Lab Report 3: Effect of UV Light on Microbial Growth
Kristin Holmes – April 2, 2013
PURPOSE: The purpose of this lab is to determine the effects of ultraviolet light on microbial growth and the effectiveness of the repair mechanisms of light repair and dark repair on UV damage.
Can Ultraviolet (UV) light be a viable form of sterilization and/or disinfection? This lab experiment will look to examine and answer that question. UV light is a form of electromagnetic radiation that is invisible to the human eye. It has a short wavelength and is considered high energy which allows it to pass through some materials. The biological effects are potentially devastating based on the ...view middle of the document...
The two mechanisms for repair that are being examined in this experiment are Light repair and Dark repair. Light repair is also known as photoactivation. Light repair uses photolyase which detects and attaches to the abnormal pyrimidine binding sites with single protons and single electrons projecting towards the dimer. The proton and electron projections create a cascade of reactions that nudge the mutated nucleotides back into place. One of the cool things is that the proton and electron are not lost in the reactions and return to the photolyase to be used again. Sadly, mammals do not contain photolyase any longer (lost in evolution). (Krakauer) The other method of repair is Dark repair – also known as excision repair. During Dark repair the damaged DNA is excised and a new strand of the missing DNA is copied in by DNA polymerase. (Menard)
This experiment will examine the effects of timed exposure to short-wave UV light and the repair effects of both methods of repair mentioned above on the bacteria Serratia marcescens.
MATERIALS AND METHODS:
Organisms: Serratia marcescens cultured in broth.
Medium: 7 Nutrient Agar (NA) plates.
Miscellaneous: Sterile cotton swabs, 5 filter paper templates, aluminum foil, Bunsen burner and UV light box.
This was a “bench effort” experiment. To begin with the 7 NA plates were labeled with initials, bacteria, the date of 3/26/13 and each numbered 1 through 7. Using sterile swabs, each NA plate was swabbed with S. marcescens bacteria (to completely cover the agar).
Plates 1 & 2 were exposed to UV light for 30 seconds, with the lids removed and covered by a filter paper mask. Plate 3 was then exposed to the UV light for 3 minutes without the lid and covered by a filter paper mask. Plates 4 & 5 were exposed to UV light for 3 minutes as well; however the lids remained on and they were each covered with a paper filter mask. Plates 6 & 7 were not exposed to UV light at all.
Plates 2, 5, & 7 were stacked together, wrapped in aluminum foil, and set closest to the wall under the window. The remaining plates were spread in a row giving them exposure to the natural light coming in through the window. All plates were left on the worktable for 48 hours.
Plate | Growth in exposed areas of mask | Growth in area covered by mask | Interpretation |
1 | Several Single colonies | Confluent | Damage, some repair |
2 | Very little growth – a few single colonies of bacteria have grown near edges of mask cover. | Confluent | Damage, very minimal repair |
3 | Confluent
(This plate should have shown extensive damage and no regrowth.) | Confluent | Inconclusive – It appears the lid may have not been removed. Should have seen no growth in exposed area of mask with much more defined edges between exposed and covered areas. |
4 | Confluent | Confluent | No damage |
5 | Confluent | Confluent | No damage |
6 | N/A | Growth | Control |
7 | N/A | Growth | Control...