Activity of Brazilian propolis extracts incorporated into chitosan nanoparticles against mixed Candida albicans and Klebsiella pneumoniae biofilms

Background Candida albicans and Klebsiella pneumoniae represent a severe clinical challenge, particularly in immunocompromised patients, since systemic candidiasis increases the risk of co-infection and mortality by K. pneumoniae , a multidrug-resistant hospital bacteria. The interaction of these pathogens enables the formation of polymicrobial biofilms, where they form a community developing a mutualistic relationship, rendering the infection even more resistant to conventional therapies. Methods This study evaluated the antimicrobial activity of several Brazilian propolis extracts via Minimum Inhibitory Concentration (MIC) assays. The extracts with the best results were selected: Red Propolis (RP) and Mandaguari Propolis (MP) for nanoparticle encapsulation. Chitosan nanoparticles (CNPs) were developed via ionic gelation to improve the efficacy of the crude extracts and decrease their cytotoxicity, while remaining a natural and biodegradable compound. The nanosystems were characterized by Dynamic Light Scattering (DLS), Zeta Potential, and Atomic Force Microscopy (AFM). Antimicrobial and antibiofilm activities were evaluated against mono- and polymicrobial biofilms (simultaneous and pre-formed models) via biomass quantification (crystal violet), extracellular matrix quantification (safranin), metabolic activity (XTT), Colony Forming Unities (CFU) counting, and Scanning Electron Microscopy (SEM). Finally, cytotoxicity was evaluated in MRC-5 human fibroblast cells. Results Both propolis extracts demonstrated potential antimicrobial activity even in their crude form; however, the chitosan nanoparticle system significantly increased their efficacy and decreased cytotoxicity. The system exhibited a mean diameter among 202–280 nm and a positive zeta potential ( ~ + 30 mV), indicating colloidal stability and enhanced membrane permeation. In mixed biofilms, both nanoencapsulated extracts reduced biomass by approximately 80% and the extracellular matrix by over 70%. Notably, nanoparticles loaded with RP showed superior antibacterial activity against K. pneumoniae , while nanoparticles loaded with MP demonstrated potent antifungal activity against C. albicans . Conclusions Brazilian propolis extracts demonstrate efficacy and preliminary safety for the treatment of microbial infections by polymicrobial biofilm formation; however, they present superior results when incorporated into the chitosan nanosystem. The encapsulation of propolis extracts in chitosan nanoparticles overcomes the limitations of crude extracts, providing effective prototype for adjuvant therapy against complex polymicrobial infections.