Antimicrobial properties of phenolic compounds from hops, Humulus lupulus L.: fight against multidrug resistance

Research on new antibacterials is a real and timely challenge, particularly for the treatment of infections caused by multidrug resistant (MDR) clinical isolates. Methicillin-resistant Staphylococcus aureus (MRSA) is the most worrying pathogen [1,2]. Despite advances in the knowledge of resistance mechanisms, it is difficult to find new effective therapeutic agents [3]. Hops (Humulus lupulus L., Cannabaceae) is a nutritional and medicinal plant. Female inflorescences of hops are used in the brewing industry for their bitterness, but also for their antiseptic properties. The antibacterial activity of female cones on Gram-positive bacteria has been well documented [4,5]. This activity is particularly attributed to acylphloroglucinols, known as bitter acids. Prenylchalcones, including xanthohumol, also show a broad anti-infective spectrum [4,5]. This potential is not exploited against multidrug resistant bacteria. In this study, the antibacterial properties of hops have been confirmed. For this purpose, hydro-alcoholic extracts of leaves, stems, roots and female cones were tested against many human pathogenic bacteria by Steers technique. No activity on Gram-negative bacteria was observed. Female hop cones showed a inhibitory activity against Gram-positive strains, with a minimum inhibitory concentration (MIC) of 19.5 to 39 µg · mL^-1 for S. aureus strains including MRSA. Some phenolic compounds were purified from the active parts, by centrifugal partition chromatography and high-performance liquid chromatography. Preliminary results show that chalcones are more active than acylphloroglucinols. A special interest is focused on xanthohumol, which has a MIC of 9.8 µg · mL^-1 toward the multi-resistant strain S. aureus T28.1, isolated from an osteitis. Various experiments are underway to increase knowledge of the mechanism of action, and to identify synergies with antibiotics. The purification of minor chalcones and the evaluation of their activity are in progress.

Acknowledgements: The authors wish to thank CUMA (University of Lille 2, France, Pr. J.F. Goossens) and LARMN (University of Lille 2, France, Pr. N. Azaroual) for access to equipment and the members of these platforms for their skillfull technical assistant, as well as some members of the laboratory of bacteriology (LIRIC) for their advice. The authors gratefully thank the Region Nord-Pas-de-Calais and the University of Lille 2 for the funding of the PhD fellow Laetitia Bocquet.

Keywords: phenolic compounds, antimicrobial, hops, MRSA.

References:

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No conflict of interest has been declared by the author(s).