Secreted material and antimicrobial activity of Salvia cacaliaefolia Benth.

Description

The genus Salvia is known to possess bioactive constituents, commonly found in the complex secretion product of the aerial parts of these plants (1). The study of the chemical constituents of this surface exudate material (2) may thus be useful for the discovery of biologically active compounds, particularly terpenoids (3), that have shown antioxidant, anti-bacterial and anti-fungal activities against human pathogens (4). In a previous screening of antimicrobial activity of the surface exudates of various Salvia species, Salvia cacaliaefolia showed promising activity on selected multi-drug-resistant Gram-positive strains such as Staphylococcus epidermidis, Staphylococcus aureus, Enterococcus faecium and Enterococcus faecalis. Aim of our study is to describe the morphology of the glandular trichomes, the histochemistry of the secretion products, the isolation of the isoprenoid constituents from the exudate and the evaluation of their antimicrobial activity. S. cacaliaefolia Benth. is a Mexican perennial herb, included into subgenus Calosphace, sectio Standleyana (5). Light and scanning electron microscopy were used to determine the morphology of the glandular hairs; histochemical procedures were used to localize the secreted substances within and on the surfaces of the hairs. The glandular hairs occurred as peltate and capitate (1). The peltate trichomes consisted of one basal epidermal cell, one stalk cell and a broad head of four secretory cells. The capitate trichomes appeared in two different types: type I, made up of a basal epidermal cell, a short stalk cell and a round head of one or two secretory cells, and type II, with a basal epidermal cell, a stalk cell, a neck cell and a secretory head of one elongated cell. In all the mature organs, the histological findings revealed a complex secretion product that appeared as an emulsion, with flavonoids and terpenoids, as reported for other Salvia species (6). The extraction of the secreted material afforded four abietane diterpenes (carnosol, 7-methoxyrosmanol, carnosic acid and 12-methoxycarnosic acid) identified by comparison with authentic samples, three triterpenes (betulinic acid, ursolic acid, ursolic aldehyde), and guaiol, identified by their physical and spectroscopic data which were largely consistent with those published in the literature (7-9). The GC-MS analysis of the secreted material revealed the presence of guaiol, - and -eudesmol, camphor, borneol, -copaene, and caryophillene oxide as major components. For some of these compounds the antibacterial activity was already characterized (10-13), while for 7-methoxyrosmanol (MR) and 12-methoxycarnosic acid (MCA) it was first reported here. These compounds were particularly active on several methicillin resistant and susceptible S. aureus and S. epidermidis strains (MIC: 8 μg/ml for MR on both species; 16 μg/ml and 16-32 μg/ml for MCA on S. aureus and S. epidermidis, respectively). Studies aimed to determine the mechanism of action of these diterpenes on the susceptible bacterial species are ongoing. S. cacaliaefolia can be considered as a source of potential antibacterial compounds with activity against resistant or multi-resistant human pathogens.

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