Antibacterial iridoid glucosides from Eremostachys laciniata

Eremostachys laciniata (L) Bunge (family: Lamiaceae alt. Labiatae; subfamily: Lamioideae) is one of the 15 endemic Iranian herbs of the genus Eremostachys. A decoction of the roots and flowers of E. laciniata has traditionally been taken orally for the treatment of allergies, headache and liver diseases. Three antibacterial iridoid glucosides, phloyoside I (1), phlomiol (2) and pulchelloside I (3) have been isolated from the rhizomes of this plant. The structures of these compounds were elucidated unequivocally by a series of 1D and 2D NMR analyses. The antibacterial activity and brine shrimp toxicity of these compounds were assessed using the resazurin microtitre assay and the brine shrimp lethality assay, respectively. All three iridoid glycosides 1-3 exhibited from low to moderate levels (MIC = 0.05-0.50 mg/mL) of antibacterial activity. Of these compounds, compound 3 was the most active, and displayed antibacterial activity against 9 of 12 different strains tested. The most noteworthy activity of 3 was against Bacillus cereus, penicillin-resistant Escherichia coli, Proteus mirabilis and Staphylococcus aureus with an MIC value of 0.05 mg/mL.