j. Soc. Cosmet. Chem., 42, 393-394 (November/December 1991) Letters to the Editor TO THE EDITOR: The conclusions drawn in the Korting et al. paper on skin pH (J. Soc. Cosmet. Chem., 42, 147, 1991) require critical teevaluation. In the authors' opinion, the results "support the hypothesis that acidity or alkalinity can influence both the surface pH and propionibacteria as one of the major components of the skin flora in the long term." The authors assert that "the only difference between the two [test] preparations consisted of the pH (5.5 with preparation A 8.5 with prepa- ration B)." The chemical and physical properties of several major components common to both A and B are altered by pH. As a result, the ingredients' interaction with skin and with microorganisms are also altered. The observed differences in the results be- tween A and B should not have been attributed exclusively to alteration in the activity of hydrogen ions. The authors also failed to provide data on the time elapsed between skin washing and their measurements, a critical issue according to Sauermann et al. (J. Soc. Cosmet. Chem., 37, 309, 1987). Specifically, sodium lactate, citric acid, the vitamins, amino acids, and disodium EDTA are not the same chemicals at pH 5.5 and pH 8.5. More importantly, the reactions between an alkylether sulfate and skin protein (sorption, denaturation, etc.) depend on pH. The substantivity of the betain in the product is dependent on pH. Finally, the preservative system (which reportedly included only benzyl alcohol and methylisothia- zoline but not methylchloroisothiazoline) exhibits its stability and its optimal efficacy (perhaps against skin bacteria) in a reasonably narrow pH range. It would be a formidable task to sort out all potential interactions that might contribute to the observed differences in the propionibacteria population. An acidic skin pH per se might indeed be a poor environment for the survival of certain bacteria and may benefit acne patients, but the evidence presented to date by Korting et al. suggests that the presumptive benefit should more likely be attributed to pH-controlled alterations of one or more of the product's components. M. M. Rieger Morris Plains, NJ 07950 TO THE EDITOR: I am grateful to Dr. Rieger for his letter, as it may encourage broader discussion on the role of the pH of skin cleansers or even cosmetics in general. In fact, this has been a 393

394 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS subject of debate for decades, but unfortunately one has not come to definite conclusions so far. A broader debate at the beginning of the nineties facing new experimental evidence forwarded recently might at least contribute a lot to, if not solve, the entire problem. Unfortunately, a few statements made by Dr. Rieger are not entirely correct from my point of view. In the first place, we have not "failed to provide data on the time elapsed between skin washing and... measurements": it is clearly stated in the section entitled "Time Course of Investigations" that measurements were performed "at about the middle of the application interval, i.e., at least six hours after the last cleansing procedure." In the second place, we do not totally underscore the potential role of the pH of the cleansing preparations used on its ingredients. We do state in the Discussion section that "in particular, differences in the pH value might affect substantivity of various ingredients and moreover direct antimicrobial properties of ingredients such as benzyl alcohol and methyl-isothiazolinone itself." Such a statement, however, has to be based on theoretic considerations rather than defined knowledge insofar as a particular complex preparation is concerned. For this reason, it seems to be wise not to include the statement in the Materials and Methods section. The core of the criticism by Dr. Rieger, however, seems to be that a variety of different factors might be the basis for the differential effect of the two preparations compared. He speaks of a "formidable task" to obtain more insight in vivo, and in fact it is. Therefore, it seems much more helpful just to take the two following pieces of evidence from the present experiment: 1) The long-term use of skin cleansers differing in their pH value influences the pH of the skin surface, and 2) the number of propionibacteria on the skin surface differs accordingly. The idea of a causal relationship between both phenomena probably can be much better substantiated in continuous culture experi- ments with the various microorganisms found on the skin. At the time when the present trial was performed we already had evidence from batch culture that there is a sharp rise in the specific growth rate of Propionibacterium aches, changing the pH of the medium from pH 5.5 to pH 6.0, while this is not the case with coagulase-negative staphylococci. This experience has been cited in the Introduction section of the paper. In the mean- time, however, continuous culture experiments have been performed using a chemostat. They have shown that again Propionibacterium aches, but not Staphylococcus epidermidis demonstrate a critically different growth behavior just within the comparatively small pH range to be influenced by skin washing procedures (1). REFERENCE (1) H. C. Korting, A. Lukacs, M. Vogt, J. Urban, W. Ehret, and G. Ruckdeschel, Influence of the pH-value on the growth of Staphylococcus epidermidis, Staphylococcus aureus and Propionibacterium acnes in continuous culture, Zbl. Hyg. Umweltmed., in press. H. C. Korting 8000 Munich 2, Germany