Modelling Antifungal Resistance Through Yeast Exposure
Exploring How S. Cerevisiae Adapts To Common Antifungal Medications Over Time
We’re so used to thinking that any medication is a solution, but what would we do when the medications we use now stop working?
In our world today, medication resistance is becoming one of the most urgent public health threats. In 2019 alone, antimicrobial resistance (AMR) directly caused 1.27 million deaths and contributed to almost 5 million deaths across the world (that’s more than HIV or malaria-related deaths of that year!) But this doesn’t include bacteria and fungi, which get much less attention.
Antifungal resistance plays a big part in this issue. Infections that are easy to treat at one point with just a topical or ear medication are becoming harder to manage in both humans and animals.
Recently, I got the opportunity to work at a veterinary clinic to gain insight into the medical field with hands-on experience. I noticed the different antifungals they used there. With interest in the AMR space, I started looking into the different anti-fungals used, specifically in the vet field, and how they connect to human medicine.
To deepen my understanding of this space, I decided for the past 4 weeks to run an experiment using S. cerevisiae (also known as baker yeast 🥖) and expose it to two different types of anti-fungal medication, Aurizon (a common anti-fungal, antibiotic and steroid, used for animals 🦮) as well as Canesten (a common topical anti-fungal cream🦶), to test out its tolerance towards the medication when exposed for long periods of times.
Before we go deeper into what my project was and its details, let’s go over what antifungal resistance is and how it develops on a scientific level
🤔 What is Antifungal Resistance?
Antifungal resistance happens when a fungus evolves to survive treatment that stops it from growing or kills it entirely. This means that even if you use the medication as indicated, the drug doesn’t work the way it’s intended to. Distance makes the infections last longer and return more often, sometimes making it almost impossible to cure.
You can also think of this in another way; just like how vaccines train our immune system to recognize and fight disease, fungi, when exposed to anti-fungal medication, do that as well, but more negatively. Vaccines give your body a small and very safe dosage, just enough to adapt and learn how to be strong when shown more substantial amounts of any disease, essentially building immunity that protects you.
When fungi are repeatedly exposed to antifungal medication, they can also learn, like your immune system. But instead of building immunity to help us, they develop a resistance to protect themselves and survive the stress given by the medication.
This resistance is developed in terms of fungi through natural selection, essentially when the fungal populations evolve against repeated exposure to antifungal medications. Most fungal cells are killed when a fungal infection is treated with a drug such as Clotrimazole (an ingredient in Aurizon and Canesten- the medications I used for this experiment).
However, we are dealing with a vast population. In that case, a few individual cells might survive due to random genetic mutations, better selectors, or other small advantages.
These specific fungi are not lucky by chance to stay alive; they carry the traits that make them less vulnerable to the drug’s mechanism. These traits could be slightly changed versions of the enzymes the drug targets or the cell membranes that absorb less of the medication. When these survivors reproduce, they pass their resistance traits onto the next generation.
When this is repeated enough times, and the drug is being used repeatedly or not being used correctly (such as stopping the medication duration too soon or being reused inconsistently), the population of the fungi present is made up of more of these resistant traits.
These fungi can exist in medications in a couple of ways.
- Mutations of the target enzymes → I mentioned this before. Still, for example, if we look at the azole antifungals (like Clotrimazole), they usually have an enzyme called ERG11, which, in simple terms, helps the fungi build their cell membrane. If this ERG11 enzyme mutates, the drug might not be able to bind as effectively, which in turn allows the fungus to keep growing 😢
- Efflux pumps → These are almost like a defence system ejecting an intruder or the immune system of the yeast. This helps the fungi develop protein that actively pumps the exposed drugs outside of their cells before they can take any effect
- Biofilm formation → This is another one I previously mentioned, but essential fungi can group up and form these biofilms, which are thickly layered communities that act as a protective wall, preventing the drug from going deeper into the most vulnerable cells
- Overproduction of target molecules → Even if the drug somehow binds to its target, some fungi can balance it out by making more of that target, diluting the drug’s effectiveness (making the fungi always one step ahead)
In my experiment, I simulated this pressure seen in fungi’s evolution by exposing the yeast to Aurizon and Canesten cream. I saw firsthand how resistance might emerge in real-world scenarios by repeatedly exposing these yeasts to antifungals over multiple generations.
