If S. epidermidis is cultured with Lactobacillus acidophilus metabolite solution in LB broth and another solution with only LB broth, then the bacteria exposed to Lactobacillus acidophilus will have less growth because the lactic acid in the metabolites will deter the the growth of S. epidermidis.

Skin Microbiome 

The skin microbiome is the community of trillions of microorganisms, such as mites, fungi, bacteria, and viruses, that naturally live on the human skin. The skin is the human body’s largest organ and is home to a diverse range of microbes (Wiginton, 2025). These microbes grow, reproduce, and adapt to specific skin conditions, whether it be oily, dry or moist (Grice & Serge, 2013). The microbiome fights off infections, where certain bacteria behave like antibiotics and decrease the pH of the skin, helping the immune system by warning the brain of any viruses present, and helps maintain inflammation and heal injuries (Wiginton, 2025). 

Most of the microorganisms present on the skin surface are beneficial or harmless to an individual. However, if the balance between good and bad bacteria is disturbed, known as dysbiosis, the human host may be more susceptible to skin diseases such as eczema, acne, or rosacea (Grice & Serge, 2013). An example of a bacteria that may do this is S. epidermidis. S. epidermidis is harmless when balanced on the skin. However, in large amounts, it can aid the growth of acne and trigger inflammation (Lee & Anjum, 2023). This example shows that skin health depends on maintaining a balanced microbial community rather than eliminating bacteria entirely. This is also why sterilization is avoided. This process may reduce the amount of harmful bacteria, but also decreases the population of the good bacteria (Grice & Serge, 2013). Conversely, holistic approaches to acne management include supporting the skin barrier by moisturizing regularly, and assisting the skin-gut-microbiome connection by balancing the diet and exercising consistently (Wiginton, 2025). However, holistic approaches are not sufficient to manage the condition in all individuals. As a result, treatments that maintain the natural balance of the skin microbiome are in high demand (Wiginton, 2025).

Acne

Acne is a skin condition in which dead skin cells and skin oil block hair follicles. In fact, 83% of girls and 95% of boys at the ages of 15-16 experience it, and many adults also struggle with acne (Chilicka et al., 2022).  It can appear in many areas of the skin, including on one’s face, shoulders, and chest. There are 4 main factors that contribute to acne growth (Acne: Risk Factors, n.d.):

1. Follicular hyperkeratinization: Hair follicles blocked by dead skin cells and sebum. This plug creates a breeding ground for bacteria to thrive in.
2. Increase in sebum production: Sebaceous glands expand in size and produce more sebum, especially during puberty. This sebum increases the chances of blockages if not properly cleared.
3. Growth of C. acnes: Cutibacterium acnes, or C. acnes, is a bacteria on healthy human skin. The sebum is the source of its nutrients. An increase of C acnes and S. epidermidis can favour an increase of inflammation on the skin. Since there is more growth of C. acne, bacteria fight for nutrients and to survive, leading to a further disbalance on the skin microbiome.
4. Inflammation: May cause follicles to rupture, creating red pimples. This red pimple is also known as a “tender”.

Antimicrobial Resistance

Antibiotic resistance is a growing health concern worldwide (Dessinioti & Katsambas, 2022). This occurs when a bacteria or disease adapts and becomes immune to the medication, making it harder to treat, an issue that has also arisen in acne medications (Dessinioti & Katsambas, 2022), where skin bacteria adapt and find other ways to thrive in harsh conditions.

C. acnes is a type of bacteria found on healthy human skin, but may contribute to acne when present in large amounts. Antibiotics have been used to treat it, however, overuse of medication has led to the adaptation of certain bacteria, including C. acnes (Dessinioti & Katsambas, 2022). Additionally, antibiotics such as erythromycin and clindamycin disturb the natural balance of the skin microbiome. A balanced amount of bacteria can keep viruses or harmful bacteria from overgrowing (Dessinioti & Katsambas, 2022). Antibiotics affect that and put the skin at more risk (Dessinioti & Katsambas, 2022).

