Hydroxyapatite (HA), a mineral naturally found in bone, is used therapeutically to replace bone and to coat prosthetics, but can become infected once in the body. Eliana Cristina da Silva Rigo and her research group have investigated antibacterial compounds from propolis, a bee product used in traditional medicine, transferred onto HA. Rigo and her team extracted antibacterial compounds from red and green Brazilian propolis, applied it to HA, and evaluated the antibacterial activity (Biomed. Mater. 13 025010).
Propolis is a glue that bees produce to fix their hives, seal alternative hive entrances and ward off microbes such as fungi and bacteria, and also mites. It has been used in traditional medicine for a long time and has only recently been analysed scientifically. Of its various antimicrobial activities, the antibacterial activities are understood best. The team of researchers from the Universidade de São Paulo found that antimicrobial peptides extracted from propolis can be applied to HA used in implants.
Can propolis prevent infections?
HA is a bioceramic material that is mainly used in the clinic to fill bone defects. New knees, hips, teeth and the like are necessary to maintain quality-of-life in old age, as well as after diseases and injuries, but they bear a risk of infection. Antibiotics are given to avoid infections, however many bacteria, especially the prevalent Staphylocci and Escherichia coli, have recently acquired multidrug resistances. HA loaded with antimicrobials such as those extracted from propolis might provide a way of preventing these infections.
The authors found that antibacterials released from propolis-loaded HA can kill Staphylococcus aureus, which – together with Staphylococcus epidermis – accounts for 50 to 60% of infections in joint replacements. In their experiments, all bacteria were killed within one hour. Whether this holds true for other bacteria and in vivo experiments remains to be investigated.
As opposed to a previous study that used HA pellets, in this work, the team used a HA powder that allows the antimicrobials to be applied by spray drying rather than by immersion. This seemed to cause a difference in the timing of the release of the antimicrobial compound. Therefore, different packaging modes of loaded HA might provide a means to control the speed of release of antimicrobial activity.
Red or green propolis?
While propolis is generally brown, its colour can veer towards red, green, black or white according to the season, hive and geographical location of the bees. These colour differences seem to be due to different tree resins that the bees find and use to make propolis. Of the red and green propolis investigated in this study, red propolis had the stronger bactericidal activity, although green propolis also exhibited potent bactericidal effects.
As propolis in general, and specifically red propolis, exhibits potent antimicrobial activity, it will be exciting to see future work that tests these effects in vivo and that moves from HA powder to prosthetics, such as joint replacements.