Acetic, propionic, and oleic acid as the possible factors influencing the predominant residence of some species of Propionibacterium and coagulase-negative Staphylococcus on normal human skin.

The minimum inhibitory concentration (MIC) of acetic and propionic acid for resident bacteria on normal human skin, such as Propionibacterium acnes and Staphylococcus epidermidis, was 25 mg/mL or more at any pH tested (pH 5.5-6.8). While the MIC of these acids for most of the transient bacteria was markedly decreased by lowering the pH of the media and at pH 5.5, the mean pH value of the normal human skin, the MIC was 6.25 mg/mL or less. The MIC of oleic acid for some strains of Gram-positive transient bacteria of Streptococcus, Micrococcus, or Bacillus was 100 micrograms/mL or less at all pH's tested. Staphylococcus aureus was resistant to this acid at pH 6.8, but became as sensitive as Streptococcus when the pH was lowered. The growth of P. acnes, the most predominant resident bacterium, was enhanced markedly and reached a maximum level at 6.25 mg/mL of propionic acid, 12.5 mg/mL of acetic acid, and 50-100 micrograms/mL of oleic acid. On the basis of these results, we presumed that acetic, propionic, and oleic acids are factors influencing the predominant residence of some species of Propionibacterium and coagulase-negative Staphylococcus on normal human skin.