Lactobacillus Acidophilus Lactobacillus acidophilus is a rod-shaped, gram-positive bacterium found in the human digestive system and is commonly used as a probiotic to promote healthy gut bacteria, supporting both immune and digestive functions (Definition of Lactobacillus - NCI Dictionary of Cancer Terms - NCI, n.d.). It produces lactic acid, which helps maintain an acidic environment that inhibits harmful bacteria in the gut (Batt, 2014) and, when applied to the skin, dissolves dead skin cells, hydrates the skin barrier, and limits cell buildup (Batt, 2014). Additionally, L. acidophilus produces hydrogen peroxide, which further deters the growth of certain bacteria (Batt, 2014). When lactic acid is applied or produced on the skin, it lowers the skin’s pH and removes dead skin cells, which hydrates the skin, as shown in the study Epidermal and dermal effects of topical lactic acid (Smith, 1996). By creating slightly acidic conditions (pH 4.5–5.5), lactic acid favors the growth of beneficial bacteria, which are more resilient than harmful bacteria due to stronger membranes and chemical defense systems (Khalfallah et al., 2021). Lactate, naturally present in human skin tissue, contributes to maintaining this acidic environment, known as the skin’s acid mantle, which supports barrier function and regulates which microorganisms can grow on the surface (Petersen, 1999; Lamber et al., 2006). Many skin-associated bacteria are sensitive to pH changes, and more acidic conditions inhibit pathogenic growth (Lamber et al., 2006). Furthermore, lactate plays a role in skin immune regulation and metabolism (Ruan et al., 2025), and although it is unclear whether probiotic-derived metabolites reach the skin from the gut in significant amounts, lactate remains biologically relevant to skin health (Ruan et al., 2025). As a result, lactic acid may suppress the growth of S. epidermidis on the skin while still allowing beneficial bacteria to grow.

Staphylococcus Epidermidis

Staphylococcus epidermidis, or S. epidermidis, is a gram positive spherical bacteria that forms grape-like clusters (Lee & Anjum, 2023). While it is harmless in its natural habitat, an imbalance of S. epidermidis can lead to the increase in diseases in the human body (Claudel et al., 2019). Past researchers believed that C. acnes was the sole bacteria in the aid of forming acne. However, newer studies discovered that the balance and imbalance between C. acnes and S. epidermidis may be the cause of acne formation on the human skin (Claudel et al., 2019). Research has not identified a specific ratio between these species for healthy skin, but rather the change in the interactions between C. acnes and S. epidermidis when the skin microbiome is disrupted (Eckhart & Sancar, 2020). 

The Gut-Skin Axis

There is significant evidence that suggests that there is a connection between the skin and the gut. The gut-skin axis refers to the two-way relationship between the digestive system and the skin, influenced by immune responses, systemic inflammation, and changes in gut bacteria (Singla et al., 2025). The main mechanism that the gut participates in is the inflammatory immune response (Sánchez-Pellicer et al., 2022). mTOR, a nutrient regulator that aids in cell growth and the strengthening of the skin barrier, affects how the body handles diseases located on the human skin. Studies have shown that the types of food an individual takes can interact with the mTOR pathway (Sánchez-Pellicer et al., 2022). If an individual has a diet containing high glycemic loads, it may affect mTOR and lead to the increase of inflammation and irritate acne present (Sánchez-Pellicer et al., 2022).

Leaky gut syndrome is a theoretical condition that is based on intestinal permeability (Cleveland Clinic, n.d.). A human gut is semi permeable, which allows nutrients to enter our bloodstream. However, some individuals have hyperpermeability, meaning that their bloodstream absorbs more than necessary nutrients (Cleveland Clinic, n.d.). This may allow for larger or toxic molecules to enter the bloodstream, triggering an inflammatory response (Cleveland Clinic, n.d.).

Previous Studies There have been studies done on similar topics. Kober and Bowe, in 2015, found that probiotics can lead to an increase in the skin’s hydration, as well as an improvement in the skin barrier function. They reported that retinoids, benzoyl peroxide, and other acne antibiotics have side effects on the skin, such as itchiness, redness, and dryness. Probiotics decrease these effects while keeping the skin healthy and strong.

