Purpose: To determine the efficacy and the antiviral mechanisms of action of six plant antimicrobials against murine norovirus type 1 (MNV-1), a human norovirus surrogate.
Methods: MNV-1 was used in tests with oregano oil (OO), carvacrol (CV), lemongrass oil (LO), citral (CI), allspice oil (AO), and clove bud oil (CBO). In addition to a cell infectivity assay, an RNase protection assay, a cell binding assay, and transmission electron microscopy (TEM) were performed to elucidate the antiviral mechanisms of action.
Results: OO, CV, and AO produced significant (P ≤ 0.05) reductions in MNV-1 of 1.0-1.8-log within 15-30min of exposure. CV was the most effective, resulting in a 3.9-log reduction within 1h. LO, CI, AO, and CBO required 24h to achieve similar reductions (2.7-,3.0-,3.4-,2.9-log, respectively). OO only achieved a 1.1-log reduction after 24h. OO, CV, AO, and CBO had no effect on virus adsorption to host cells; LO and CI caused indiscriminate binding to host cells/culture plates. OO, CV, AO, and CBO all caused a loss of MNV-1 capsid integrity, exposing the genome to the effects of the RNase enzyme. The MNV-1 treated with LO and CI was unaffected. Under TEM, all six antimicrobials caused expansion of the virus particles from ≤35nm to up to double in size (OO, AO, CBO) or larger (500-800nm; LO, CI, CV). With CV and CBO, virus capsid disintegration was directly observed.
Significance: These antimicrobials appear to act upon the non-enveloped MNV-1 through a variety of mechanisms such as the disintegration of the virus capsid, degradation of the RNA genome, and coating of the capsid, thereby preventing specific adsorption to host cells. Understanding the mechanisms of such antiviral activity is important in order to predict the efficacy of similar compounds against foodborne viral pathogens.