The antimicrobial activity of a methanolic extract of the common edible fruit Prunus cerasus L. was evaluated. HPLC analysis of the extract revealed the abundant presence of anthocyanins. Minimum Inhibitory Concentrations (MIC) for Gram-positive and Gram-negative pathogenic bacteria was in the range 2–6.6 mg mL, whereas the time-kill assay revealed that the bactericidal effect was exerted only at concentrations higher than 2× MIC. Interestingly, at concentrations lower than MIC, P. cerasus L. extract exerted a stimulating effect on bacterial proliferation and on the ability to form biofilms. In particular, a multidrug-resistant Acinetobacter baumannii strain showed a 30% increase in growth at low concentrations. In the light of this finding, we speculate that the use of berry extracts (rich in anthocyanins) may present some risks to human health when used as preservatives in food and cosmetic products.