BACKGROUND: Cranberry fruits possess many biological activities partly due to their various phenolic compounds; however the underlying modes of action are poorly understood. We studied the effect of cranberry fruit extracts on the gene expression of Staphylococcus aureus to identify specific cellular processes involved in the antibacterial action.
METHODS: Transcriptional profiles of four S. aureus strains grown in broth supplemented or not with 2 mg/ml of a commercial cranberry preparation (Nutricran90) were compared using DNA arrays to reveal gene modulations serving as markers for biological activity. Ethanol extracted pressed cakes from fresh fruits also produced various fractions and their effects on marker genes were demonstrated by qPCR. Minimal inhibitory concentrations (MICs) of the most effective cranberry fraction (FC111) were determined against multiple S. aureus strains and drug interactions with -lactam antibiotics were also evaluated. Incorporation assays with [(3)H]-radiolabeled precursors were performed to evaluate the effect of FC111 on DNA, RNA, peptidoglycan (PG) and protein biosynthesis.
RESULTS: Treatment of S. aureus with Nutricran90 or FC111 revealed a transcriptional signature typical of PG-acting antibiotics (up-regulation of genes vraR/S, murZ, lytM, pbp2, sgtB, fmt). The effect of FC111 on PG was confirmed by the marked inhibition of incorporation of D-[(3)H]alanine. The combination of -lactams and FC111 in checkerboard assays revealed a synergistic activity against S. aureus including strain MRSA COL, which showed a 512-fold drop of amoxicillin MIC in the presence of FC111 at MIC/8. Finally, a therapeutic proof of concept was established in a mouse mastitis model of infection. S. aureus-infected mammary glands were treated with amoxicillin, FC111 or a combination of both; only the combination significantly reduced bacterial counts from infected glands (P0.05) compared to the untreated mice.
CONCLUSIONS: The cranberry fraction FC111 affects PG synthesis of S. aureus and acts in synergy with -lactam antibiotics. Such a fraction easily obtained from poorly exploited press-cake residues, may find interesting applications in the agri-food sector and help reduce antibiotic usage in animal food production.