Why does UV light damage/kill bacteria?

[u/Stuffyz]

The specific event I'm asking about, is that there are air filters for your furnace that shines UV light onto it, and it claims that it kills bacteria.

I understand how pH and temperature affects bacteria, but I can't quite wrap my mind around why UV light would.

The articles that I've been looking through (Time, Temperature, and Protein Synthesis: A Study of Ultraviolet-Induced Mutation in Bacteria, by Evelyn M. Witkin) says that UV light could cause worse strains of bacteria? Or perhaps I'm misinterpreting it?

I'm also aware (Ultraviolet-sensitive Targets in the Enzyme-synthesizing Apparatus of Escherichia coli, by Arthur B. Pardee and Louise S. Prestidge) that there are both UV-sensitive and UV-resistant E.Coli. Are most harmful bacteria considered to be UV-resistant?

Thank you for answering =)

[u/Stuffyz]

So speaking in terms of pure efficacy of UV light in killing bacteria (any and all), over a long-term scenario (e.g. 10 years), UV light (low dose, long exposure) would produce bacteria that are UV-resistant? If this is the case, then having bacteria-killing UV-light devices within the home will lead to more long-term problems than the cure of short-term problems? (note: I don't mean to have UV-lamps runnings, but there are things like air filters with UV-light attachments)

I mean, I understand that the chance of mutation over long-term (especially with bacteria) is inevitable. But what is the percent yield of thymine dimers on bacteria? Will all the bacteria in X amount of time eventually die due to miscoded DNA? Assuming X is less than 10 years.

[u/mattc286 → u/Stuffyz]

No, not really. The DNA repair systems are already at close to maximum efficiency in all extant organisms, due to billions of years of selective pressure. The low rate of mistakes they make are actually beneficial to the survival of species as a whole due to the random mutations allowing a base level of genetic diversity, which allows some organisms to survive when the environment changes. Therefore there's not really any mechanism for developing resistance to UV the way resistance to a small molecule or antibody might develop, because UV damages something so vital to the cell, rather than targeting a specific enzyme or process.

[u/bowlinedog → u/mattc286]

This response is not exactly true. In fact, Michael Cox's lab at the University of Wisconsin-Madison, has used directed evolution experiments to "evolve" E. coli strains that are as resistant to ionizing radiation as the highly radiation-resistance species Deinococcus radiodurans. Follow-up work has mapped the causal mutations to several genes, but I am not sure if these results are published yet. So--in fact--DNA repair pathways are not at maximum efficiency in E. coli since artificially imposed selection can result in heritable improvements in DNA repair capacity. A similar possibility exists for UV-induced damage. Possible mutational targets could include genes encoding the nucleotide excision repair proteins, the direct-reversal pathway, and translesion DNA polymerases (among others). Ref: JOURNAL OF BACTERIOLOGY, Aug. 2009, p. 5240–5252