Sublethal Concentrations of Silver Nanoparticles Stimulate Biofilm Development

Although silver nanoparticles (AgNPs) are used as antimicrobial agents in a wide variety of commercial products, sublethal exposure can counterproductively promote the development of biofilms. We observed by fluorescence microscopy denser biofilm growth with mixed cultures from a wastewater treatment plant after exposure to 21.6 mu g/L 10 nm AgNPs. To further understand biofilm promotion mechanisms, experiments were conducted with a pure culture of Pseudomonas aeruginosa PAO1. Sublethal exposure of PAO1 to AgNPs (10.8 and 21.6 mu g/L) also enhanced biofilm development and upregulated quorum sensing, lipopolysaccharide biosynthesis, and antibiotic resistance (efflux pump) genes. An increase in the sugar and protein contents of the PAO1 biofilm matrix (by 55 +/- 3 and 114 +/- 32%, respectively, relative to unexposed controls) corroborated the transcriptional upregulation of PAO1 biofilm-related genes. Enhanced biofilm development by exposure to low AgNP concentrations might accelerate biofouling and biocorrosion and harbor pathogens that pose a risk to public health.