Phytochemical analysis and antimicrobial activity of blackberry (Rubus fruticosus) fruit extract against Gram-negative multidrug-resistant bacteria isolated from clinical samples

While antibiotics remain foundational in treating microbial infections, their overuse has accelerated the emergence and global spread of multidrug-resistant (MDR) bacterial strains. This study aimed to characterize the phytochemical composition of a concentrated blackberry (Rubus fruticosus) extract and evaluate its antimicrobial potential against clinical MDR strains of Acinetobacter baumannii, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Enterobacter cloacae. Phytochemical quantification was performed using spectrophotometric, titrimetric, and HPLC analyses, while antimicrobial activity was determined via the well diffusion method and minimum inhibitory concentration (MIC) assays. The blackberry extract contained 0.54% total phenolic compounds and 4.60% organic acids; in comparison, a green tea leaf extract exhibited higher phenolic content (10.10%) but lower organic acids (1.60%). The total anthocyanin content of the blackberry extract was 159.81 mg/100 g, with cyanidin-3-O-glucoside (134.56 ± 0.10 mg/100 g) identified as the predominant constituent. In silico molecular docking studies indicated that neither individual antibiotics nor isolated anthocyanins could effectively inhibit the full array of resistance mechanisms in Gram-negative bacteria. In contrast, in vitro assays demonstrated that the whole blackberry extract significantly inhibited the growth of all tested MDR strains. These integrated findings suggest that combating complex MDR pathogens requires a multi-target strategy. The broad-spectrum efficacy of the crude extract, likely arising from synergistic interactions among its constituents, underscores the potential of complex phytochemical preparations. Such natural formulations represent a promising avenue for the development of novel antimicrobial agents and may offer a viable strategy to restore the efficacy of existing antibiotics through adjunctive therapy.