The Gut Microbiome

The human gut microbiome is home to trillions of microorganisms, including but not limited to fungi, viruses, and bacteria that mainly reside in the large intestine. It plays an important role in digestion, metabolism, immune system development, and inflammation regulation. These microbes break down carbohydrates and fiber that the human body can not digest on its own, produce beneficial compounds (SCFAS - short chain fatty acids), and help produce vitamins like vitamin B and vitamin K (Cleveland Clinic, 2023). The gut microbiome also strengthens the gut barrier. This prevents that absorption of harmful substances or toxins into the blood stream (Mann et al., 2020). Research shows that when the balance of gut bacteria present is disrupted (also known as dysbiosis), it can lead to an increase in inflammation and improper immune responses (Mann et al., 2020). For example, a high in fat and low in fiber diet can lead to the alteration of gut composition and reduce diversity present, which can trigger inflammation among the body and lead to weight gain (Mann et al., 2020). The guts also contain a large amount of immune cells. The beneficial bacteria helps train the immune system to distinguish between harmless and harmful bacteria (Cleveland Clinic, 2023). 

Recent studies show a connection between inflammatory skin diseases (eczema, psoriasis, and rosacea) and the gut microbiome. Individuals that suffer from these diseases often have different gut bacteria composition, having an increase in harmful bacteria and a decrease in beneficial microbiomes (Mann et al., 2020). Dysbiosis can increase gut permeability, which allows toxins to spread throughout the body and affect other organs, including the skin (Cleveland Clinic, 2023).  This imbalance may lead to more skin inflammation. Some researchers suggest that probiotics may help restore the microbial balance that is necessary for a healthy microbiome, which may improve skin health (Mann et al., 2020).

Introduction

In this study, I tested whether metabolites produced by Lactobacillus acidophilus can reduce the growth of Staphylococcus epidermidis. To do this, I measured bacterial growth in two ways: OD600, which shows how cloudy the liquid culture is and indicates overall bacterial density, and CFU/mL, which counts the number of live bacterial colonies on plates. The treatment used tropical L. acidophilus metabolites, applied directly to the bacterial cultures. While it’s not yet confirmed that these metabolites survive gut travel, there is some suspicion they could, making it interesting to test their effect directly. I hypothesized that the test groups receiving the metabolites would show less growth than the controls as the metabolites would inhibit the growth of S. epidermidis.

Table 1: Different types of Variables

Materials Note: Proper PPE is required, as well as biohazardous disposal. All materials are sterilized in an autoclave beforehand.  Table 4: Different Materials Required

Procedure Note: All plates and LB broth were prepared in advance using standard microbiology procedures, as well as the sterilization of all the materials used under a flow cabinet. This project is also a multi-day experiment, so plan accordingly.

Proof of Concept Procedure

1. Open a Lactobacillus acidophilus capsule and dissolve the powder in 5 mL sterile water.
2. Add the mixture to the prepared broth and incubate at 37°C for 24–48 hours to grow the bacteria.
3. Centrifuge the culture at 8,000 RPM for 15 minutes, then collect the supernatant (liquid) using a sterile syringe.
4. In a separate flask, grow E. coli in broth at 37°C for 18–24 hours.
5. Dilute the S. epidermidis culture 1:10 — mix 1 mL of culture with 9 mL of sterile broth.
6. Prepare four sterile test tubes: label two “A” (control) and two “B” (test).
7. For tubes A (control): add 1 mL diluted E. coli + 1 mL sterile water.
8. For tubes B (test): add 1 mL diluted E. coli + 1 mL Lactobacillus supernatant.
9. Incubate all tubes at 37°C for 1–2 hours.
10. Measure bacterial growth at 600 nm (OD600) using a spectrophotometer with 1:4 dilution (750 ul bacteria and 2.250 mL of Luria Broth)
11. Plate 100 μL from each tube on nutrient agar and label clearly.
12. Incubate plates at 37°C for 24–48 hours.
13. Count bacterial colonies on each plate.
14. Compare colony numbers between control and treated plates and OD600 between control and treated plates
15. Repeat the experiment two times for accuracy.

