(485c) Bacterial Colonization of Surfaces Displaying Adhered Silver Nanoparticles

Silver nanoparticles (AgNPs) are among the most commonly employed engineered nanoparticles in consumer products, where the broad spectrum biocidal properties of silver are exploited to endow products with a measure of antimicrobial performance.  Silver ions have well established microbial toxicity.  In some cases AgNPs are embedded in a matrix; in others they are deposited on a surface.  In either case, AgNPs provide a source of dissolved silver ions.  The possibility exists as well for toxic consequences of direct microbe contact with the AgNPs, although this appears to be rather context-dependent.  Often, materials that display AgNPs are envisioned as a means to inhibit biofouling or to enhance sterility. This can be particularly attractive in medical or food processing situations.  AgNP leaching from consumer products under normal usage provides a pathway to environmental exposure, mainly via wastewater treatment plants.  Concern over the possibly detrimental effects of engineered AgNPs in soil and aquatic ecosystems has motivated active research into the potential toxicity of these nanomaterials to microbes, small organisms and plants, as well as the chemical and physical transformation processes and transport processes that ultimately dictate the potential for broad environmental impact of the introduced nanoparticles.

While AgNPs can be highly toxic to planktonic bacterial cultures, recent work indicates that established bacterial biofilms, which represent the vast majority of naturally occurring populations, are considerably more tolerant to the introduction of AgNPs and the dissolved silver they release.  Instead of AgNP effects on established biofilms, this presentation focuses on the ability of planktonic bacterial populations to colonize a surface that has been coated by adhesion of AgNPs from aqueous suspension before the introduction of bacteria to the system.  Silver nanoparticles in this study were stabilized by poly(vinylpyrrolidone).  Biofilm growth and viability were monitored on silica glass substrates, with or without adherent AgNPs, that were immersed in agitated suspensions of Pseudomonas fluorescens. The critical parameter that dictates whether or not the AgNP-decorated surface inhibits biofilm formation is the ratio of silver to planktonic biomass, or equivalently, the total cell population for a fixed amount of surface-adhered AgNPs. Biofilm formation is only prevented when the initial planktonic bacteria population is sufficiently low to suffer 100% lethality.  Higher bacterial populations may initially suffer substantial loss of viability, but if this is less than 100%, the population recovers and establishes a thriving biofilm on the AgNP-decorated surface.  An important aspect of this phenomenon is the binding of silver ions to suspended bacteria, which makes it unavailable to other bacteria.  In this way, bacteria populations can overwhelm the release of toxic silver ions from AgNPs by sequestering silver ion in a non-bioavailable form. This enables the recovery from initial viability losses when the population grows at a faster rate than silver ion can be replenished in solution.  Having sequestered most of the dissolved silver ions, the bacteria then readily colonize the AgNP-decorated surface.  A particularly noteworthy effect is the preferential colonization of bacteria at early times on surface regions that have abundant AgNPs compared to AgNP-free regions.  After several days, AgNP-rich and AgNP-free surface regions are indistinguishable in terms of biofilm biomass and viability. The AgNPs themselves provide no direct deterrence to biofilm formation.  The system is dominated by dissolved silver toxicity, which can be readily overcome at high biomass:silver ratios. This work implies that even AgNPs that have managed to avoid any of the expected chemical transformations that are known to decrease silver toxicity, a “worst case scenario”, may have little detrimental effect on bacterial biofilms in the environment.