Antiviral efficacy and mechanisms of action of oregano essential oil and its primary component carvacrol against murine norovirus

AIMS: To investigate the antiviral efficacy of oregano oil and its primary active component, carvacrol, against the nonenveloped murine norovirus (MNV), a human norovirus surrogate. METHODS AND RESULTS: Along with an observed loss in cell culture infectivity, the antiviral mechanisms of action were determined in side‐by‐side experiments including a cell‐binding assay, an RNase I protection assay and transmission electron microscopy (TEM). Both antimicrobials produced statistically significant reductions (P ≤ 0·05) in virus infectivity within 15 min of exposure (c. 1·0‐log₁₀). Despite this, the MNV infectivity remained stable with increasing time exposure to oregano oil (1·07‐log₁₀after 24 h), while carvacrol was far more effective, producing up to 3·87‐log₁₀reductions within 1 h. Based on the RNase I protection assay, both antimicrobials appeared to act directly upon the virus capsid and subsequently the RNA. Under TEM, the capsids enlarged from ≤35 nm in diameter to up to 75 nm following treatment with oregano oil and up to 800 nm with carvacrol; with greater expansion, capsid disintegration could be observed. Virus adsorption to host cells did not appear to be affected by either antimicrobial. CONCLUSIONS: Our results demonstrate that carvacrol is effective in inactivating MNV within 1 h of exposure by acting directly on the viral capsid and subsequently the RNA. SIGNIFICANCE AND IMPACT OF THE STUDY: This study provides novel findings on the antiviral properties of oregano oil and carvacrol against MNV and demonstrates the potential of carvacrol as a natural food and surface (fomite) sanitizer to control human norovirus.