Exercise 17 ultraviolet light

Most Effective Wavelength of UV Light for Killing Bacteria

Pyrimidine dimers

Type of Damage Caused by UV Light

pyrimidine dimers,

Repair Mechanisms in Cells

Light Repair (Photoreactivation):

• Uses the enzyme photolyase, which is activated by visible light.

• Photolyase breaks apart pyrimidine dimers, restoring normal DNA structure.

Dark Repair (Nucleotide Excision Repair):

• A more complex process that does not require light.

• Enzymes remove damaged DNA segments and replace them with new nucleotides.

• DNA polymerase fills the gap, and ligase seals it.

Factors Affecting Germicidal Activity of UV Light

Time of exposure: Longer exposure increases bacterial death.

Wavelength: 260 nm is the most lethal.

Presence of barriers: UV light does not penetrate plastic, glass, or solid surfaces well.

Type of organism: Some bacteria have better DNA repair systems.

Spore formation: Endospores (e.g., Bacillus species) are more UV-resistant than vegetative cells.

Why Are Lids Removed Before Exposure?

Removing the lid ensures direct exposure to UV light

Purpose of the Index Card

The index card is used to block UV light from a portion of the plate, creating a control area that allows comparison between exposed and unexposed bacterial growth.

This helps determine the effectiveness of UV treatment.

Purpose of Keeping the Petri Dish Lid On During UV Exposure

If the lid is kept on, it blocks UV light, reducing its effectiveness.

This can be used as a control to show how barriers prevent UV penetration.

Identifying Damage with Repair vs. Without Repair

With repair: If the bacteria can repair DNA damage, colonies will regrow after exposure.

Without repair: If DNA repair fails, no growth will be seen in UV-exposed areas.

Determining UV Resistance in Different Organisms

If bacteria A survives longer under UV exposure than bacteria B, bacteria A is more resistant.

Resistance can be measured by comparing growth patterns at different exposure times.

Example: If bacteria A survives 5 minutes of UV but bacteria B dies at 2 minutes, bacteria A is 2.5 times more resistant.

Name and Differences Between the Two Bacteria in This Lab

Bacillus megaterium (endospore formation)

Staphylococcus aureus (vegetative cell)

Why Are Exposure Times Different for Different Organisms?

Some bacteria are naturally more resistant to UV light due to DNA repair mechanisms, pigments, or spore formation.

Spore-forming bacteria (e.g., Bacillus subtilis) require longer UV exposure because spores have protective coatingsthat resist damage.

Non-spore-forming bacteria (e.g., Staphylococcus aureus) are more sensitive to UV light and require shorter exposure times to kill them.

Which bacterium appears to be more resistant to the effects of UV radiation

bacillus megaterium ,have endospores, takes longer to destroy cells

What specific mutation or damage does UV light cause to DNA?

Thyamine dimers