Recently, a new promising compound for the oral treatment of acne and other dermatologic disorders became available [7,8]. This substance, 13-cis retinoic acid (Ro 4-3780), is presently being evaluated in clinical trials. A group of 18 patients with severe acne, unresponsive to previous treatment, participated in a controlled clinical and pharmacologic study [9,12]. From this group five patients were randomly selected to investigate possible effects of oral treatment with 13-cis retinoic acid on the bacterial skin flora. Clinical data of five patients are listed in Table 1. During the trial no additional systemic or topical treatment was administered. Clinical improvement was assessed by counting the skin lesions of the patients prior to initiation of treatment and every 4 weeks thereafter.
Samples from the center of the forehead and from the skin overlying the fourth thoracic vertebra were obtained by the "detergent scrub" technique of WiUiamson and Kligman [14]. This procedure was carried out immediately before and 4, 8, 12, and 16 weeks after initiation of the treatment regimen. The patients were asked to bathe and to wash their faces with soap and water the evening before the examination and no further cleansing was allowed until after the samples were taken.
Samples were processed and cultured as described previously [13]. Aerobic coryneform bacteria and anaerobically growing cocci were rarely present in substantial numbers and were therefore disregarded. Aerobic cocci and propionibacteria were identified by colony morphology and Gram stain. Staphylococcus aureus was identified by coagulase test. No further distinction was made between coagulase negative staphylococci and micrococci or between types of propionibacteria.
To evaluate a possible antibacterial effect of 13-cis retinoic acid in vitro, a minimal inhibitory concentration (MIC) assay was performed using a standard solid medium technique [1]. 13-cis retinoic acid was dissolved in acetone and added to TSYT medium (3 ~ Tryptone Soya Broth, Oxoid, 1% Yeast Extract, 0.5 % Tween 80, 1.2% agar) in 16 test concentrations up to a maximum concentration of 2 mg/ml. The acetone was allowed to evaporate prior to pouring of the plates. Three strains of each S. aureus, S. epidermidis, and P. acnes were randomly selected from routine samples. Standardized inoeula of these isolates were cultured on the test media aerobically (overnight) and anaerobically (7 days). Plain TSYT plates served as controls.