Trial 1 Procedure

1. Open a Lactobacillus acidophilus capsule and dissolve the powder in 5 mL sterile water.
2. Add the mixture to the prepared broth and incubate at 37°C for 24–48 hours to grow the bacteria.
3. Centrifuge the culture at 8,000 RPM for 15 minutes, then collect the supernatant (liquid) using a sterile syringe.
4. In a separate flask, grow Staphylococcus epidermidis in broth at 37°C for 18–24 hours.
5. Dilute the S. epidermidis culture to 0.4
6. Prepare four sterile test tubes: label two “A” (control) and two “B” (test).
7. For tubes A (control): add 1 mL diluted S. epidermidis + 1 mL LB
8. For tubes B (test): add 1 mL diluted S. epidermidis + 1 mL Lactobacillus supernatant.
9. Incubate all tubes at 37°C for  24 hours.
10. Measure bacterial growth at 600 nm (OD600) using a spectrophotometer with 1:4 dilution (500 ul bacteria and 1.5  mL of Luria Broth)
11. Plate 100 μL from each tube on nutrient agar and label clearly.
12. Incubate plates at 37°C for 24–48 hours.
13. Count bacterial colonies on each plate.
14. Compare colony numbers between control and treated plates and OD600 between control and treated plates
15. Repeat the experiment two times for accuracy.

Trial 2 and 3 Procedure Preparation of Bacteria

1. Open a Lactobacillus acidophilus capsule and dissolve the powder in 5 mL sterile water in a 50 mL flacon tube. Add 5 mL LB broth into this mixture.
2. In another 50 mL falcon tube with 10 mL LB broth, inoculated with S. epidermidis.
   1. If the culture is dehydrated, first rehydrate the bacteria using the proper protocol provided by the manufacturer. In this case, the protocol provided by Innovating Science.
3. Incubate both cultures at 37 ̊ C at 200 rpm for 24 hours.
4. After the incubation period, plate 50 uL of each culture onto agar plates using a sterile spreader (one plate per culture) in order to maintain it for future use. Restreak every 1 - 2 weeks.
5. When ready to start trial, inoculate one colony from the plate of each bacteria strain into 10 mL of LB broth and incubate at 37 ̊ C at 200 rpm for 24 hours in sterile 50 mL falcon tubes.

Day 1: Trial Set Up

1. Check the OD600 using the spectrophotometer using the following steps.
   1. Add 2 mL of fresh LB into a cuvette and blank the spectrophotometer.
   2. Measure 500 uL of L. acidophilus into a cuvette and add 1.5 mL of LB broth. (Creates a 1:4 dilution). As spectrophotometers are not accurate above an OD600 of 1.0, this will help to ensure appropriate readings (Wang et al, 2024).
   3. Place the L. acidophilus cuvette into the spectrophotometer and measure the optical density of it. Multiply the reading by 4 to get the OD600 of the original culture.
   4. Record the OD600 in the labbook
   5. Repeat the same steps with S. epidermidis.
2. Centrifuge the L. acidophilus culture at 8 000 rpm for 5 min, then take the OD600 of the supernatant using the spectrophotometer and the steps above.
3. Prepare four sterile 50 mL falcon tubes: label C1 and C2 (for control group) and T1 and T2 (for test group).
4. For tubes C: Add 3 mL LB broth and 2 mL S. epidermidis.
5. For tubes T: Add 2 mL LB broth, 1 mL L. acidophilus supernatant, and 2 mL S. epidermidis.
6. Incubate all tubes at 37 ̊ C at 250 rpm for 24 hours.

Day 2: Serial dilutions and plating

1. Take the OD600 of all the trial tubes (c1, c2, t1, t2) using the spectrophometer.
2. Prepare 2 sterile 15 mL tubes for each tube (Total of 8). Label them as follows: C1 1:10^3, C1 1: 10^6, C2 1: 10^3, etc.
3. Add 4.955 mL of LB broth to each tube.
4. Add 5 uL of C1 to C1 1:10^3 tube, using a pipette to thoroughly mix it.
5. Remove 5 uL of C1 1:10^3 and add it to C1 1: 10^6. Mix it thoroughly and expel 5 uL of C1 1: 10^6 to keep the same volume.
6. Repeat steps 5 and 6 for the other 3 tubes.
7. Label the plates with the tube and the dilution factor: C1 1:10^3, C1 1: 10^6, etc. One plate for the 10^3 dilution and two plates for the 10^6 dilution (3 plates total for each starting tube).
8. From the appropriate tube, pipette 50 uL onto each plate and spread until dry with a sterile spreader.
9. Wrap and incubate the plates at 37 ̊ C for 24 hours.

Day 3: Counting of colonies

1. Count the bacterial colonies on the most countable plate (1:10^6) for each tube.
2. Calculate the CFU/mL in the original culture.
3. Compare colony numbers between control and test group.

Repeat the experiment multiple times for greater accuracy.

