Abstract
Abstract The interaction of the aqueous extract of green propolis with the bacterial envelope was investigated using atomic force microscopy on E. coli and S. aureus, as Gram-negative and Gram-positive model microorganisms. The bacteria were incubated in the extracts with variable proportions of water/ethanol, i.e., in different concentrations of total phenolic and flavonoids, for 4 and 12 hours. All extracts exhibited antibacterial action with higher activity (lower MIC) for full ethanolic extract (100% alcohol) against both strains, what was attributed to a synergistic interaction of ethanol and phenolic compounds presented in greater concentration in this extract. According to AFM analysis, it was evidenced the propolis main mechanism of action as rupture and lysis of bacterial cells. Changes in bacterial morphology following treatment indicate membrane imbalances with the internal contents leaking out accompanied by cell swelling, due to possible water uptake through cell lysis. The intensity of such damage was found to be time and specie dependent.
Highlights
Propolis is a well-known natural compound collected by bees from plant fluids and exudates and is used to protect and seal parts of the hive
The aims of this study are: i) prepare aqueous and ethanolic extract from green Brazilian propolis; ii) to identify the main phenolic constituents; iii) to determine their minimum bacterial inhibitory concentration; and iv) evaluate the effects of extracts exposing extracts to the bacteria cell walls using atomic force microscopy in an effort to help clarify the nature of the molecular interactions involved in propolis antibacterial activity
After resting and spontaneous precipitation the extracts were filtered in 0.2 μm pore size PVDF membranes (Milipore, MA, USA) and identified as WPE, EPE30 and EPE50, EPE70, EPE for ethanol concentration of 50, 70 and 100% respectively
Summary
Propolis is a well-known natural compound collected by bees from plant fluids and exudates and is used to protect and seal parts of the hive. Raw propolis consists essentially of resin (around 50% of de mass) and wax (30%), with fractions of essential oils (10%), pollen (5%) and several other organic-derived compounds and impurities (Pietta et al, 2002). The proportion of these compounds is strongly dependent on botanic source and geographic origin (Bankova et al, 2014). Phenolic and flavonoid compounds are reputed to have the highest biological and pharmacological activities (Vennat et al, 1995; Marcucci et al, 2001)
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