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J. Soc. Cosmet. Chem., 36, 313-318 (July/August 1985) Method for determination of the microbiological assay of imidazolidinyl urea A. POLTRONIERI, Lab. Analytical Research, STAR S.p.A.. Agrate B.za, Milan, Italy, S. PRIVITERA, Urelain S.r.l.. via Gatdone 7, Milan, Italy, and A. SALVI, Biolab S.r.l., Segrate, Milan, Italy. Received August 28, 1984. Synopsis The preservative imidazolidinyl urea is available from several manufacturers. Work with imidazolidinyl urea suggested its antimicrobial activity was variable. Therefore an assay based on zone of inhibition measurements using agar plates seeded with PJe//domonaJ aer/•ginosa was done to assess imidazolidinyl urea activity. Forty samples obtained from five vendors were assayed. in some cases substantial variability in antimicrobial activity was found. Results obtained by this assay method may, however, be used to adjust the preservative concentration in cosmetic products compensating for its variable activity. INTRODUCTION In recent years there has been a notable increase in the number of companies which offer under their own brandnames the antibacterial defined by the Cosmetic, Toiletry and Fragrance Association as imidazolidinyl urea. This study arose from our goal to determine via microbiological testing whether the antibacterial capacity of the preservative varies depending on its source, since traditional chemical analysis, viz. formaldehyde titration (1) or nitrogen assay (by Kjeldahl method) (2) apparently cannot be relied upon to indicate the antibaterial capacity of the preser- vative. METHODS OF EVALUATING THE ANTIBACTERIAL ACTIVITY OF PRESERVATIVES Standard methods of evaluating preservative activity are generally very simple, although they do require good laboratory technique and rigorous standardization. The methods are scalar (broth) dilution and agar diffusion (3). We chose agar diffusion for our study. The preservative activity was judged by inhibition of the growth of Pse•domonas aer•- ginosa strain ATCC 15442. It was stored for no longer than 14 days. It was transplanted by means of a streak onto the slanted solid medium (4 cc per 16 X 150 mm test tube, screw-capped). Incubation was for 48 hours at 30-32 ø C. The culture medium was Plate Count Agar (Difco) poured into test tubes 16 x 150 mm (4 cc per tube) and into bottles. It was sterilized at 121 ø C for 20 minutes. Seven cc of physiologically 313