The key reference for this post is a paper titled “Nucleic Acid Photolysis by UV254 and the Impact of Virus Encapsidation”. This paper was published in Environmental Science and Technology in 2018 and reports the outcomes of work undertaken by the research group of Dr Krista Wigginton at the University of Michigan.
This paper provides fundamental insights to what’s going on when we use UV radiation to inactivate viruses. We can use these insights to begin to infer the likely susceptibility of the virus SARS-CoV-2 to inactivation by ultraviolet (UV) radiation during wastewater or drinking water treatment.
First, some UV disinfection basics:
UV radiation in the “UVC” wavelength range (200-280 nm) is known to be effective for inactivating (“killing”) many types of microorganisms, including viruses, bacteria and protozoa. Thus it is increasingly used in wastewater and drinking water treatment plants for this purpose.
There are two main types of lamps used to produce the UVC radiation for water treatment plants, some of which produce a narrow band of UV wavelengths and some produce a broader range of wavelengths. But in all cases, UV radiation at 254 nm is produced and this wavelength is known to be highly effective for microbial inactivation. Thus UV of 254 nm wavelength (known as UV254) is the focus of practically all UV-related water treatment, process control, regulation and scientific studies.
When UV radiation is used to cause direct chemical disruptions to microorganisms (or to chemical contaminants), we refer to this as “direct UV photolysis”. Often, more indirect pathways are also important; whereby the UV causes reactions with other chemicals present in the water, the products of which then go on to inactivate the microorganisms. We call these processes “indirect UV photolysis”.
We know that UV inactivation of microorganisms is primarily achieved by the UV radiation causing chemical disruptions by direct UV photolysis of the microorganisms’ genetic material (DNA or RNA), known as nucleic acids.
DNA and RNA are polymers, composed of monomer units called nucleotides. Key components of these nucleotides are ‘nucleobases’. There are two structural forms of nucleobases, known as pyrimidine nucleobases (thymine (T), cytosine (C), and uracil (U)) and purine nucleobases (adenine (A) and guanine (G)). These nucleobases strongly absorb UV254 radiation, leading to photochemical reactions, including the formation of chemical bonds between two consecutive nucleobases (a process known as ‘dimerization’). The most studied are the formation of TT dimer products, but TC, CT, CC, and UU dimer products have also been reported.
These dimerization reactions disrupt essential genetic processes, such as transcription and replication, and ultimately lead to inactivation of the microorganism. Other photochemical reactions can also contribute to inactivation, such as hydration, protein-nucleic acid linkages, covalent cross-links between complementary strands, and nucleic acid backbone breakages.
The experiments undertaken by Wigginton and her team:
In this study, the authors measured the direct UV254 photolysis kinetics of four carefully selected model viruses, representing four types of viral genetic material:
- Single-stranded RNA
- Double-stranded RNA
- Single-stranded DNA
- Double-stranded DNA
They examined the reactions of these four types of genetic material in two different forms:
- As “naked nucleic acids” (meaning just the DNA or RNA, without the rest of the virus structure)
- As native virus particles (including their full virus structure, such as in a protein-based ‘shell’, called a capsid).
When studying naked nucleic acids, single-stranded DNA was observed to undergo the most rapid photolysis reactions, followed by single-stranded RNA and double-stranded DNA. The slowest photolysis reactions were observed for double-stranded RNA, indicating a higher level of resistance for these types of viruses to UV254 photolysis.
The authors hypothesised that for native virus particles, with the genetic material compressed inside a protein capsid, photo-reactivity might be reduced. This hypothesis was more related to structural conformations that might limit the opportunity for dimerization reactions, rather than a significant protective function of the capsid itself.
However, they found that for three of the four virus types tested (single-stranded RNA, double-stranded RNA and single-stranded DNA), encapsidation did not impact the reaction kinetics of the nucleic acids. Furthermore, they observed slightly faster reaction kinetics (~1.2x) for double-stranded DNA when encapsidated, although the statistical significance of this observation was marginal.
So what could this mean for SARS-CoV-2?
SARS-CoV-2 is a single-stranded RNA virus with both a protein capsid and a lipid envelope. Other work by the same research group has shown that the lipid envelope doesn’t provide any significant protection from UV254 photolysis. From the current work, we also know that the capsid does not play a significant role in susceptibility to UV254 photolysis. The most significant feature is the nature of the genetic material, which is single-stranded RNA.
Single-stranded RNA was observed to react less quickly than single-stranded DNA, roughly the same rate as double stranded DNA, and significantly more quickly than double-stranded RNA. Using this information, we can make a rough estimation of the likely susceptibility of SARS-CoV-2 to UV254 photolysis, compared to other known waterborne viruses, which are regularly targeted for inactivation by UV254 photolysis.
Doing that, we might suppose that SARS-CoV-2 has:
- Roughly the same susceptibility to UV photolysis as other single-stranded RNA viruses (e.g. Hepatovirus A, Polioviruses, Noroviruses, Coxsackieviruses, and MS2 bacteriophage);
- Roughly the same susceptibility to UV photolysis as double-stranded DNA viruses (e.g. Adenoviruses);
- Greater susceptibility to UV photolysis than double-stranded RNA viruses (e.g. Rotaviruses).
Of course, these suppositions will need to be validated before we can be certain of their accuracy. However, until such direct validation is available, we can assume that they are based on the best available information.
A logical conclusion is that current UV disinfection practices, targeting waterborne viruses such as those listed above, can be assumed to be (at least) similarly effective for the inactivation of SARS-CoV-2.
You are welcome to respond to this conclusion in the comments section below.
Qiao Z, Ye Y, Chang PH, Thirunarayanan D and Wigginton KR (2018) Nucleic Acid Photolysis by UV254 and the Impact of Virus Encapsidation. Environmental Science & Technology, 52(18), 10408-10415.