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The present study describes the antibacterial behavior and the bacterial resistance analysis of extremophile Pseudomonas aeruginosa in contact with copper nanoparticles (CuNPs). For this purpose, green synthesis of CuNPs was performed by combined ultrasound-assisted and chemical reduction methods, obtaining semispherical CuNPs ranging from ca. 4-9 nm. Antibacterial activity (AA) of biosynthesized CuNPs demonstrates an antibacterial inhibition of 85 % (LD85) at 400 µg/mL and a minimum bactericidal concentration (MBC) of 800 µg/mL after 3 h of contact. Bacterial adaptation in contact with CuNPs was observed through the consecutive exposition of microorganisms, presenting a significant increase of LD85 values from 400 µg/mL to 6400 µg/mL after 11 expositions. This behavior demonstrates the bacterial growth adaptation with high-dose of CuNPs. The bacterial resistance mechanism was determined through the overproduction of pyocyanin, associated with oxidative stress events, the genomic polymorphism of resistant bacteria obtained by PCR-RAPDs, and the morphological interaction between P. aeruginosa and CuNPs evidenced by transmission electron microscopy (TEM) micrographs. Our results suggest that under controlled CuNPs exposition, extremophile P. aeruginosa can generate bacterial resistance mechanisms, an important issue for the effective design of antimicrobial nanomaterials. [ABSTRACT FROM AUTHOR]