antibacterial activity. The results presented demonstrate that V. crotonoides, for the first time studied by this propose, can represent a promising new source of antibacterial agents.