Aqueous Leaf Extracts of Peppermint (Mentha × piperita) and White Snakeroot (Ageratina altissima) Exhibit Antibacterial and Antiviral Activity

With new emerging diseases such as COVID-19 and an increasing incidence of cancer, there remains a significant need for investigating new therapeutic options to treat a wide range of ailments and disorders. Peppermint (<i>Mentha</i> × <i>piperita</i>) and white snakeroot (<i>Ageratina altissima</i>) have been used medicinally by native people in the Midwestern United States for centuries. However, the antiproliferative and antimicrobial properties of the aqueous extracts of these plants remain unclear. In this study, we evaluate the therapeutic potential of peppermint and white snakeroot aqueous leaf extracts by examining their activity against mammalian cancer cells, bacteria, and viruses. Both peppermint and snakeroot extracts showed no reductions in viability at concentrations lower than 25 mg/mL and 10 mg/mL, respectively, in two different cancer lines, HEp-2 and DBT-9 cells, in vitro. While treatment with the snakeroot extract resulted in significant disruption to cytoskeletal organization in HEp-2 cells at a concentration of 10 mg/mL, peppermint and snakeroot extracts did not have a major impact on the viability or proliferation of the cancer cell lines tested. Peppermint and snakeroot were then evaluated for antibacterial activity against four different bacterial pathogens. Significant inhibition of bacterial replication was observed for <i>E. coli</i> (at concentrations greater than 0.1 mg/mL) and <i>S. aureus</i> (at concentrations greater than 1 mg/mL) treated with either peppermint or snakeroot extracts. No significant activity was observed against the bacterial strains <i>P. aeruginosa</i> and <i>S. pyogenes</i>. Peppermint (EC₅₀ = 2.36 mg/mL) and snakeroot (EC₅₀ = 2.64 mg/mL) significantly reduce infectivity and replication (at concentrations above 0.2 mg/mL) of the major human pathogen, human respiratory syncytial virus (hRSV). However, testing for antiviral activity against a mouse coronavirus (murine hepatitis virus, MHV) showed no impact on replication at concentrations up to 2.5 mg/mL. Lastly, chemical analysis of the extracts identified several prominent compounds, which were subsequently evaluated for their biological contributions to the observed plant extract phenotypes. Two of the identified compounds, 1,8-cineole (Eucalyptol) and menthol, show significant antimicrobial activity. We report that aqueous extracts of peppermint and white snakeroot exhibit specific antibacterial and antiviral activities that support further investigation for therapeutic potential.