Antimicrobial and ATP synthase modulating activity of the surface constituents of Salvia tingitana Etl. (Lamiaceae)

Description

During our research on plant compounds from Salvia species able to face the constantly increasing resistance to antibiotics (1), the antimicrobial properties of the dichloromethane extract obtained from the surface of the aerial parts of S. tingitana were investigated following a bioassay-oriented fractionation approach. The surface extract showed moderate antimicrobial activity against Gram positive multi-resistant bacterial strains; no activity was observed against the Gram-negative strains or Candida albicans. The hexane insoluble and soluble fractions of the crude extract, as well as the semi purified fractions obtained by silica gel column chromatography (Ia-VIa and Ib-VIIb) were evaluated at doses corresponding to that of the crude extract. Among the tested fractions, fraction Ia, IVa, Va and VIa were active against Staphylococcus aureus and S. epidermidis. Fractions IIIb, IVb and were active against S. aureus, S. epidermidis, Enterococcus faecium and E. faecalis, while Vb was active against the two Staphylococcus species. The separation of the fractions afforded eight new and five known sesterterpenes and one new norsesterterpene, along with other known compounds including five labdane and one abietane diterpenes, one sesquiterpene and four flavonoids, identified by IR, 1D and 2D NMR, HR-MS analysis. The antimicrobial activity of the isolates was analysed on several Gram positive multi-resistant bacterial strains, also of marine origin (S. aureus, S. epidermidis, S. haemoliticus, S. hominis, S. capitis, S. lugdunensis, S. saprophyticus, S. mitis, E. faecium, E. faecalis, E. durans, E. gallinarum, E. casseliflavus, E. gallolyticus). The compounds showed antimicrobial activities against Gram positive bacteria with MIC values ranging from 32 to 128 μg/mL, with the exception of manool and sclareol, which exhibited MIC values ranging from 4 to 64 μg/mL. Moreover, taking into account that the modulation of ATP synthase has been described as the base of the activity of several compounds against multidrug-resistant mycobacteria and Gram-positive pathogens and that the overall structure and energy transduction mechanism of the ATP synthase are well conserved from bacteria to mammalians (2), the inhibitory action of the more active compounds on the ectopic F1Fo-ATP synthase (ATP synthase) of purified rod outer segments (OS), as a subcellular system naturally expressing extra-vesicularly the molecular machinery for the oxidative phosphorylation, was also evaluated (3, 4). The compounds that displayed the best antibacterial activities i.e. salvileucolide methylester, sclareol and manool showed ATP synthase inhibition of 60, 70 and 80%, respectively. Interestingly, their time killing curves, performed against the susceptible bacterial species, showed bacteriostatic activity.

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