Characterization of antimicrobial resistance and biofilm formation in Escherichia coli isolated from wild boars.

The wild boar (Sus scrofa) is a widely distributed and ecologically adaptable species, increasingly recognized as a useful sentinel for monitoring antimicrobial resistance (AMR) at the human-livestock-wildlife interface. This study investigated the occurrence of multidrug-resistant (MDR) Escherichia coli, with particular focus on biofilm-forming ability and associated resistance determinants, in wild boars from Southern Italy. Nasal swabs were collected from healthy wild boars harvested during the 2023 hunting season in the Campania Region. E. coli isolates were identified by MALDI-TOF MS and characterized for phenotypic and genotypic AMR profiles. Biofilm production was evaluated using the crystal violet assay, while biofilm-associated genes and selected resistance genes were detected by polymerase chain reaction (PCR). A total of 81 E. coli isolates were analysed. Biofilm formation was detected in 66 isolates (81.5%), mainly as weak producers, and was positively correlated with the presence of biofilm-associated genes (r = 0.79, p < 0.0001), predominantly fliC (13.6%), fimH (9.9%), papC (8.6%), and agn43/flu (6.2%). Carbapenemase genes were predominantly detected among biofilm gene-positive isolates, including the combined presence of blaIMP, blaVIM, and blaNDM in three strains. Significant associations (p < 0.05) were found for blaPER, mphA, and blaVIM when comparing biofilm gene-positive and -negative strains. Most isolates exhibited a MDR phenotype, with a multiple antibiotic resistance (MAR) index ranging from 0.20 to 0.39. These findings highlight wild boars as potential reservoirs of multidrug-resistant E. coli and reveal the complex interplay between biofilm formation, antimicrobial resistance, and wildlife, underscoring the need to include wildlife in integrated One Health antimicrobial resistance surveillance strategies.