Immobilization of antimicrobial silver nanoparticles (AgNPs) on surfaces has been proposed as a method to inhibit biofouling or as a possible route by which incidental releases of AgNPs may interfere with biofilms in the natural environment or in wastewater treatment. This study addresses the ability of planktonic Pseudomonas fluorescens bacteria to colonize surfaces with pre-adsorbed AgNPs. The ability of the AgNP-coated surfaces to inhibit colonization was controlled by the dissolved silver in the system, with a strong dependence on the initial planktonic cell concentration in the suspension, i.e., a strong inoculum effect. This dependence was attributed to a decrease in dissolved silver ion bioavailability and toxicity caused by its binding to cells and/or cell byproducts. Therefore, when the initial cell concentration was high (∼1×10(7)CFU/mL), an excess of silver binding capacity removed most of the free silver and allowed both planktonic growth and surface colonization directly on the AgNP-coated surface. When the initial cell concentration was low (∼1×10(5)CFU/mL), 100% killing of the planktonic cell inoculum occurred and prevented colonization. When an intermediate initial inoculum concentration (∼1×10(6)CFU/mL) was sufficiently large to prevent 100% killing of planktonic cells, even with 99.97% initial killing, the planktonic population recovered and bacteria colonized the AgNP-coated surface. In some conditions, colonization of AgNP-coated surfaces was enhanced relative to silver-free controls, and the bacteria demonstrated a preferential attachment to AgNP-coated, rather than bare, surface regions. The degree to which the bacterial concentration dictates whether or not surface-immobilized AgNPs can inhibit colonization has significant implications both for the design of antimicrobial surfaces and for the potential environmental impacts of AgNPs.

Silver nanoparticles (AgNPs) released from consumer products may enter the environment and possibly harm microbial communities. Prior research showed that surface-adherent AgNPs inhibit bacterial surface colonization, a precursor to biofilm formation, only when planktonic bacterial inoculum concentrations are less than a threshold level ( Wirth and co-workers, J. Colloid Interface Sci. 2016 , 467 , 17 - 27 ). This inoculum effect is due to a decrease in free silver ion concentration associated with sublethal binding to bacteria. Natural organic matter can be an additional silver sink in environmental systems. Using Pseudomonas fluorescens as a model biofilm-forming bacterium, we find significant increases in minimum bactericidal concentrations for AgNP suspensions and Ag in solution when adding humic acid (HA) to bacterial suspensions. When HA is present, planktonic bacteria survive and colonize AgNP-laden glass surfaces at lower bacterial inoculum concentrations than were needed for survival and colonization in its absence. This occurs despite the observed tendency of HA to inhibit colonization on bare glass surfaces when silver is absent. Results are interpreted through equilibrium Ag binding isotherms to HA and suspended bacteria. These results indicate that silver ion sinks may lessen AgNP impacts on natural microbial ecology relative to the disruption observed in pristine laboratory conditions.

Tenebrio molitor, a species of darkling beetle, is capable of metabolizing expanded polystyrene (EPS), a durable single-use plastic, when in its larval stage, mealworms. EPS consumption studies were conducted on mealworm, beetle, and co-culture habitats with a one-week acclimation period and a three-week experimentation period to compare the EPS consumption rate at each stage. Subsequent experiments tested various beddings (oats, beads), supplemental nutrients (spinach, protein powder paste, cucumbers, lemon slices), and EPS pre-treatments (lemon-lime soda, lemon juice, tomato paste) to determine the effect of those variations on mealworm EPS consumption. It was concluded that one mealworm consumes EPS at least fifty times faster than one beetle. EPS biodegradation rates were estimated under the assumption that 48.2% of consumed EPS was biodegraded by mealworm gut bacteria into carbon dioxide and biomass. It was found that EPS biodegradation rates increased 34.8% in habitats with inedible bead beddings compared to those with an edible oat bedding. EPS biodegradation rates by mealworms were also seen to increase 482% in habitats where the EPS was pre-treated with lemon-lime soda and 125% in habitats with a spinach supplemental nutrient compared to EPS biodegradation rates in a habitat with no supplemental nutrient or pre-treatment. Each supplemental nutrient and EPS pre-treatment variation in an individual inedible bead bedding improved the mealworm consumption rate of EPS. It was determined that approximately four mealworms could degrade EPS at least as quickly as natural biodegradation. These findings suggest that habitat conditions strongly affect EPS consumption rates by mealworms. Highlights • Mealworm consumption rates of EPS are fifty times greater than those of beetles.• Mealworm EPS consumption rates are lower in edible beddings than inedible beddings. • EPS consumption rates by T. molitor improved with cups pre-treated in acidic media.