The Dirty Truth: What’s the Grossest Thing in My House?

Our project, titled "The Dirty Truth: What’s the Grossest Thing in My House?," is an investigation using scientific methods to identify which common household surfaces show the highest levels of bacterial growth, measured by colony-forming units (CFUs).
Hafsah Shittu, Iman Sharaf
Calgary Islamic School, Akram Jomaa Campus
Grade 9

Hypothesis

If bacteria grow best in warm, moist, and nutrient-rich environments, then the kitchen sponge will have the highest average bacterial colony growth (CFU) compared to other commonly used household surfaces, after 5 days of incubation on nutrient agar. This is because sponges stay moist, trap food particles, and are used frequently.

Research

Bacteria grow in nutrient-rich environments. Bacteria are living organisms, which means they grow and multiply. Bacteria are everywhere and can be found on every surface. Warm, moist areas are ideal for bacterial growth, making items used for cleaning or food preparation particularly susceptible. Common household items harbor lots of bacteria, often more than a toilet seat. These include kitchen sponges, dishcloths, cutting boards, coffee maker reservoirs, and refrigerator handles due to damp, warm environments and contact with food or hands. Electronics like phones, keyboards, and remotes, as well as toothbrush holders, pet bowls, and countertops, also rank high as germ hotspots.

Variables

Independent Variable: The household surface we tested (TV remote, door handle, kitchen sponge, toilet seat, etc.)

Dependent Variable: The Number of bacterial colonies (CFU) after 5 days.

Controlled Variable: Amount of time allowed for bacterial growth, incubation temperature, swabbing method, type of growth medium, incubation time, type of cotton swabs, size and type of petri dishes, number of trials per surface, observation method,

Procedure

Label Petri dishes with surface name and trial number. Swab each surface for 5–10 seconds with consistent pressure, using a new sterile swab for each surface and trial. Streak swab on nutrient agar in a zig-zag pattern; seal the dishes with tape or plastic wrap. Incubate dishes upside down for 5 days at 25–30 °C in a controlled environment. Observe bacterial growth daily without opening the dishes. Count colonies (CFU) using a colony counting grid for each trial; for very dense growth, estimate by averaging one section of the grid. Record data in a table and calculate average CFU for each surface. Dispose of Petri dishes safely, wash hands thoroughly, and sanitize the work area.

Observations

Bacterial colonies grew on all the household items we tested, with the highest growth being the kitchen sponge, consistently reaching over 1000 colony-forming units (CFU), too numerous to count accurately. The tooth brush had the lowest growth, averaging at only 7(CFU) across three trials. The rest of the items showed average bacterial growth, with the TV remote at 78 CFU, the door handle at 89 CFU, the toilet seat 108 CFU, the light switch 69 CFU, the keyboard 116 CFU, and the iPad 99 CFU. These results showed us that the sponge was the most contaminated surface, while the toothbrush was the least contaminated among the items tested.

Analysis

Throughout the three trials, the sponge showed the highest bacterial growth, based on having the highest number of colonies, the largest colony sizes, and the fastest spreading pattern. Most likely because sponges are full of holes and pockets, each providing a niche for a community of microbes to settle down in. The Keyboard had the second highest growth, which was surprising since we thought the toilet seat would be second because, many people think the toilet seat is one of the dirtiest places in the house because it is connected to germs and human waste. However, it is cleaned often and made of materials that do not hold bacteria well. We did notice patterns in bacterial growth with the sponge’s pattern being dense, fast, and quickly spreading colonies. The Keyboard’s bacterial growth pattern was many medium-sized colonies, with circular, smooth edges. The iPad, TV remote, door handle, and light switch showed moderate growth patterns, growing little bit by little everyday. The toothbrush had very little bacteria with a small number and limited size of colonie growth compared to the other household surfaces we tested, because most toothbrushes dry quickly and are rinsed often. Bacteria does also need water and moisture to survive. Unlike the sponge, the toothbrush doesn’t store the moisture, but rather dry up almost instantly. Although we did not identify specific bacteria, previous studies show that over 75% of kitchen sponges can harbor harmful bacteria like Salmonella and E. coli (Healthline, 2023). Our results support our hypothesis that the sponge would show the highest bacterial growth because it is moist, traps food particles, and frequently used, showing the importance of regular cleaning.

Conclusion

Our results showed that the sponge had the most bacterial growth among all household items we tested. This supports our hypothesis that If bacteria grow best in warm, moist, and nutrient-rich environments, then the kitchen sponge will have the highest average bacterial colony growth (CFU) compared to other commonly used household surfaces, after 5 days of incubation on nutrient agar. This is because it stays moist, collects food particles, and is used frequently. Items like the toothbrush had less bacteria because they dry out quickly and are usually rinsed after use. This suggests that bacteria grows in moist areas and it is important to keep your space clean. Future experiments could improve upon this by using a microscope to identify each bacterial colony and get more accurate findings. For future experiments, using a microscope to identify and classify the bacterial colonies could improve accuracy by distinguishing between different species, including harmful bacteria.

Application

My experiment found that the kitchen sponge contained significantly more bacteria than any other household item tested. This has a major real-world application for home hygiene: it proves that items used for cleaning can actually spread germs if not disinfected. Based on these findings, families should avoid using sponges for more than a week or switch to reusable cloths that can be laundered in high heat to prevent the spread of bacteria.

Sources Of Error

Although we were able to successfully complete the experiment, a few errors may have affected the results. 1. Counting errors: Our main source of error we faced while doing our experiment was counting each colony accurately\, especially because some bacterial colonies were overlapping\, thus leading us to estimate the number of colonies (TNTC). Inconsistent swabbing: It can be tricky to apply the same pressure and area when swabbing each surface, causing variation in bacteria collected.

Acknowledgement

We would like to thank our science teacher for helping and encouraging us during this project. We also thank our parents and friends for their advice, and support. This project wouldn’t have been possible without them, and their support helped us complete the experiment effectively.