Background: Extracts of medicinal plants are increasingly of interest as novel drugs for antimicrobial and antiviral agents, since microorganisms might develop resistance to commonly used antimicrobial or antiviral agents. Materials and Methods: Ethanolic extracts from Lamiaceae plants prunella, peppermint, rosemary and thyme were phytochemically characterised. The inhibitory activity of four 20% ethanolic plant extracts and four 80% ethanolic extracts against herpes simplex virus (HSV) strains was tested in cell culture. Results: Rosmarinic acid, a typical compound in Lamiaceae species, was identified in the extracts except for thyme 20% ethanolic extract. In addition, some other phenolic compounds such as apigenin- and luteolin-derivatives were identified in different amounts. All extracts exhibited high and concentration-dependent levels of antiviral activity against free acyclovir-sensitive and acyclovir-resistant HSV-1 strains with 50% inhibitory concentrations of 0.05–0.82 μg/ml. Mechanistically, exposure of free virions as well as host cells to prunella and peppermint 80% ethanolic extracts at maximum non-cytotoxic concentrations prior to infection reduced plaque formation drastically. Thus, both extracts revealed a dual mode of action similar to aqueous lemon balm extracts. Conclusions: Since infectivity of acyclovir-susceptible and acyclovir-resistant HSV strains was significantly reduced with Lamiaceae extracts, the results obtained indicate that ethanolic plant extracts affected herpesvirus prior to and during adsorption and in a different way than acyclovir. Based on its dual mode of action, e.g. antiviral effect against free virions and blocking virus attachment to host cells, prunella and peppermint 80% ethanolic extracts are promising antiviral agents in recurrent herpes labialis for topical therapeutic applications.

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