Which organism is commonly considered part of normal human skin flora?

• A. Staphylococcus epidermidis
• B. Escherichia coli
• C. Streptococcus mutans
• D. Bacillus subtilis

Answer: (A)

Staphylococcus epidermidis is a typical resident of human skin because it thrives on the dry, salty surface and forms part of the normal skin microbiota. It is a gram-positive cocci that commonly colonizes the outer skin layers and nasal passages, helping to occupy ecological niches and often inhibiting the growth of potential pathogens through competition and biofilm formation on surfaces. While it is usually harmless, it can act as an opportunistic pathogen in settings like implanted medical devices. In contrast, Escherichia coli is primarily a gut bacterium, Streptococcus mutans is associated with dental plaque in the mouth, and Bacillus subtilis is more environmental and not a standard skin resident. Therefore, Staphylococcus epidermidis is the organism commonly considered part of normal human skin flora.