🧑🔬 Breaking down my experiment
I tried to simulate how antifungal resistance might emerge through repeated medication exposure for my experiment. From cases I’d seen during my veterinary co-op placement, I wanted to use Aurizon as it was often used to treat canine ear infections caused by yeast. I found a common over-the-counter antifungal medication, Canesten topical cream, to work well for this project. Then, to test everything, I chose Saccharomyces cerevisiae, which is a safe, easily accessible type of fungus that would respond to similar antifungal pressure over multiple generations.
Even though it’s not the same species found in common fungal infections, such as Candida albicans in humans or Malassezia pachydermatis in animals (especially in ear infections), it still can be a useful way for studying how fungi may respond to antifungal pressure over time
📌 All my materials used:
- Fungus: Baker’s yeast (S. Cerevisiae) ~ 1 tsp used to start with
- Growth medium: Sugar water (dissolved white sugar) ~ 4g of sugar mixed with 60 ml of distilled water
- Heating mat: Keep the yeast at about 30–35 degrees Celsius to help with faster growth
- Anti-fungal medications tested:
- Aurizon (Veterinary, Rx) ~ 2 drops per trial
- Canesten Cream (Human OTC Clotrimazole) ~ pea-sized amount per trial - Control samples: Received no antifungal treatment
- Measurement method: Microscopic yeast counts per high power field (×400 magnification), 10 random fields per slide
Note: I adjusted doses based on the manufacturer's instructions:
- Aurizon is applied in drops, so I used 2 drops per test tube (approximating canine ear treatment scaled to volume)
- Canesten is applied topically in small amounts, so I used a pea-sized portion per trial
- No antifungal was used in controls to measure natural yeast growth without drug pressure
🧪 The Experimentation Process
Generation One: The Baseline
Generation one was simple: I prepared a sterile sugar water solution with 60 ml of water and 4g of sugar dissolved thoroughly. The yeast was then added to the sugar-water mixture and then mixed through. This step was repeated for the control, the Aurizon and the Canesten samples.
Get Manaal Lakhani’s stories in your inbox
Join Medium for free to get updates from this writer.
After the yeast-sugar-water was set up for all of them, the new samples were set up with the Aurizon bottle with just two drops of Aurizon and the Canesten with a pea-sized blob of the cream, then both were stirred until dissolved.
To observe the yeast cells per high power, I looked at all of the samples under the microscope, and all of them averaged around >100 cells per high power. The three samples of generation 1 of the yeast were then set in a dark kennel on top of a heating pad to grow for 4–5 days.
Generation Two: The Exposure Begins
Each time I switched generations, I took approximately 3 mL of each sample. I transferred them to a new, fresh sugar-water solution (to provide them with fresh food to help them grow) and added the exact medication dosage again.
The amount of yeast decreased significantly, but still, there were yeast alive and floating around in the samples.
Generation 3: Survival of the Fittest
In this generation, the numbers decreased quite a bit, and in the medication sample, there was often little to none left. The yeast had been exposed to much of the medication, but some were still alive and thriving. Again, I repeat the same process as generation two, transferring about 3 ml of a new fresh sugar-water solution and providing the same medication dose.
Generation 4: The results
The fourth generation had fewer outcomes, but still quite a few remained. I saved part of generation 3 to see if they would multiply more without more of the medication being exposed (simulating how if you were to stop the medication before the prescribed duration), and they ended up thriving a lot more. The numbers increased, and budding yeast cells formed fascinatingly in the bottles and under the microscope!
🔬 Analysis
I examined 10 random fields per slide for each generation under high-power magnification. I recorded the cell count and averaged it for each group to get a better understanding, but I further analyzed the data I collected through the process.
(Note: Generation 4 yeast hasn’t been collected yet, as it is still growing, and I will update it as soon as I can get the new data.)
All this data collected shows that repeated exposure to different antifungal medications reduces the use of populations over time, both with Aurizon and Canson cream, proving it effective. After just two rounds of exposure, the yeast counts in the treated groups dropped quite dramatically compared to the control group, which remained higher despite natural fluctuations in general.
Aurizon, being a veterinary medication, consistently showed the most potent suppression of yeast growth. Canesten was also effective but slightly less so in the earlier generations.
By generation 3, both medications had driven yeast counts to almost zero, indicating neither treatment had lost effectiveness, and no measurable resistance was seen. Over time, but as generation 3 developed, it showed some resistance as the counts got higher after some point.
This suggests that the different antifungal medications can remain effective over repeated exposure when used consistently and adequately. When I did the part where I let the yeast samples alone by not increasing the generation three medication, we saw that the yeast or fungal infection could grow when we gave it some time.
After talking to my supervisor at my veterinary clinic, he said that through his experience, this resistance gap could easily come back again when people don’t continue through the prescribed medication.
