This study examined the suitability of commercial U.S.P. insulin as a bacterial growth medium. Purified port insulin (Iletin II, Eli Lilly & Co., Indianapolis, Indiana) was inoculated with approximately 500,000 bacteria per ml of: (1) Staphylococcus epidermidis, (2) Staphylococcus aureus, or (3) Escherichia coli. All three bacterial cultures were sterilized by the insulin within 24 h at 37°C. However, Staph epidermidis was the most sensitive and Escherichia coli was least sensitive to the bactericidal effects of commercial U.S.P. insulin. Components of commercial U.S.P. insulin were then examined for their bactericidal activity against Escherichia coli. Phenol (230 mg/dl), glycerol (1.6 d/dl), and zinc cations (4.0, 2.0, and 1.0 mg/dl) demonstrated bactericidal activity, whereas dialyzed insulin demonstrated minimal effects. We conclude that insulin contaminated with 5 × 105 bacteria commonly found on the skin will self-sterilize within 24 h. This effect is secondary to the additives placed in the insulin and not to the insulin itself.

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