Reactivity of Enveloped Virus Genome, Proteins, and Lipids with Free Chlorine and UV254

The current understanding of SARS-CoV-2 is that waterborne transmission is unlikely to be an important mode of infection. While the virus RNA has been reported in untreated sewage (and many more such reports can be anticipated throughout 2020 and beyond), these reports do no imply that viable, infective virus is present. Very few studies have attempted to cultivate SARS-CoV-2 from human excreta or untreated sewage and those which have, have mostly been unsuccessful.

Nonetheless, it is important that water managers have a good understanding of the expected disinfection performance of widely used drinking water and wastewater treatment processes. Even if survivability of SARS-CoV-2 in water is expected to be low, understanding disinfection performance remains a key component of overall public health risk assessment and risk management.

Two very important water treatment processes, used for viral disinfection, are inactivation with free chlorine (“chlorination”), and irradiation with ultraviolet (UV) radiation, typically at a wavelength of 254 nm (UV254).

This paper reports the outcomes of experiments investigating the performance of free chlorine and UV254 for the inactivation of a surrogate virus, Pseudomonas phage Phi6. See this earlier blog-post on the applicability (and limitations) of Phi6 as a surrogate for other enveloped viruses.

Arguably, the most interesting observations from this paper are largely qualitative, rather than quantitative. That is, this paper reveals evidence regarding which parts of the virus (lipids, protein structures, genetic material) are primarily impacted by the two disinfection processes, thus providing insights as to how particular viruses (such as enveloped coronaviruses) might respond, compared to other more heavily studied viruses.

The authors found that free chlorine readily penetrated the lipid envelope to react with proteins in the nucleocapsid and polymerase complex (which is internal to the nucleocapsid). The most reactive Phi6 peptides were approximately 150x more reactive with free chlorine than the most reactive peptides reported in non-enveloped coliphage MS2.

The authors initially hypothesised that the increased susceptibility of Phi6 to free chlorine inactivation compared to non-enveloped viruses was due to reactions in the membrane proteins. However, their results showed that in Phi6 some membrane proteins reacted slower than the nucleocapsid proteins and polymerase complex proteins. This suggests that free chlorine molecules readily penetrate the lipid membrane and react with proteins in the more internal structures of the virus. The authors noted that similar findings have been reported in bacteria, where non-dissociated HOCl molecules could penetrate a negatively charged bacterial membrane to react with intracellular structures.

As the “weakest link” in the overall virus structure to withstand the impact of free chlorine, it is these rapidly oxidised proteins that determine the overall inactivation rate of the virus during chlorination.

The inactivation kinetics of Phi6 by UV254 was comparable with those of nonenveloped adenovirus and coliphage MS2. This was found to be a consequence of the observation that the “weakest link” during exposure to UV254 was reactions with genetic material (DNA or RNA) of the virus. It was observed that the virus envelope provided little protection against (and little enhanced susceptibility to) the UV254 radiation. Thus the overall inactivation rate of the virus was determined by the rate of genetic disruptions (photolysis) caused by UV254.

UV light induces damage to virus genomes, breaking bonds and forming dimeric lesions in DNA and RNA. These lesions prevent both transcription and replication and ultimately lead to inactivation of the virus.

Since enveloped viruses appear to be have similar susceptibility to UV254 as non-enveloped viruses, we can begin to understand that well-studied non-enveloped viruses (such as coliphage MS2) can provide a reasonable surrogate inactivation performance for photolysis of enveloped viruses such as SARS-CoV-2.

REFERENCE:

Ye, Y., Chang, P. H., Hartert, J. and Wigginton, K. R. (2018) Reactivity of Enveloped Virus Genome, Proteins, and Lipids with Free Chlorine and UV254. Environ. Sci. Technol., 52(14), 7698-7708.

https://doi.org/10.1021/acs.est.8b00824

Published by Stuart Khan

Professor of Civil & Environmental Engineering, University of New South Wales

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