However, it should be noted that clinical or medical fungi in these fungal infections can often be more complex and genetically diverse, and they have been exposed to these inconsistent treatments, which gives them more opportunity to adapt. Because the yeast strain S. cerevisiae, which is classified as non-pathogenic (meaning that it's not capable of causing disease), does not naturally evolve in antifungal resistance quickly.
The lack of resistance here doesn’t rule out the entire risk. It shows us that slowly and silently, in the background, it can develop that resistance. With more time, stress, and complexity, the resistance could eventually show up. This experiment captured it in the early stages of this process, which shows how effective antifungal pressure is initially. Still, in the later stages, the resistance can deepen and be more harmful in the future.
💭 Reflection and Future Improvements
This project gave me hands-on experience in looking at antifungal treatments and how they impact fungal populations over time. It helped me connect real-world veterinary and human medicine to actual experimental data. Seeing how yeast dropped from hundreds to just a few cells per field showed how powerful these medications can be. But resistance isn’t always obvious immediately; it builds over time and often quite sneakily.
Working in the veterinary clinic made this project feel relevant. Aurion was one of the most common antifungals used in practice, which made it very efficient and very good to see its effect within this lab project, even when using baker’s yeast.
(It also gave me a good appreciation of what vets or doctors generally deal with when people stop using medications throughout their treatment when they’re supposed to be using them!)
That all being said, there were a couple of limitations, such as using baker’s yeast (S. cerevisiae), which is not the same species found in veterinary or human fungal infections, such as Candida or Malassezia pachydermatis. Also, my yeast only went through three whole generations of exposure. If I were to have more time, I would’ve extended this experiment to around 5–6 more generations to see the early signs of resistance begin to emerge.
Another improvement I could have made would be testing different dosages if I were to test one medication that’s of my choice or a combination of different anti-fungals to see how yeast responds under stronger or various types of pressure, which reflects what could happen in multiple practices in vet or human medicine when they have to adjust the treatment based off of the responses of the fungal infection.
Lastly, a more advanced version of this project could include genetic or biochemical testing, which I initially wanted to do but didn’t seem practical in my time constraints. So, look for mutations or resistance markers currently out of reach in a co-op lab setting. If you know anything about this field, feel free to contact me. I’d love to talk more about it and do something in a professional research environment!
Conclusion
This experiment showed me how antifungal medications like Aurizon and the Canesten Cream significantly reduce yeast population overexposure. While no resistance developed in the shorter term, my study successfully modelled that the early stages of antifungal pressure showed how easily drug-resistant traits can emerge if the conditions allow it. Hands-on lab work and insights from my veterinary co-placement allowed me to explore the different medical concerns of antibiotic resistance in a controlled, safe, and accessible way.
🎉 Shout out to my co-op supervisor, Alex Hau, who guided me and gave his veterinary insight into this whole process :)
The different findings helped me understand the importance of using anti-fungal medications consistently and correctly in human and animal care to delay or prevent resistance.
Ultimately, this whole project deepened my understanding of how resistance involves not all at once but slowly and silently. It also highlighted how small-scale models like yeast can help us explore bigger problems like antimicrobial resistance and how we need to take action and create awareness for these problems to stay ahead of them and develop solutions for the future.
If you’re interested in more research or information about antibiotic resistance, I will publish a review paper soon highlighting a more technical analysis of the entire antibiotic and antimicrobial space. Make sure to follow me, and stay tuned!
Citations
- The Lancet. (2022). Global burden of bacterial antimicrobial resistance in 2019: A systematic analysis. The Lancet, 399(10325), 629–655. https://doi.org/10.1016/S0140-6736(21)02724-0
- Public Health Agency of Canada (PHAC). (2023). Candida auris — Fungal diseases. Government of Canada. https://www.canada.ca/en/public-health/services/diseases/fungal-diseases/candida-auris.html
- Vetoquinol. (n.d.). Aurizon. CVP Service. https://veto.cvpservice.com/product/view/1234329
- Plumb, D. C. (2015). Plumb’s Veterinary Drug Handbook (8th ed.). Wiley-Blackwell.
- GMPC Akademie. (n.d.). Antifungal Resistance and Clinical Significance in Small Animals. German Journal of Veterinary Research. https://gmpc-akademie.de/articles/gjvr/single/71
- Nature Reviews Microbiology. (2022). Tackling the emerging threat of antifungal resistance to human health. https://www.nature.com/articles/s41579-022-00720-1
- Alex, Veterinary Technician. (2025, May 27). Personal interview. 16th Avenue Animal Hospital.
