Threshold-Dependent Collapse of Escherichia Coli Survival Under Ultraviolet Radiation

My name is Yashita Ghore, I'm a grade 11 student from Strait Area Nova Scotia.

UV radiation is biologically harmful to DNA as it can prompt mutations called pyrimidine dimers. Pyrimidine dimers occur when adjacent pyrimidine bases (cytosine and thymine) are covalently bonded with carbon atoms. These dimers distort DNA double helix structure, creating physical barriers that alter transcription and translation as well as mutating DNA.

Cells have strong repair mechanisms, but they’re not unlimited. My research investigates how bacterial survival is altered under increasing UV exposure. I wanted to determine a point at which DNA damage overwhelms repair mechanisms, causing survival to rapidly decline.

Understanding this threshold allows for efficiency and optimization. Knowing exact points means, we know the effective minimum dose to avoid unnecessary energy use or overexposure. In situations where elimination is the goal, knowing where survival rapidly declines allows us to ensure that harmful microorganisms do not survive in the context of food safety, hospitals, and water treatment.

There are several fields in which understanding how microorganisms respond to UV exposure are important:

Limits of DNA repair systems

Applications in sterilization and public health

Environmental relevance

Astrobiology and space exploration

Through my project specifically, I want to explore DNA and the limits of repair systems. As well as how the role of UV radiation can be improved in the context of healthcare, (ex. Radiation Oncology, Sterilization).

Prior to beginning my research, I completed the PHAC biosafety lab training to ensure I could handle and work with E. coli safely.

Research question

How does increasing UV exposure time affect survival rates of E. Coli, and at which point does DNA damage exceed cellular repair capacity?

Hypothesis

If the bacterium is exposed to increasing durations of UV radiation, survival will decrease exponentially, suggesting DNA will be damaged which will interfere with replication and exceed repair mechanisms at higher doses. I want to find a threshold exposure point beyond where survival declines, but to where no bacteria is able to survive; this is due to damage accumulating faster than cellular repair systems can compensate.

Procedure

Pipette 20 mL of E. Coli broth into a sterile petri dish.

Remove a 1.0 mL sample for the control group and place in a test tube containing 0.85% saline solution.

Place the petri dish inside the UV box.

Expose the sample for the indicated amount of time.

Remove 1.0 mL from the petri dish and pipette into saline solution.

Mix each tube to ensure there are no cell aggregates remaining and the sample and saline are thoroughly mixed.

Dilute each sample by creating six dilution banks.

Plate each dilution, and spread with glass beads.

Wait 24 hours for cultures to grow, collect and analyze data.

After allowing my cultures to grow for 24 hours, I counted the colonies in each series. When counting colonies, anything below 30 is insignificant while anything over 300 is regarded as being too numerous to count (TNTC).

I calculated the Colony-Forming Units per milliliter (CFU/mL) for each countable sample to quantify bacterial concentration, allowing accurate comparison of survival across different exposure levels.

CFU/mL= # of colonies observed from 0.1mL sample/

Dilution of sample

Purple Bars shows # of colonies for each countable plate.

Blue Line shows CFU/mL

Dotted line shows trend line.

The trend line shows a dose-response relationship between increasing UV exposure and how it leads to a loss in survival at a threshold point. It shows a primarily linear decrease until ~5 minutes where survival crashes. This indicates a point at which DNA repair mechanisms are no longer able to compensate for the damage being caused by the UV exposure.

This research reveals a clear decrease in bacterial survival when UV exposure increases, which suggests a threshold-driven response to UV radiation, where bacterial survival remains relatively stable at lower exposures but collapses rapidly beyond a critical time point (5 minutes). This suggests that once DNA damage surpasses the capacity of cellular repair mechanisms, viability is lost in a disproportionate and irreversible manner.

My project provides insight into how DNA repair mechanisms respond to increasing UV exposure. Ultraviolet radiation induces lesions (pyrimidine dimers). My results suggest that pyrimidine dimers do not occur gradually, but instead follow a threshold-dependent response: cells are initially able to tolerate and repair damage, but beyond a critical dose (5 minutes of exposure), repair systems are overwhelmed and survival rapidly declines. This highlights a key concept of genetic resilience, the capacity of DNA to maintain structural integrity under stress.

Before this threshold, cells can recover and preserve structure and function; beyond the threshold, accumulated damage leads to mutations or cell death. This has broad implications for predicting how organisms respond to radiation in different environments, from natural UV exposure to controlled medical applications.

This research enhances comprehension of how DNA damage leads to biological consequences. By linking radiation dose to survival, the project helps model how resilience breaks down under increasing stress. This insight is foundational to fields concerned with genomic stability, including cancer biology, where failures in DNA repair pathways are a defining feature.

In the context of Radiation Oncology, the same dose-dependent principle underlies cancer treatment. Radiation therapy aims to induce DNA damage, particularly lesions that overwhelm repair pathways in order to selectively eliminate tumor cells. My findings model how survival declines as accumulated damage surpasses a biological threshold, reinforcing the importance of delivering sufficient dose to ensure irreversible cellular failure. This aligns with clinical challenges in optimizing treatment plans: balancing tumour elimination while preserving healthy tissue.

I am building on these findings by directly quantifying DNA damage using molecular techniques such as PCR, and gel electrophoresis. This will allow for a deeper understanding of the mechanisms underlying the observed threshold response. I will then apply findings towards Radiation Oncology. Incorporating fractionated radiation exposure, where total dose is delivered in smaller intervals with recovery periods. This would allow further understanding and investigation of repair mechanisms, which is a key principal in cancer treatments. Together, these extensions will help model how radiation dose and timing can be optimized to maximize elimination of target cells in patients.

