The antimicrobial activity of essential oil of Hyssopus ocinalis L. var decumbens (Jordan & Fourr.) Briq. from France (Banon) and Hyssopus ocinalis L. from Italy (Piedmont) was studied taking account of their chemical composition determined by GC and GC–MS. Pinocamphone and isopinocamphone are present in H. ocinalis (4.4% and 43.3%, respectively), according to the ISO 9841 Standard (1991 E) but they are lacking in var. decumbens, where linalol (51.7%), 1,8-cineole (12.3%) and limonene (5.1%) instead are
predominant. The disc diusion tests carried out on Gram-positive (Staphylococcus aureus and Enterococcus spp.) and Gram-negative bacteria (Klebsiella oxytoca, Escherichia coli, Proteus mirabilis, Pseudomonas spp. and two strains of Salmonella spp.) showed an antimicrobial activity generally negligible for H. ocinalis, but broader, and in a few cases more evident (Enterococcus spp. and E. coli), for var. decumbens. All yeasts (seven strains of Candida albicans, C. krusei and C. tropicalis) were strongly inhibited by both species. In liquid medium the MIC of H. ocialis was always 41.2% v/v for bacteria and between 0.6 and 1.2% v/v for yeasts, while the MIC of var. decumbens was between 0.15 and 0.6% v/v for the Gram-positive bacteria, 0.3 and 1.2% v/v for the Gram-negative bacteria and 0.15 and 0.3% v/v for the yeasts. The effect of var. decumbens was generally bactericidal. Linalol, and in a lesser way, 1,8-cineole, may contribute to the greater antimicrobial activity of var. decumbens in comparison with H. ocinalis, while limonene may be responsible for the antimycotic action observed in both oils, as suggested by results of the disc diusion tests carried out on the pure reference substances.