Acylhydrobenzoquinones influence the susceptibility of Staphylococcus aureus towards abietanes and the speciation within Plectranthus sensus lato

The genus Plectranthus sensu lato is known to produce abietanes, which exhibit significant potency against Gram-positive bacteria. It is hypothesized that abietanes exert their antibacterial effects by disrupting microbial cell walls. Recent studies have classified Plectranthus s.l. abietanes into six distinct groups. However, the impact of this chemical diversity on the response of abietanes against Gram-positive bacteria remains unexamined. Therefore, a structure-activity relationship study on abietanes from species of Plectranthus sensu stricto and Coleus was conducted to identify active fragments or pharmacophores in abietane-type diterpenoids that could be responsible for their activity against the Gram-positive bacteria Staphylococcus aureus. The study followed a bio-guided approach and included chemical profiling of the extracts, performed using NMR spectroscopy, and comparative analysis, using LC-MS in both positive and negative ionization modes. Results revealed all six classes of abietanes occurred as major constituents of the extracts, with distinctive chemical markers differentiating between Plectranthus s.s. and Coleus species. Oxidation of the B-ring in acylhydrobenzoquinones was identified as a key factor contributing to the strong antibacterial activity of Plectranthus s.l. derived compounds against S. aureus. Analysis of 28 species of Plectranthus s.l. revealed four main clusters based on the accumulation of various abietane classes. The active acylhydrobenzoquinones were a defining factor in the separation of one of the groups of plants while another cluster in both Plectranthus s.s. and Coleus separated based on flavonoid contents. It would be of interest to examine how this trend might be broadened to encompass all species within each genus.