Dimeric acylphloroglucinol with activity against resistant forms of S. aureus

Introduction. The genus Hypericum is represented by ca. 500 species, distributed all over the world. In the state of Rio Grande do Sul there are about 20 species, all of them housed in Brathys and Trigynobrathys sections. These plants accumulate several classes of bioactive compounds, principally phloroglucinol derivatives. Some Hypericum extracts and isolated phloroglucinols has exhibited antimicrobial activities. Therefore, the aim of this study is to determine the antimicrobial activity of n-hexane extract from H. caprifoliatum Cham. & Schlecht. and the main phloroglucinol isolated from this species. Experimental. The aerial parts of H. caprifoliatum at flowering stage were collected in the Rio Grande do Sul state, in November 2016. A competent botanist identified the plant and the collection was authorized by SisGen-Brazil, access registration A347956. The vegetal material was dried at room temperature and extracted with n-hexane. Waxes were removed from the extract with cold acetone. The extract was submitted to successive chromatographic columns using n-hexane and dichloromethane mixtures as mobile phases, aiming the isolation of phloroglucinols derivatives. The antimicrobial assays were performed with four strains: Enterococcus faecalis (ATCC 29212) Staphylococcus aureus (ATCC 25923), Methicilin Resistant S. aureus (MRSA) and S. aureus (MLSB). The minimum inhibitory concentration (MIC) was determined by macrodilution, using different concentrations of the extract and isolated compounds (1.9 – 500 µg/mL). The tubes were incubated at 35 ºC by 24 h, and the results were expressed as the concentration that inhibited the microbial growth. All the assays were carried out in duplicate. Results and Discussion. The phytochemical analysis resulted in the isolation of a dimeric acylphloroglucinol, identified by NMR techniques as hyperbrasilol B. The antimicrobial assays demonstrated that the most susceptible strains were the MRSA and the S. aureus (MLSB), with MIC values from the extract of H. caprifoliatum (≤ 31.3 µg/mL and ≤ 7.8 µg/mL). Hyperbrasilol B was also tested in the same bacteria strains, however the MIC values were lower than those obtained with the extract. All the tested strains were susceptible to the treatment with this phloroglucinol derivative (MIC values of 31.3 µg/mL). Previous studies demonstrated the antimicrobial activity of Hypericum extracts and isolated compounds. Several dimeric acylphloroglucinols were also active and some chemical feature of the compounds demonstrated to be important for the antimicrobial activity. This study included resistant forms of clinical important bacteria and, interestingly, H. caprifoliatum n-hexane extract showed higher activity against the two resistant forms of S. aureus.