Data

Table 3: OD600 of 24 hour inoculation cultures prepared for trial setup

Optical density at 600nm of cultures grown overnight in luria broth at 37C and 250rpm. A 1:4 dilution was performed using 1.5mL of broth and 500ul of culture before readings were taken on the spectrophotometer. Data represents the OD600 of the original culture.

Table 4: OD600 of Test and Control Tubes after Incubation

Optical density at 600 nm of control and test tubes at 37C and 250 rpm. Each test tube contained 2 mL of S. epidermidis, 1 ml L. acidophilus metabolites, and 2 mL lurea broth while each control tube contained 2 mL of S. epidermidis and 3 mL of luria broth. A 1:4 dilution was performed using 1.5 mL of luria broth and 500 uL of test/control tube. Data represents the OD600 of original cultures.

Table 5: CFU/mL for each Tube

Colony forming units per milliliter of cultures at 37C. Test and control tubes were serially diluted and plated on luria-bertani agar. For plates with high colony amounts (Trial 3), quadrant counting was used and values were multiplied by 4. In Trials 2 and 3, 2 plates were used per last serial dilution and averaged. CFU/mL values represent the bacterial concentration after all calculations.  Figure 1. Initial OD600 of 24-hour inoculation cultures used for trial set-up. A 1:4 dilution was performed using 1.5mL of broth and 500ul of culture before readings were taken on the spectrophotometer. Data represents the OD600 of the original culture.

Figure 2. Final OD600 readings of control and test tubes following incubations at 37C and 250 rpm. Tubes were prepared using a 1:4 dilution (1.5 mL LB and 500 uL of culture) prior to the readings. Data represents the OD600 of original cultures.

Figure 3. Colony-forming units per milliliter of control and test tubes after incubation at 37C. Samples were serially diluted and plated on LB agar. In Trials 2 and 3, 2 plates from the final serial dilutions were averaged. For Trial 3, quadrant counting was used for over populated plates and values were multiplied by 4. Each individual tube was accounted for (a and b in each trial). The yellow line represnets the percent change among each pairing (C1 vs T1 and C2 vs T2 in each trial). CFU/mL values represent bacterial concentrations after all calculations were made.

Table 6: Changes in Bacterial Growth (OD600) in Control and Test Tubes

Optical density at 600 nm of control and test tubes was measured after incubation at 37C and 250 rpm. A 1:4 dilution was performed using 1.5 mL Luria broth and 500 uL of test or control tube. Each trial contained two control and two test tubes, and percent change was calculated for each tube (hence a and b for each trial). Data represents growth changes across all trials.  Figure 4. Percent Change in OD600 Over All Trials after incubation at 37c and 250 rpm. Samples were diluted 1:4 using 1.5 mL of Luria broth and 500 uL of the test or control tube. Each trial contained two control and two test tubes, and percent changes in OD600 were calculated for each tube (a and b in each trial). Data represent bacterial growth changes across all trials.

P value (CFU/mL): 0.038 & P value (OD600): 0.0354 (Results are significant)

Observations

The antimicrobial effect of L. acidophilus metabolite on S. epidermidis was measured through OD600 and CFU/mL. After incubation, the control tubes showed more bacterial growth than in the test tubes containing L. acidophilus metabolites. The control tubes had OD600 values of 3.188 and 3.068, while the test tubes had less growth, with values at 2.720 and 2.848. Additionally, after the growth had been spread onto agar plate and incubated overnight, the control tubes had higher colony counts with 1.21 × 10⁹ CFU/mL and 1.64 × 10⁹ CFU/mL. The test tubes had less growth with 6.74 × 10⁸ CFU/mL and 7.54 × 10⁸ CFU/mL. In trial 2, the results were not consistent. The OD600 values were similar to each other. The control tubes had OD600 values of 2.616 and 2.6000, while the test tubes had variation in the values at 2.560 and 3.348. CFU/mL had different values, with control tubes containing 6.50 × 10⁸ CFU/mL and 7.29 × 10⁸ CFU/mL, while test tubes contained higher growth values of 1.20 × 10⁹ CFU/mL and 1.10 × 10⁹ CFU/mL. In trial 3, the control tubes had OD600 values of 3.704 and 3.780, and test tubes had OD600 values of 3.304 and 3.211. CFU/mL for control tubes were 1.26 × 10¹⁰ and 1.15 × 10¹⁰, while test tubes were slightly lower at 1.08 × 10¹⁰ and 8.40 × 10⁹.

