372 JOURNAL OF COSMETIC SCIENCE solution consisted of 0.075 M phosphate buffer with 0.1% Triton X-100 detergent at pH 7.9 and letheen as the neutralizer. Letheen has been previously validated and long used in this laboratory as a suitable neutralizer for formulations containing TCC. The samples were serially diluted in half-strength (0.0375 M) buffer in ten-fold dilutions to 10- . Aliquots (0.1 ml) of each undiluted and diluted specimen were spread in triplicate on the surface of trypticase soy agar with 5% sheep blood (TSA/B). All plates were incubated aerobically for 48 + 4 hours at 35 ø + 2øC. The number of CFUs per dilution was determined. RESULTS In both studies, the surviving CFUs of bacteria for each subject were enumerated and these values were 1og•o transformed. The mean loglo CFU counts for both the immediate and prolonged (24-hour) efficacy studies are presented in Table I. The difference in the loglo CFU counts between the antibacterial bar soap and the placebo are also shown in this table. The log•o CFU counts were compared using a Wilcoxon signed-rank test to estimate which of the bars (antibacterial or placebo) had the greatest antibacterial activity. A binomial sign test was also used to determine any significant differences between the number of subjects experiencing a reduction in CFUs with the antibacterial bar soap relative to the bar soap vehicle. The antibacterial soap was significantly (p • 0.05) more effective than the vehicle at controlling the growth of S. aureus on the skin at the 30-minute, two-hour, and five-hour occlusion times (Table I). Even when the challenge was 24 hours after the final wash, the residual benefit of the antibacterial soap was significantly (p • 0.05) more effective than that observed immediately after use. The binomial data for subjects with fewer CFUs is presented in Table II. A significantly (p • 0.05) greater number of subjects had fewer surviving organisms on the arm washed with the antibacterial soap than on the arm washed with the placebo soap at all contact times. This pattern continued even 24 hours after the final wash. DISCUSSION Washing with either an antibacterial soap or with a non-antibacterial soap will remove bacteria from the skin due to the surfactancy of the soap base and the mechanical action of the wash procedure. Still, washing is often incomplete, and the surviving bacteria can readily recover. However, antibacterial soaps are generally formulated to deposit an antimicrobial agent on the skin that may remain effective for an extended period after washing. As demonstrated in this work, this residual efficacy can inhibit the growth of organisms that survive on the skin or come in contact with the skin subsequent to washing. Within two to five hours after contact with bacteria, individuals who had recently washed exhibited greater than 1 log difference between the antibacterial soap and the soap vehicle. These results are consistent with those reported by Finkey eta/. (9) and Scala et aL (10), in which they observed a residual difference of approximately 1.3 logs and 2.0 logs, respectively, between an antibacterial soap containing 1.5% TCC and

ANTIBACTERIAL SUBSTANTIVITY 373 •o tim o +1 +1 +1 +1 +l +1 v v v +1 +l +1 +1 +1 +1 • • • c'-,1 c'-,1