2-12: UV light

kills by direct contact

what does short-wave (UV-C) light do

causes T-T dimer adducts which result in DNA mutations (nucleotide mutations)

what does 260 nm UV-C do

- bacteriocidal

- death occurs when the concentration of T-T dimers overwhelms the prokaryotic DNA repair machinery and a resulting frameshift mutation occurs in an essential gene

is 260 nm UV-C bacteriocidal or bacteriostatic at sufficient UV intensities

damages protein shapes

what does 280 nm UV-C do

- 280 nm is very efficiently absorbed by aromatic ring amino acids (tyrosine, tryptophan, phenylalanine)

- as aromatic ring amino acids absorb the energy contained in UV light, this causes protein shape changes that block protein function

describe what occurs when a microbe is exposed to 280 nm UV-C

- bacteriostatic because it causes slow growth

- it can become bacteriocidal if enough UV light blocks protein shape and function for a long enough time (the overall metabolism of the cell will be greatly diminished causing cell death)

is 280 nm UV-C bacteriocidal or bacteriostatic at sufficient UV intensities

- they are ionizing meaning they have enough energy to remove tightly bound electrons from atoms (are considered the highest energy photons because shorter wave length = higher power)

- the high-energy photons can strike DNA and break covalent bonds (phosphodiester bonds) which severs one of two of the strands (chromosomal mutation)

are X-rays and gamma-rays ionizing or non-ionizing types of radiation; how do they mutate DNA

1. enzyme - uvcA operon codes for photolyase

2. cellular - will form endospores

3. 4x genome DNA repair system

state the 3 ways microbes will try to combat UV-C radiation

Bacillus subtilis (1 day old) and Escherichia coli

give examples of organisms we used in class that use photolyase to combat UV-C radiation

Bacillus subtilis (7 days old)

give examples of organisms we used in class that use endospores to combat UV-C radiation

Deinococcus radiophilus

give example of the organism in class that uses 4x genome DNA repair system to combat UV-C radiation