Overall, Trials 1 and 3 showed a noticeable reduction in OD600 and CFU/mL values in the test groups compared to the controls. For context, OD600 measures how cloudy a liquid culture is, which indicates how many bacteria are present, while CFU/mL counts the number of viable bacterial colonies on a plate. Statistical analysis confirmed that these differences were significant, with a p-value of 0.035 for OD600 and 0.038 for CFU/mL, suggesting that the reductions were unlikely to be due to chance. This supports the idea that metabolites produced by Lactobacillus acidophilus can inhibit the growth of Staphylococcus epidermidis under in vitro conditions. The reduction in growth is likely due to antimicrobial compounds such as lactic acid and hydrogen peroxide, which create a more acidic environment that is less favorable for bacterial growth. Initially, Trial 2 appeared to show inconsistent results, but it was later found that the labels had been flipped, and once corrected, the data aligned with the other trials. This confirms that the overall trend is consistent across experiments. Overall, the data supports the hypothesis that L. acidophilus metabolites reduce S. epidermidis growth, but additional trials and tighter control of variables would help strengthen the conclusions and confirm these findings under different conditions.

The results of this experiment support the hypothesis that L. acidophilus metabolites reduce the growth of S. epidermidis. In trial one, the control tubes had approximately 50 % more bacterial growth than the test tubes. In trial two, the plates were very likely to have been switched, and trial three supports that claim. The trend of reduced bacterial growth with L. acidophilus metabolites was observed in trials one and three, and most likely trial two as well, with the test tubes showing less growth compared to the control tubes. This decrease in bacterial growth is likely due to the antimicrobial effects of L. acidophilus metabolites (lactic acid and hydrogen peroxide), that create an acidic environment and disrupt bacterial growth

These findings suggest that probiotic metabolites may have the potential to be used as an alternative to control skin bacteria that is related to acne while still keeping microbiome balance. Probiotics could be encouraged in skin care products to help reduce breakouts on teenage and adult skin. Future studies could also include additional trials, testing against other acne related bacteria, and finding out if different concentrations of probiotics deters bacterial growth. Additional trials can also allow for statistical analysis of the data.  

Several limitations and possible errors may have influenced my results. One limitation was the small sample size and limited number of trials, which affects reliability. Another issue was in the setup of the experiment. The test tubes contained 2 mL of LB broth, while the control tubes contained 3 mL. Although the total volume was kept at 5 mL, this difference meant the control groups may have had slightly more available nutrients, which could have affected bacterial growth. In the test tubes, the 1 mL containing the supernatant was collected from the overnight culture. This supernatant would have used the nutrients present in their LB in the first growth period. As a result, the test groups may have had less nutrients available compared to the controls, which would influence the bacterial growth in both tubes (control would have an advantage). This experiment was also conducted in a liquid solution rather than under real skin conditions, so it does not fully replicate the complexity of the human skin microbiome. There may have also been pipetting inaccuracies during serial dilutions and bacterial transfers. Changes to the dilution process between trials could have introduced variability. In Trial 2, plates may have been mislabelled, which could explain inconsistent results, although results are reported as if there was no error. Small variations in incubation time or temperature may have also influenced growth.

