Deciphering the biological activity of propolis and its constituents

Propolis is a varied combination of plant resins scavenged by worker bees for use within the hive. A central challenge lies in establishing correlations between its chemical composition and biological activities. This study aimed to investigate the biological activity of propolis using three global samples relating chemical composition and the potential interaction with antibiotics using an industry sample of propolis. Methods utilised included microbial, biochemical, and RNA expression analyses. HPLC data showed notable disparities in concentrations of key propolis constituents between regions, with North Portuguese (NP) propolis exhibiting the highest levels of all standards compared to those of UK (Leeds) and Brazilian origin. Antimicrobial testing revealed that the propolis MIC was lower for gram-positive bacteria; S. aureus and E. faecalis (MIC 0.03-0.06% for both UK and NP) when compared to gram-negative bacteria, with E. coli exhibiting an MIC of 0.125% and P. aeruginosa 1%. Sub-inhibitory concentrations of propolis extended the lag phase most notably against S. aureus and E. faecalis. Biofilm assays demonstrated propolis' efficacy in inhibiting biofilm formation, whilst kill curves demonstrated bactericidal activity across all three propolis samples and antioxidant testing revealed potent antioxidant activity, with NP exhibiting the greatest activity. Evaluation of propolis-antibiotic interactions revealed increased zones of inhibition for MRSA and MSSA isolates when combined with 6 out of 8 antibiotics tested. E-tests confirmed a MIC reduction. Chequerboard assays revealed the nature of the relationship. RNA sequencing showed evidence of bacterial stress, with downregulation of cellular metabolic processes. In conclusion, findings highlight the multifaceted nature of propolis' biological activities, such as the exhibition of antimicrobial activity, however HPLC analysis of 13 standards is not sufficient to fully predict biological activity. Chequerboard assays confirmed either a synergistic or additive activity with three antibiotics highlighting the potential of propolis to be used in the fight against antibiotic resistance.