I would like to thank my family and friends for their unwavering support throughout my project, this would not have been possible without their encouragement.

I extend my gratitude to the St. Francis Xavier University biology department for giving me access to the lab. Dr. Moira Galway, and Mrs. Leah Rogers, for their guidance and support throughout this research.

I'd like to thank Sue Hawkes of the Coady International Institute for helping me film my ProjectBoard video.

Finally I’d like to thank the SRSF for providing me with this incredible opportunity to present my work, Ms. Shannon MacLennan, and Mr. Andrew Clarey for their support leading up to the CWSF.

Centre for Biosecurity / Centre de la biosûreté. “PHAC Biosafety 101,” April 8, 2024.

https://www.youtube.com/watch?v=teqPRedfDJo

Socorro Sánchez Correa, María del, María el Rocío Reyero Saavedra, Edgar Antonio Estrella Parra, Erick Nolasco Ontiveros, José del Carmen Benítez Flores, Juan Gerardo Ortiz Montiel, Jorge Eduardo Campos Contreras, et al. 2023. ‘Ultraviolet Radiation and Its Effects on Plants’. Abiotic Stress in Plants - Adaptations to Climate Change. IntechOpen. doi:10.5772/intechopen.109474.

Ries, G., Heller, W., Puchta, H., Sandermann, H., Seidlitz, H. K., & Hohn, B. (2000). Elevated UV-B radiation reduces genome stability in plants. Nature, 406(6791), 98–101. https://doi.org/10.1038/35017595

Drew, H. R., Wing, R. M., Takano, T., Broka, C., Tanaka, S., Itakura, K., & Dickerson, R. E. (1981). Structure of a B-DNA dodecamer: conformation and dynamics. Proceedings of the National Academy of Sciences of the United States of America, 78(4), 2179–2183. https://doi.org/10.1073/pnas.78.4.2179

Elisabeth G Richard, “UV Radiation,” ScienceDirect, January 2020. https://www.sciencedirect.com/science/article/abs/pii/S0733863519300816

Lindgren, Jukka & Gehrmann, William & Ferguson, Gary & Pinder, John. (2008). Measuring effective vitamin D 3-producing ultraviolet B radiation using Solartech's Solarmeter® 6.4 handheld, UVB radiometer. Soc. 43. 57-62.

https://read.dukeupress.edu/environmental-humanities/article/9/2/378/133024/Astrobiology-and-the-Ultraviolet-World

Ekta Yadav and Katherine Vanta, “The Truth About Thymine Dimers and Their Role in the Development of Non-Melanoma Skin Cancers,” journal-article, by Remedy Publications LLC., Clinics in Oncology, vol. 7, 2022, https://www.clinicsinoncology.com/open-access/the-truth-about-thymine-dimers-and-their-role-in-the-8782.pdf.

https://www.sciencedirect.com/topics/chemistry/thymine-dimer

https://www.canada.ca/en/health-canada/services/sun-safety/what-is-ultraviolet-radiation.html

Svobodová, Alena & Walterova, Daniela & Vostálová, Jitka. (2006). ULTRAVIOLET LIGHT INDUCED ALTERATION TO THE SKIN. Biomedical Papers. 150. 25-38. 10.5507/bp.2006.003.

Cho H, Misra R.2021.Mutational Activation of Antibiotic-Resistant Mechanisms in the Absence of Major Drug Efflux Systems of Escherichia coli. J Bacteriol203:10.1128/jb.00109-21.https://doi.org/10.1128/jb.00109-21

Mbonimpa, E. G., Blatchley, E. R., III, Applegate, B., & Harper, W. F., Jr. (2018). Ultraviolet A and B wavelength-dependent inactivation of viruses and bacteria in the water. Journal of Water and Health, 16(5), 796–806. https://doi.org/10.2166/wh.2018.071

Cleveland Clinic. (2022). Radiation Therapy. Cleveland Clinic. https://my.clevelandclinic.org/health/treatments/17637-radiation-therapy

Beck, S. E., Ryu, H., Boczek, L. A., Cashdollar, J. L., Jeanis, K. M., Rosenblum, J. S., Lawal, O. R., & Linden, K. G. (2017). Evaluating UV-C LED disinfection performance and investigating potential dual-wavelength synergy. Water research, 109, 207–216. https://doi.org/10.1016/j.watres.2016.11.024

National Cancer Institute. (2020, October 26). Radiopharmaceuticals Emerging as New Cancer Therapy - National Cancer Institute. Www.cancer.gov. https://www.cancer.gov/news-events/cancer-currents-blog/2020/radiopharmaceuticals-cancer-radiation-therapy

Beluli, Valdrin & Kaso, Aleksander. (2019). Destruction of DNA Through Ultraviolet Radiation (UV-C, UV-B, UVA-2, UVA1) in the Sterilization of Polymer Packaging (ISO: 1043 - PET, LDPE, HDPE) in Fermented Milk Products. 6. 8-16. 10.1501/nuclear_0000000048.

Books:

Allott, AA. Mindorff, DM. 2023. Oxford Resources for IB Diploma Programme BIOLOGY Course Companion (2023 Edition)

Photos:

Britannica. (2019). polymerase chain reaction. In Encyclopædia Britannica.

https://www.britannica.com/science/polymerase-chain-reaction

Pyrimidine dimer - Alchetron, The Free Social Encyclopedia. (2017, August 18). Alchetron.com. https://alchetron.com/Pyrimidine-dimer