Photoreactivation:

 
Photoreactivation is a biological process observed in certain organisms, particularly bacteria and viruses. Exposure to visible light following damage from UV triggers enzymatic repair mechanisms within the organism’s DNA.
 
When microorganisms are exposed to UV light, particularly in the UV-C range, the UV photons cause damage to the DNA structure, resulting in the formation of pyrimidine dimers. However, in the presence of visible light, specific enzymes called photolyases become activated. These photolyases can recognize and bind to the damaged DNA sites, facilitating the reversal of the DNA damage. Essentially, photoreactivation allows microorganisms to repair the DNA damage caused by UV radiation light, increasing their chances of survival.
 

Dark Repair:

 
Dark repair, also known as nucleotide excision repair, is another biological process observed, particularly in bacteria. It involves repairing DNA damage caused by UV radiation in the absence of visible light.
 
In the absence of visible light, specific enzymes and repair pathways become activated to identify and repair the damaged DNA segments. During dark repair, enzymes recognize the lesions in the DNA strand and excise the damaged portion, replacing it with newly synthesized DNA and restoring the original DNA sequence.
 
Dark repair is an essential mechanism for microorganisms to maintain genome integrity and ensure survival following exposure to UV.
 
These mechanisms can revive a considerable portion of the microorganisms originally killed by UV-C light, reducing the overall effectiveness of the disinfection process.
 

Example:
A study has shown that as much as 60% of microorganisms originally killed with UV-C can be reversed.
(Song, Mohseni and Taghipour, 2019)⁹
 
This study was conducted with E. coli but many other bacteria contain enzymes and repair mechanisms that may allow for photoreactivation and photorecovery from UV exposure.
(Kowalski, 2009)¹⁰

Antimicrobial Resistance:

Repair mechanisms alter the DNA/RNA, which creates a risk of microorganism mutations during the repair process. These mutations could render microorganisms more pathogenic or resistant to future disinfection attempts. This could potentially diminish the effectiveness of UV-C further over time.
(Shibai et al., 2017)¹¹

As a result, it has been recommended that UV-C disinfection may not be entirely reliable as a stand-alone disinfection method and requires the implementation of complementary disinfection strategies to ensure thorough microbial eradication.
(Demeersseman et al., 2023)¹²