J. Soc. Cosmetic Chemists, 17,343-353(1966) Use of Human Subjects for Product Evaluation: An Evaluation of Antibacterial Soap Bars JOHN A. KOOISTRA, Ph.D., ELMER A. BANNAN, M.S., and R. OWEN CARTER, Ph.D.* Presented September 21, .1965, Midwest Chapter Synopsis--It is shown that in vitro •nicrobiological testing of antibacterial-containing soaps is frequently unreliable and is influenced by the strain of the organisms used. On the other hand, in vivo testing under laboratory controlled or consumer testing conditions can be used to establish the degerming efficacy of sanit/zing soap bars. Several techniques for con- ducting such tests are described. For many years efforts have been made to improve the skin de- germing potential of skin cleaning agents by enhancing the inherent anti- bacterial power of soap through the incorporation of specific antibacte- rial ingredients. Many antibacterial chemicals were tried. It was not, however, until the introduction of hexachlorophene in the mid-forties that a satisfactory agent was found to achieve this purpose. With the demonstration that skin degerming through the use of specific antibac- terials incorporated into soap was feasible, the search for new and better antibacterial agents was intensified. The apparent simplicity of the degerming process is likely to be mis- leading. Actually, the suppression of the cutaneous micro flora through washing of the skin with surface active agents containing bacteriostatic compounds is a complex process. It involves a number of interactions, * The Procter & Gamble Company, Ivorydale Technical Center, Cincinnati, Ohio. 343

344 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS most of which are understood incompletely at best, among the cutaneous microorganisms, the agent, and the environment. Individual micro- organisms of the skin differ greatly among themselves in regard to sen- sitivity to a given antibacterial agent. Agents differ in their ability to deposit from the washing solution on the stratum corneum and to pene- trate into the appendages of the skin. The effect of the matrix and the solubility of the antibacterial agent in the liquid phases of the skin sur- face influence the degree to which the bacterial population is affected. These factors play a role in determining the extent to which a practical objective is achieved that is, to decrease the number of skin bacteria that can lead to infection and to axillary and other so-called body odors. In view of such a multifaceted situation, methodology is obviously critical in evaluating new antibacterial agents and in comparing one material or product with another in regard to degerming efficacy. The purpose of this paper is to discuss some approaches currently used for evaluation of skin degerming efficacy and to highlight their advantages and limitations. METHODOLOGY AND •DISCUSSION In Vitro Testing In vitro procedures, because of their relative simplicity, are widely utilized. In order for an agent to warrant consideration at all as a candidate antibacterial ingredient, obviously it must first have anti- bacterial activity in the matrix in which it is to be used. In vitro pro- cedures can be used for screening purposes to supply such information and thus exclude from further consideration compounds with inadequate activity. By their nature, however, these techniques rely on arbitrary selections of test organisms and exposure conditions. They can give reproducible results however, extrapolation from in vitro data to more complex or practical situations has been found by experience to be un- reliable. Many in vitro procedures have been advocated, but the type that is most widely used at the present time involves determination of the minimum inhibitory concentration (MIC). Part of the problem inherent in basing over-all conclusions regarding degerming efficacy on MIC data is illustrated by the information of Table I. This table presents MIC data comparing two antibacterial soaps against a group of 20 different Staphylococcus aureus strains iso- lated from pyogenic infections. Antibacterial Soap A was a milled bar