154 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS when the washings with preparation B began, and a sharp fall when the washings with preparation A began. Figures 5 and 6 demonstrate the findings in more detail. At the forehead, propionibacteria were significantly fewer in the presence of the acidic cleansing preparation on day 31 at the forearm the same applied to days 10, 17, 24, 31, and 59. INTERDEPENDENCE OF BACTERIAL DENSITY AND pH While the number of coagulase-negative staphylococci was only correlated with the skin pH values in a significant manner at the forehead and not at the forearm, propionibac- terial counts and skin surface pH correlated significantly at both sites, the correlation coefficient also being higher, especially at the forehead (Figures 7-10). DISCUSSION The pH values found at the skin surface of the forehead and the forearm in our study population lie well within the range given in the pertinent literature (for review, compare 19). As in our previous investigation, the mean pH value again tends to be lower than 5.5, which is considered as a representative value by most authors. Washing 8.5 8 7.5 ? 6.5 6 5.5 5 4.5 4 3.5 Y - .766 +.G?9 X r-.47B (slg., p-.BS) 95• : .3B8 r .618 N- 180 .... I . . I I I I I I I I .... I .... I .... 4.5 5 5.5 6 6.5 [pHI Figure 7. Correlation of the counts of coagulase-negative staphylococci and pH values on the forehead from washing with synthetic detergent preparations at pH 5.5 and 8.5.

pH EFFECTS ON SKIN 155 n 4.5 4 3.5 2.5 1.5 ! ,5 N Y - 3.161 -.849 X r--.831 (n.s., p.85) 95• • -.228 r .169 N- N N N 4.5 5 5.5 6 6.5 [pHI Figure 8. Correlation of the counts of coagulase-negative staphylococci per square centimeter and pH values on the forearm from washing with synthetic detergent preparations at pH 5.5 and 8.5. the skin of the forehead and the forearm repeatedly with an acidic synthetic detergent preparation influenced both skin surface pH and the bacterial resident flora, as in the previous study (7). This observation supports the hypothesis, which still is the subject of considerable debate, that washing habits not only influence the skin surface pH in the short term but also in the long term, and that there are associated changes with respect to the cutaneous microflora. Moreover, it is of particular interest that the biological effects on both parameters are similar with the repeated application of both an alkaline soap and an alkaline syndet, but remarkably different in the case of an acidic syndet, even if the chemical composition of both syndets is nearly identical, except for pH. Certainly, a trial of the present type does not allow one to attribute the biological effects found solely to the pH value of a skin cleansing preparation. Indirect effects are still possible. In particular, differences in the pH value might affect substantivity of various ingredients and, moreover, direct anti- microbial properties of ingredients such as benzyl alcohol and methylisothiazolinone itself. This, however, should affect both propionibacteria and staphylococci in a similar way and should not selectively affect the former component of the cutaneous resident flora. A direct effect of the skin surface pH by itself is more likely, as there is a major difference in the specific growth rate of propionibacteria but not of staphylococci in the