Soap, a Friend or Foe of Skin Bacteria

Overview

Abstract Human skin is home to the skin bacterium Staphylococcus epidermidis (S. epidermidis) which helps protect against harmful bacteria. Soap and alcohol can temporarily remove skin bacteria, but long-term use of soap may have allowed S. epidermidis to adapt to soap exposure, whereas alcohol use is relatively recent. This project tested whether soap or alcohol better maintains a healthy skin microbiome. I hypothesized that soap washing would allow S. epidermidis to persist more than the non-skin bacterium Escherichia coli (E. coli), while alcohol would remove both bacteria at similar rates. Pig skin was tested for background bacteria levels. Approximately equal numbers (colony-forming units, CFUs) of S. epidermidis and E. coli were applied to the pig skin, and survival of the bacteria was measured after washes with water, soap or 70% alcohol. For additional controls, S. epidermidis and E. coli were tested for resistance to soap and alcohol, and for survival when co-incubated. I found that washing with soap increased the relative number of S. epidermidis to E. coli on the pig skin compared to a water wash (paired t-test, p < 0.01, n = 3 biological trials). In comparison, 70% alcohol removed both bacteria equally (paired t-test, p > 0.05, n = 3 biological trials). For controls, soap did not significantly reduce the number of S. epidermidis or E. coli, while 70% alcohol eliminated both bacteria (paired t-test, p < 0.05, n = 3 biological trials). The two bacteria did not affect each other’s survival during co-incubation and background bacteria on the pig skin were minimal. In the short term, soap preserves beneficial skin bacteria better than alcohol, which may help support skin health.

Competition history

• CSEF 2026 Microbiology (Junior Division) · Entry J-16-09

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