Kareena Narula | SQ Vol. 20 Research Features (2022-2023)
Water, made up of only two elements, appears as simple as its chemical structure. Yet, water comprises over 60 percent of the human body,1 indicating its crucial role for our ability to carry out biological processes. However, despite the importance that water plays in human health, the ability to access uncontaminated, clean water does not extend to everyone. According to the World Health Organization, over 663 million people currently do not have access to clean water and suffer from water contamination globally.2
One of the most frequent sources of contamination is the spread of Escherichia coli (E. coli) on the surfaces of stagnant water sources.3E. coli is a common bacteria found in most corners of the planet, ranging from the human gut to the largest bodies of water. However, ingesting water containing harmful variations of E. coli can cause severe gastrointestinal issues such as diarrhea, vomiting, fever, and even death.4 Without a doubt, there is an urgent need for strategies to target and prevent the spread of E. coli in potable water sources. Developing a better understanding of the conditions that allow E. coli to thrive will enable the creation of innovative public health initiatives to predict the occurrence of contamination in potential drinking water sources.
Growing up in Houston, Texas, Leanne Liaw always had a natural curiosity that extended beyond the walls of her high school science classrooms. A truly visual learner, she was drawn to public health because it allowed her to see how the environment plays a vital role in governing real-world health outcomes. Interested in how the proliferation of bacteria in water could be mitigated by electromagnetic radiation to improve its potability, or safety, she designed an experiment that focused on examining the influence of E. coli in water contamination. Now an undergraduate student at UC San Diego, Liaw’s research demonstrates the influence of ultraviolet (UV) light on the proliferation of E. coli populations.
All forms of light are a type of radiation that falls under the electromagnetic spectrum. The electromagnetic spectrum organizes different levels of radiation based upon their physical wavelengths, and contains categories such as gamma rays, UV light, and visible light. High-energy radiation signals such as UV light have shorter wavelengths, and are therefore more harmful than lower radiation signals.5 These shorter wavelengths allow UV radiation to penetrate cellular membranes. Extended exposure can break the helical structure of DNA, damaging crucial information that regulates cellular functions and processes. In humans, damage to DNA caused by UV radiation can cause long-term health issues that arise in the forms of skin cancer, cataracts, and other serious conditions.6 However, the energy emitted from UV light has been harnessed in microbiological and environmental research to study the influence of UV radiation on the survival of microorganisms, including E. coli. In Liaw’s work, she examines the effect of UV light exposure on the proliferation of E.coli colonies to develop future methods to mitigate E.coli growth in these water sources.
UV radiation can be broken down into three forms: UV-A, UV-B, and UV-C. UV-A has a longer wavelength, rendering it ineffective for sanitation purposes. While UV-C light has the shortest wavelength of the three forms, it is mostly absorbed by the atmosphere and results in minimal impact to surface water. For her study, Liaw chose to use UV-B radiation as it has a shorter wavelength (290 nm-320 nm) in comparison to UV-A and can therefore penetrate cellular membranes. In addition, UV-B radiation closely reflects the intensity of sunlight that penetrates the atmosphere and typically has the most influence on bacterial growth in water sources.
A particular strain of E. coli, known as K-12, was selected for its close genetic resemblance to E. coli strains that grow in stagnant bodies of water. However, K-12 has non-pathogenic qualities, therefore minimizing the risk of unintended infectious exposure to the researchers during experimentation. These samples of E.coli were treated with one of three conditions for a total experimental duration of 72 hours: constant UV-B light exposure, intermittent UV-B light exposure (12 hours per day), and no UV-B light exposure. Liaw first grew the K-12 E. coli on a starter petri-dish to ensure the viability of the selected bacterial samples. The next day, Liaw returned to find the dish dotted with hundreds of white spots. Bacterial colonies that appeared larger and bright showed signs of strong growth. These colonies were selected to be transferred to test tubes of nutrient broth that simulated the aquatic conditions that E. coli naturally occur in. Each test tube was then placed under their respective UV modulation.
After cultivating the colonies, Liaw quantified the amount of E. coli present in each sample by determining its Nephelometric Turbidity Units (NTU) value. NTUs quantify the amount of suspended particles in a solution by measuring using the exact amount of scattered light in the tube in proportion to the amount of liquid inside of the tubes, which can be observed by the solution’s “cloudiness.” A high turbidity value indicates a high concentration of particles, or E.coli suspended within the solution, while a low turbidity value indicates a low concentration of suspended particles. The initial turbidity of the tubes was calculated prior to UV-B exposure, then after 72 hours under their respective UV-B exposure condition to determine a final turbidity value.
After 72 hours, Leanne returned to collect her samples and was excited by her findings. She observed that the group with no UV-B exposure exhibited the largest turbidity value (~500 NTU) and the tubes appeared the most cloudy, while both groups with some form of UV-B exposure exhibited significantly lower turbidity values (~17-200 NTU). This vast difference between NTU values of each condition suggests that higher levels of UV-B exposure towards K-12 E. coli are more effective at reducing the proliferation of bacterial populations. Liaw performed an ANOVA statistical analysis to assess significant variance in the data in relation to the mean, or average, turbidity values collected from each of the three groups. The resulting ANOVA test between the three groups suggested that the measured differences in bacterial growth between the different UV-B exposure conditions are statistically significant, and that these differences are likely due to the different exposure conditions tested.
By comparing the growth of E.coli when exposed to different amounts of UV-B light, Liaw was able to better understand the conditions that encourage and discourage the proliferation of E.coli colonies. On a larger scale, these results contribute to developing a better understanding of how environmental conditions influence E.coli growth in potential drinking water sources. When asked about any future directions she hopes to take her research, Liaw hopes to further elucidate how such antimicrobial effects are induced by additional physicochemical processes. As an aspiring MD-PhD, Liaw also expresses that her deep commitment to public health and her findings will hopefully contribute to ongoing initiatives related to improving water potability and food safety guidelines to bolster community health. Most of all, in countries where water decontamination using conventional chlorination and pasteurization is not always financially feasible, Liaw’s research introduces a viable resource of UV light treatment to predict bacterial proliferation in water sources, with the potential to improve the health of millions of people around the world.