Rosin is a solid resin obtained from pine trees used for various applications, including as soap. Rosin contains numerous lipid-soluble acids and has previously demonstrated antibacterial activity against Gram-positive bacteria, in particular, acting by disruption of the cell membrane.
In a research paper recently uploaded to the preprint server bioRxiv* by Bell et al. (July 19th, 2021), the potential of rosin soap as a virucide is investigated, finding a ten-thousand-fold drop in infectivity against enveloped viruses such as influenza and SARS-CoV-2 in particular.
How was the study performed?
The group infected Vero cells with influenza A virus, respiratory syncytial virus (RSV), SARS-CoV-2, or encephalomyocarditis virus (EMCV), Madin-Darby canine kidney (MDCK) cells with influenza A, and HEp-2 cells with RSV, subsequently exposing each to rosin solution at concentrations of 0.025%-2.5% for 5-30 minutes.
The cytopathic effect of infection was also assessed 72 hours post-treatment, as the severity of infection can be inferred from the degree of cell rounding and death observed, especially in influenza-sensitive cell lines such as Madin-Darby Canine Kidney (MDCK).
The acids of the rosin soap were found to be cytotoxic to MDCK cells at the highest concentration tested, though the efficacy of virus inactivation against influenza was higher.
To test the potential of rosin as a surface disinfectant, not intended to be applied directly to the hands, rosin soap at 2.5% concentration was exposed to the virus for five minutes before filtration and purification of the virus and subsequent exposure to cells. The virus was effectively inactivated, showing almost no infectivity towards the cells, while no residual cytotoxicity was observed towards the cells following the purification process.
Effect of Rosin soap treatment on a panel of virus infectivity in solution compared to mock (DMEM). Three enveloped (IAV Udorn strain; RSV and SARS-CoV-2 [SARS2]) and one non-enveloped (EMCV) virus was used. Virus suspensions were incubated with Rosin Acid solution at 37 oC for 5 minutes before residual infectivity was determined via dilution on susceptible cells (MDCK cells for IAV, Vero cells for RSV, S2 and EMCV). Infectious virus titer corresponds to the reciprocal of the final dilution giving virus-induced cytopathic effect. Background (dashed lines) delineates the dilution that the Rosin soap treatment was toxic to the susceptible cells.
Membrane disrupting activity
EMCV is a model non-enveloped virus often used in biomedical research. In identical experiments to the above virus, infectivity was not reduced by exposure to rosin soap. At the same time, it was observed that each of the enveloped viruses was affected by the soap in a concentration-dependent manner. Incubation time with the soap did not notably influence the virucidal rate. There was little observed difference in infectivity amongst any of the enveloped viruses, whether exposed for five or thirty minutes. Similarly, incubation temperature with rosin had little influence on the efficacy of the soap as an antiviral, whether kept at 37⁰C, 20⁰C, or 4⁰C.
Many human pathogenic viruses can persist on surfaces for long periods of time, providing the opportunity for direct transmission to numerous individuals. The SARS-CoV-2 pandemic has urged the search for safe, cheap, and environmentally friendly surface cleaners and personal soaps that are suitable for repeated use throughout the day. This work suggests that rosin soap could be one solution.
As the soap is ineffective against non-enveloped viruses, it likely has a mechanism of action based on disruption of the lipid membrane, as against Gram-positive bacteria. However, the group suggests that other non-enveloped viruses such as norovirus could be tested to determine if there is any effect. The precise compound, if such a single example exists, within rosin soap that exerts membrane-disrupting activity against viruses has not been identified. However, the effect is likely one of adjusted surface tensions by surfactants.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.