Effect of Radiation on Bacterial Growth

DNA sequence changes can result from cell mutations passed down from a parent organism to its children. Some cell mutations can be advantageous, but the majority are damaging because they result in the cell's ability to perform a specific function being lost. Bacteria naturally experience base pair mutations at a rate of about 10-6 per reproduction occurrence (Smith, 1976). There is a rapid increase in mutations when this happens in the presence of a mutagen. Mutagens, which can be substances like colchicine or electromagnetic radiations, speed up mutations (Jagger, 1985) Electromagnetic radiations are able to remove electrons from a cell leading to formation of ions known as free radicals. These ions can damage cell RNA or DNA through oxidation. Ultra Violet light causes mutagenesis by excitation of electrons in a cell. When the electrons in DNA are excited, there is formation of bonds in pyrimidines that are close together particularly thymine. The bond forms a pyrimidine dimer. The formed pyrimidine dimers alter the DNA shape resulting in problems in cell replication. This knowledge is very important especially in the food industry for preservation of perishable food and products and treatment of water (Schunck, 1917).

Objectives of Experiment

To understand how ultraviolet light can be used to kill bacteria.

To find out whether ultraviolet light can kill all bacteria or not. If not, why?

The hypothesis for the experiment was that ultra violet radiations can kill bacteria differently when exposed for varying durations to UV light.

Methodology

We worked in paired groups where one group was given a culture with the D. radiodurans while the other group was given culture containing E. coli bacteria. We then diluted the bacteria to 10-4. 0.1 ml of the diluted solution was placed in the TSA plate and a sterile swab was used to spread the bacteria evenly on the plate.

The bacteria was then exposed to UV light by removing the of the TSA plate. The plate was placed in the transilluminator with the open side down so that each half of each plate was on the metal edge of the box. One half was exposed to UV light while the other half was not exposed to UV light. The transilluminator was turned on and readings were recorded accurately in a table at intervals of 5, 10, 20, 40 and 60 seconds. The plates were then incubated at 37° C for 24 to 48 hours. We calculated the number of colony-forming units per ml.

Results

Table 12-1: Number of colony-forming units per ml



E. coli



D. radiodurans





CFUs/ml



CFUs/ml



Time

-uv

+uv

-uv

+uv

5s

0.003

0.002

0.002

0.002

10s

0.003

0.002

0.002

0.003

20s

0.003

0.002

0.002

0.003

40s

0.003

0.002

0.002

0.002

60s

0.003

0.002

0.002

0.002







Discussion

The petri dish plate covers are made of either thick glass or plastic UV rays cannot penetrate through them to get an adequate reaction from the bacteria solution.

The control experiment was leaving the other half of the TSA slide to observe what happens in the absence of UV light. Using D. radiodurans bacteria can also be considered a control to observe whether all bacteria are killed by UV radiations.

E. coli bacteria grew on the unexposed side of the TSA plate. It did not grow on the UV-exposed side because the UV radiation inhibited the reproduction of the bacteria colonies by damaging the DNA.

There was a difference in the number and color of E.coli colonies for the given time intervals. The longer the E.coli bacteria was exposed to UV irradiation, the more the colony-forming units decreased. There was still growth after UV exposure and this can be attributed to the bacteria developing resistance to the UV radiation and also the UV radiation wavelength was not strong enough.

D. radiodurans grew on both the exposed and unexposed sides of the TSA plate because it ionizes the UV radiation and also it has a fast DNA repair mechanism therefore its DNA repairs faster than it is destroyed.

During the various exposure times, there was a difference in the number of D. radiodurans. It increased over time as the UV radiation had no effect on it due to its resistance.

























References

Jagger, J. (1985). Solar-UV actions on living cells. New York: Praeger.

Schunck, C. (1917). A Spectroscopic Investigation of some Sources of Ultra-Violet Radiation in Relation to Treatment by Ultra-Violet Rays. Journal Of The Röntgen Society, 13(51), 25-36. http://dx.doi.org/10.1259/jrs.1917.0012

Smith, K. (1976). Aging, carcinogenesis, and radiation biology. New York: Plenum Press.