Exercise 12: Effect of UV light on Bacterial Growth

Given your results, what conclusions can you draw concerning the length of UV exposure needed for effective killing of the bacteria that you tested? Be sure to explain how your results led to your conclusions.

Based on the results of the experiment, the Serratia marcescens species needs to be exposed to UV light for more than 3 minutes in order to achieve effective killing of the bacteria. I came to this conclusion because all of the plates showed bacteria growth when exposed to UV light for 30 seconds, 1 minute and 3 minutes. Therefore, the Serratia marcescens species needs to be exposed to UV light for more than 3 minutes in order to hinder bacterial growth.

Describe the results you obtained for each plate. In other words, what did each plate look like? (Serratia marcescens)

Control plate: Significant growth of bacteria. The plate is almost completely covered with bacterial colonies. The streak marks are still visible. 30 sec UV exposure with cardboard cutout on plate: There is less bacterial growth than on the control plate but still a significant amount. The M shaped cardboard cut out is easily distinguishable as there is no growth where the cut out was but it is surrounded by growth. 1 min UV exposure with cardboard cutout on plate: There is less bacterial growth than that of the plate exposed for 30 seconds, however, the shape of the cardboard is still distinguishable from the rest of the plate as there is no growth where the cut out is. 3 min UV exposure with cardboard cutout on plate: There is less bacterial growth than that of the plate exposed for 1 minute, however, the shape of the cardboard is still distinguishable from the rest of the plate as there is no growth where the cut out is. 3 min UV exposure with cloth covering the plate: There is significant bacterial growth all over the plate. 3 min UV exposure with Petri dish lid on the plate: There is little to no bacterial growth.

which broth cultures were used

Enterobacter aerogenes, S. marcescens or S. epidermidis

The control plate in exercise 12 about UV exposure, was treated in which of the following ways?

Lawn of bacteria applied but not exposed to UV light

Describe how you will apply the bacterial to the agar to prepare your plates for exposure to UV in exercise 12.

Sterile swap dipped in the culture spread on the agar evenly and rotating the plate several times to create a "lawn" of bacteria

Did the results from the other groups look similar to yours? Was there an apparent difference in the effect of the UV on each species tested?

The Staphylococcus plates had little to no growth with or without UV light exposure. The Enterobacter aerogenes plate had far less growth than the Serratia marcescens plates after exposure to UV light.

Given your results, what conclusions can you draw concerning the ability of UV light to effectively penetrate through various substances, such as cardboard, plastic and cloth, to kill bacteria? Be sure to explain how your results led to your conclusions.

The ranking of substances from easiest to penetrate by UV light to most difficult to penetrate is as follows: cardboard, plastic, cloth. Cardboard is the easiest to penetrate because on the plate that had the cardboard cut out that was exposed to UV light for 3 minutes, there was no bacterial growth. The plate with the lid on had little growth and plastic was therefore the second most penetrable. The cloth had lots of bacterial growth and was therefore the most difficult substance to penetrate with UV light.

Briefly describe how the UV light is distorting the DNA. How does this damaged DNA affect bacterial cell division?

UV radiation directly hits and damages the DNA molecule. UV radiation can cause the formation of thymine dimers which distort the double helix of DNA inhibit proper enzyme activity necessary for DNA replication and transcription.

what changes when a thymine dimer is generated

a covalent bond is formed between the two thymine bases and the DNA double helix is distorted

what are thymine residues normally attracted to

adenine bases on the opposite strand of the double helix

what kinds effects can these mutations have depending on the location of the damage on the DNA molecule

beneficial, harmful or neutral effects

what happens when DNA is damaged by physical or chemical agents

cells acquire mutations that are passed down from one generation to the next

what did you do in this experiment

compared how the time of exposure affects cell growth and test the ability of UV light to penetrate various substances

what does this distortion inhibit

critical enzymes involved in copying the DNA molecule for protein production

what does UV light do to DNA

directly hits DNA molecule and causes physical damage

why is UV light harmful to cells

due to its effect on the DNA molecule

locations that UV light can be used for germ control include

laboratories, surgical suites and water treatment plants

The majority of DNA mutations that occur have a _______ effect on the cell?

negative or neutral

more often, mutations are...

neutral or harmful

because of this action, UV light is a form of...

non-ionizing radiation that can be used for germ control under certain conditions

harmful types of damage can lead to what kind of problems

problems with basic metabolic processes, uncontrolled cell growth and even cell death

beneficial mutations are...

rare, but can happen

if a low level of damage occurs...

the cell can repair the damaged DNA (although mutation rates go up)

if enough damage occurs...

the cell will not survive

what is essential to the proper function of all cells

the integrity of the DNA molecule

UV radiation can cause the formation of _______ which inhibit proper enzyme activity necessary for DNA replication and transcription.

thymine dimers

what is one common damage from UV light

when 2 thymine residues located next to each other in the molecule are hit by the UV light leaving a thymine dimer

procedure day 1

1. dip a sterile swab in the culture for abt 5 seconds 2. rub swab over entire surface of a nutrient plate to create a lawn (may want to use hockey stick to ensure the plate is uniformly covered) 3. repeat this for the 5 other nutrient agar plates 4. label one plate "control" and set aside (this will NOT be exposed to UV light) 5. label 3 plates: "30 sec", "1 min", and "3 min" 6. take these 3 plates to the UV light work area, REMOVE the lids, and place cardboard covers on top 7. expose the plates to UV light for 30 sec, 1 min, and 3 min by sliding them inside the UV box 8. remove the cardboard and put the lids back on 9. label the last 2 plates: "cloth" and "lid on" 10. take these 2 plates to the UV light work area, remove the lid from the plate marked "cloth" and place a cloth ring so that the ENTIRE SURFACE of the plate is covered 11. expose the "cloth" and "lid on" plates to UV light for 3 min 12. replace the original lids on the plates and invert all 6 plates in a stack, and place in the 37 degree incubator