Tackling carbapenem-resistant Acinetobacter baumannii (CRAB) and their virulence factors using biosynthesized silver nanoparticles combined with imipenem

Carbapenem-resistant <i>Acinetobacter baumannii</i> is an extremely hazardous microorganism due to its high level of resistance to a wide array of antibiotics, making it a significant public health concern. Herein, this study aimed to biofabricate silver nanoparticles using a cell-free filter derived from <i>Streptomyces</i> sp., with a focus on characterizing their physicochemical properties, and use them to combat CRAB and their virulence factors. The biofabricated Ag-NPs were predominantly spherical with an average size 50 nm, confirmed through TEM analyses, while DLS measurements showed an average hydrodynamic diameter of approximately 36.78 nm. UV-Vis spectroscopy displayed a characteristic surface plasmon resonance peak in the range of 420 nm, indicative of nanoparticle formation. XRD confirmed the crystalline structure, presenting peaks corresponding to face-centered cubic silver. FTIR spectroscopy revealed active participation of metabolite compounds derived from the <i>Streptom</i>yces cell-free filter in both reduction and stabilization processes. Eight clinical bacterial isolates were identified as CRAB using the Vitek-2 system, and biofilm formation with 100 % was assessed through Congo red and microplate assays. The MIC for Ag-NPs and imipenem (IMP) were found to be between 4 and 5 μg/mL and 13 and 15 μg/mL, respectively. Additionally, the fractional inhibitory concentration index (FICI) for the synergistic combinations of Ag-NPs and IMP ranged from 0.5 to 0.375, indicating a notable decrease in the MIC values for both IMP and Ag-NPs from 14 and 5 μg/mL to 1.75 and 1.25 μg/mL, respectively. The qRT-PCR demonstrated a significant reduction in the expression levels of the <i>Bap</i> and <i>ompA</i> genes by up to 4.0-fold (p ≤ 0.001). The time-killing assay confirmed that the bacterial strain was effectively eliminated through the synergistic action of Ag-NPs and IMP. Moreover, the cytotoxicity assessment of Ag-NPs and their combination with IMP revealed low toxicity of the combination of Ag-NPs and IMP, with an IC₅₀ of 26.13 ± 0.24 and 45.33 ± 0.21 μg/mL, respectively (p < 0.0019), indicating good biosafety, while the hemolysis rates were recorded at 0.4 and 0.7 at 12 and 24 h, respectively. We concluded that the combination of Ag-NPs with IMP could serve as a promising alternative strategy for treating CRAB.