A physiologically relevant approach to characterize the microbial response to colloidal particles in food matrices within a simulated gastrointestinal tract

Colloids on the nanometer size scale are beginning to find increased applications in drinks, foods, food-contact surfaces, and food packaging. While these particles add intrinsic value to the food industry, their potential toxicities warrant careful studies. The physicochemical changes and possible perturbations to microbial communities within the gastrointestinal tract have not been adequately studied. The purpose of this study was to design and perform a simulated digestion protocol to evaluate the effect of colloidal silver in an orange juice suspension when exposed to planktonic bacterial cultures and biofilms. The model system includes four precursor steps in which the silver is exposed to varying pH conditions and incubation times. The gastrointestinally “digested” samples were then incubated with Escherichia coli strains for up to 4 h, the average residence time of foods in the GI tract. The physicochemical changes of the colloids and their corresponding biological effects were characterized at every step. The results showed differences between (1) bacterial cultures versus bacterial biofilms, (2) “digested” versus “undigested” silver on bacteria, and (3) differences between “digested” silver nitrate versus silver colloids on bacteria. We conclude that simulated digestion, as well as manner in which bacterial cells are grown, influences the results of toxicity.