Although silver nanoparticles (AgNPs) are used as antimicrobial agents in a wide variety of commercial products, sub-lethal exposure can counterproductively promote the development of biofilms. We observed by fluorescence microscope faster and denser biofilm growth with mixed cultures from a wastewater treatment plant after exposure to 21.6 µg/L of polyvinyl pyrrolidone (PVP)-coated AgNPs (10-nm), in either a static system or a flowing system. In our flow cell biofilm bioreactor, biofilm development was assessed on different materials used for water distribution systems, and it was more pronounced on a concrete surface than on polyvinyl chloride or stainless steel surface. In all cases, a stimulatory effect on biofilm formation was exerted by low concentrations of PVP-AgNPs (4.5- to 20.5-fold increase in total organic carbon of biofilm compared to each untreated control). We also investigated the effects of AgNPs and Ag<sup>+</sup> ions on biofilm microbial community structure and their transcriptomic activity by using advanced molecular biotechnologies (e.g., pyrosequencing and quantitative real-time polymerase chain reaction), and after AgNP treatment biofilm community shifted towards silver resistant species. To further understand biofilm promotion mechanisms, experiments were conducted with a pure culture of <i>Pseudomonas aeruginosa</i> PAO1. Sub-lethal exposure of PAO1 to PVP-AgNPs (10.8 and 21.6 µg/L) also enhanced biofilm development, and up-regulated quorum sensing (QS), lipopolysaccharide (LPS) biosynthesis and antibiotic resistance (efflux pump) genes. An increase in sugar and protein contents of the PAO1 biofilm matrix (by 55±3% and 114±32%, respectively, relative to unexposed controls) corroborated the transcriptional up-regulation 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. Furthermore, considering that biofilms are more difficult to eradicate than planktonic bacteria, the observed up-regulation of antibiotic resistance genes suggests the potential for exacerbating bacterial virulence and pathogenesis, which underscores the need for judicious use of AgNPs and control of their incidental releases.

Low concentrations of silver nanoparticles stimulated biofilm development on various materials used for water distribution systems

251st National Meeting (2016)

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