Acne: Risk factors. (n.d.). Almirall: Feel the Science. Retrieved October 7, 2025, from https://www.almirall.com/your-health/your-skin/skin-conditions/acne/risk-factors#:\~:text=There%20are%20four%20basic%20factors,that%20blocks%20the%20hair%20follicle Alyoussef, A. (2024, January 3). The Impact of Consuming Probiotics and Following a Vegetarian Diet on the Outcomes of Acne. National Library of Medicine. Retrieved October 8, 2025, from https://pmc.ncbi.nlm.nih.gov/articles/PMC10835645/ Arnaout, B. (2021, June 4). The Gut Microbiome and its Effects on Human Health. NORGEN BIOTEK. https://norgenbiotek.com/blog/gut-microbiome-and-its-effects-human-health?srsltid=AfmBOorG2D-UYDyfhTroI7JF0uYecZyiYAnwlptfb5eXAh4D_rfDigih Batt\, C. A. (2014\, April 14). LACTOBACILLUS | Introduction. Encyclopedia of Food Microbiology (Second Edition). https://www.sciencedirect.com/science/article/abs/pii/B9780123847300001762 Batt, R. (2014). Probiotics for skin health: Mechanisms and applications. Journal of Cosmetic Dermatology, 13(3), 208–214. Bowe, W. P., & Logan, A. C. (2011, January 31). Acne vulgaris, probiotics and the gut-brain-skin axis - back to the future? - Gut Pathogens. Gut Pathogens. Retrieved October 9, 2025, from https://gutpathogens.biomedcentral.com/articles/10.1186/1757-4749-3-1 Campos, M. (2021, January 15). Acne: What you need to know. Harvard Health. Retrieved October 8, 2025, from https://www.health.harvard.edu/blog/acne-what-you-need-to-know-2019010315717 Chen, H., & Rowden, A. (2025, May 12). Gram-positive and gram-negative: What is the difference? Medical News Today. Retrieved January 1, 2026, from https://www.medicalnewstoday.com/articles/gram-positive-vs-gram-negative#what-they-are Chilicka, K., Dzieńdziora-Urbińska, I., Szyguta, R., Asanova, B., & Nowicka, D. (2022, March 15). Microbiome and Probiotics in Acne Vulgaris—A Narrative Review. National Library of Medicine. Retrieved October 7, 2025, from https://pmc.ncbi.nlm.nih.gov/articles/PMC8953587/ Claudel, J.-P., Auffret, N., Leccia, M.-T., Poli, F., Corvec, S., & Dreno, B. (2019, May 21). Staphylococcus epidermidis: A Potential New Player in the Physiopathology of Acne? Karger. Retrieved January 1, 2026, from https://karger.com/drm/article-abstract/235/4/287/114515/Staphylococcus-epidermidis-A-Potential-New-Player?redirectedFrom=fulltext Cleveland Clinic. (n.d.). Leaky Gut Syndrome: Symptoms, Diet, Tests & Treatment. Cleveland Clinic. Retrieved January 1, 2026, from https://my.clevelandclinic.org/health/diseases/22724-leaky-gut-syndrome Cleveland Clinic. (2022, April 5). Acidophilus (Lactobacillus Acidophilus): Uses, Benefits & Side Effects. Cleveland Clinic. Retrieved December 29, 2025, from https://my.clevelandclinic.org/health/drugs/22650-acidophilus Cleveland Clinic. (2023, August 18). What Is Your Gut Microbiome? Cleveland Clinic. Retrieved January 2, 2026, from https://my.clevelandclinic.org/health/body/25201-gut-microbiome Definition of lactobacillus - NCI Dictionary of Cancer Terms - NCI. (2022, December 1). National Cancer Institute. Retrieved October 6, 2025, from https://www.cancer.gov/publications/dictionaries/cancer-terms/def/lactobacillus Dessinioti, C., & Katsambas, A. (2022, December 22). Antibiotics and Antimicrobial Resistance in Acne: Epidemiological Trends and Clinical Practice Considerations. National Library of Medicine. Retrieved January 4, 2026, from https://pmc.ncbi.nlm.nih.gov/articles/PMC9765333/ Eckhart, L., & Sancar, A. (2020). The skin microbiome: A new actor in inflammatory acne. American Journal of Clinical Dermatology, 21(1), 18–24. https://doi.org/10.1007/s40257‑020‑00531‑1 Grice, E. A., & Serge, J. A. (2013, January 3). The skin microbiome - PMC. National Library of Medicine. Retrieved January 3, 2026, from https://pmc.ncbi.nlm.nih.gov/articles/PMC3535073/ Khalfallah, G., Gartzen, R., Möller, M., Heine, E., & Lütticken, R. (2021). A new approach to harness probiotics against common bacterial skin pathogens: Towards living antimicrobials. Probiotics and Antimicrobial Proteins, 13, 1557–1571. https://doi.org/10.1007/s12602-021-09783-7 Kober, M.