Antibacterial activity of retinaldehyde against Propionibacterium acnes

Background: Retinaldehyde has been shown to exert antibacterial activity in vitro. Aim: This study evaluates the effect of retinaldehyde on Propioniba cterium acnes both in vivo and in vitro. Methods: Microbial minimal inhibit ory concentrations (MICs) of retinaldehyde and retinoic acid were determine d on reference strains of P. acnes. In vivo activity of daily topical appli cation of 0.05% retinaldehyde on the Fi acnes density was evaluated after a pplication in a single-blind randomised study. Results: MICs of retinaldehy de were 4 mg/l for P. acnes No. CIP179 and CIP53119 and 8 mg/l for P. acnes No. CIP53117. In contrast, the MICs of retinoic acid were superior to 128 mg/l for these three strains. In vivo, retinaldehyde-treated areas displaye d a significant decrease in counts of viable P. acnes as compared with the untreated areas with a median decrease of 10(2) log P. acnes/cm(2) after 2 weeks of daily application. Vehicle alone had no effect. Conclusion: The MI C of retinaldehyde against P. acnes suggests a direct antibacterial activit y. Daily topical application of 0.05% retinaldehyde is associated with a cl ear reduction of the P. acnes density.