In vitro nano‑emodin–associated exosome‑mediated aPDT against Acinetobacter baumannii

Biofilm-forming Acinetobacter baumannii, particularly multidrug-resistant (MDR) strains, present major challenges in burn wound treatment. This study evaluated the antibacterial and antibiofilm efficacy of exosomes associated with nano-emodin (NE-Exos) combined with antimicrobial photodynamic therapy (aPDT) against A. baumannii. Antibacterial and antibiofilm activities were evaluated against the standard strain (ATCC 19606) and clinical isolates of A. baumannii, including MDR, extensively drug-resistant (XDR), and non-MDR/non-XDR strains. Minimum inhibitory and bactericidal concentrations (MIC, MBC), biofilm-inhibitory and eradication concentrations (MBIC, MBEC), drug release, hemolysis, and expression of biofilm-associated genes (abaI, bap) were evaluated. Nano-emodin and NE-Exos showed inhibitory and antibacterial activities with MIC and MBC values ranging from 15 to 39 μg/mL and 30 to 156 μg/mL, respectively. NE-Exos showed better antibiofilm efficacy compared to nano-emodin alone, especially in MDR isolates. The release of nano-emodin from NE-Exos reached 30.73% nano-emodin release at pH 5.5 after 300 min, compared to 27.56% and 23.76% at pH 7.4 and 8.5, respectively, and was associated with minimal hemolysis (0.015%). Furthermore, NE‑Exos combined with aPDT significantly downregulated the biofilm‑associated genes abaI and bap, with reductions of up to 10.96‑fold and 5.33‑fold, respectively (p < 0.001). Exosomes may have contributed to the observed antimicrobial effects, although their role appears to be as a carrier for nano‑emodin. These findings support NE‑Exos combined with aPDT as a promising in vitro strategy against A. baumannii biofilms; however, the lack of direct quantification of nano‑emodin encapsulation should be considered a study limitation.