-M., & Bowe, W. P. (2015, April 6). The effect of probiotics on immune regulation, acne, and photoaging. National Library of Medicine. Retrieved October 9, 2025, from https://pmc.ncbi.nlm.nih.gov/articles/PMC5418745/ Lambers, H., Piessens, S., Bloem, A., Pronk, H., & Finkel, P. (2006). Natural skin surface pH is on average below 5, which is beneficial for its resident flora. International Journal of Cosmetic Science. Retrieved January 5, 2026, from https://pubmed.ncbi.nlm.nih.gov/18489300/ Lee, E., & Anjum, F. (2023, April 27). Staphylococcus epidermidis Infection. National Library of Medicine. https://www.ncbi.nlm.nih.gov/books/NBK563240/ Mann, E. A., Bae, E., Kostyuchek, D., Chung, H. J., & McGee, J. S. (2020, September 29). The Gut Microbiome: Human Health and Inflammatory Skin Diseases. National Library of Medicine. Retrieved January 1, 2026, from https://pmc.ncbi.nlm.nih.gov/articles/PMC7992658/ Mayo Clinic. (2024, July 20). Acne - Symptoms and causes. Mayo Clinic. Retrieved December 28, 2025, from https://www.mayoclinic.org/diseases-conditions/acne/symptoms-causes/syc-20368047 Petersen, L. J. (1999). Interstitial lactate levels in human skin at rest and during an oral glucose load. Clinical Physiology. Retrieved January 5, 2026, from https://pubmed.ncbi.nlm.nih.gov/10361615/ Pu, M., & Garrett, E. M. (2023, November 10). Microbiology & infectious diseases. Pathology Outlines. Retrieved October 7, 2025, from https://www.pathologyoutlines.com/topic/microcutibacteriumacne.html Ruan, D., Hu, T., Yang, X., Mo, X., & Ju, Q. (2025). Lactate in skin homeostasis: Metabolism, skin barrier, and immunomodulation. Frontiers in Immunology. Retrieved January 5, 2026, from https://pubmed.ncbi.nlm.nih.gov/40046050/ Sánchez-Pellicer, P., Navarro-Moratalla, L., Núñez-Delegido, E., Ruzafa-Costas, B., Agüera-Santos, J., & Navarro-López, V. (2022). Acne, microbiome, and probiotics: The gut-skin axis. Microorganisms, 10(7), 1303. MDPI. Retrieved October 7, 2025, from https://doi.org/10.3390/microorganisms10071303 Singla, N., Singla, K., Attaubi, M., & Aggrawal, D. (2025, September 22). Gut-skin axis: Emerging insights for gastroenterologists-a narrative review. World Journal of Gastrointestinal Pathophysiology. Retrieved January 1, 2026, from https://www.wjgnet.com/2150-5330/full/v16/i3/108952.htm?appgw_azwaf_jsc=JYQo_vQ8YAjs-OjiEqyAg-Sfk7N0Ana3hDuHXcEpQcq_2q2exKqWbCmnXrc62tB1O8vuVqRcuXogzK5d0z2npSsTsaRsJWYKCRrig-PgZBCr_frGh-lwYYzfYgayWOyppEcrYwYsXUyT5qTl05Djb-z6fxkO4ao3_qRlfyWSPx9ZRGzNU3N-BHPk Smith WP. Epidermal and dermal effects of topical lactic acid. J Am Acad Dermatol. 1996 Sep;35(3 Pt 1):388-91. doi: 10.1016/s0190-9622(96)90602-7. PMID: 8784274. Wang, H., Gu, C. M., Xu, S., Wang, H., Zhao, X., & Gu, L. (2024). Measurement of optical density of microbes by multi-light path transmission method. mLife, 3(4), 565–572. https://pmc.ncbi.nlm.nih.gov/articles/PMC11686084/ What is The Skin Microbiome and Why Does It Matter? (n.d.). Absorbine. Retrieved January 3, 2026, from https://absorbine.com/pages/skin-microbiome?srsltid=AfmBOopwynuNUlZbwow4AnYdC8Pgq-fW6vHP8WcLmU8V3DtX44Ijw88o Wiginton, K. (2025, April 27). What Is the Skin Microbiome? WebMD. Retrieved January 3, 2026, from https://www.webmd.com/skin-problems-and-treatments/skin-microbiome

A huge thank you to my mentor Ms. Rachel. She taught me everything I needed to know for this project and guided me throughout the process. This project would not have been possible without her. I also would like to thank Dr. Soares for arranging CYSF and answering any questions I had or assisting me when I was unsure on what to do. My parents looked over my research and helped me make sure I was on the timeline, so I also appreciate them (as well as driving me to school during the breaks).