Sublethal effects of photoactive engineered nanomaterials on filamentous bacteriophage infection and E. coli gene expression in freshwater

Abstract

Wide application and release of engineered nanomaterials (ENMs) into the environment require an understanding of their potential ecological impacts, particularly under real environmental conditions. Previously we reported that low doses of photoexcited ENMs exert significant sublethal stress on bacterial outer membranes in a freshwater medium, potentially increasing bacterial susceptibility to viral infection and promoting microbial evolution and diversity. However, little is known about how ENMs may affect bacteriophage infection under environmental conditions. Therefore, this study investigates the effects of commonly used photoactive ENMs – n-TiO₂, n-Ag, and their mixtures – on the infection of a filamentous coliphage, bacteriophage f1, at environmentally relevant concentrations under freshwater conditions. We also interrogate cellular surface properties and the expression of key genes associated with phage–cell interactions in response to ENM exposure. Under light, n-TiO₂ or n-Ag increases bacteriophage infection, consistent with trends showing increased outer membrane permeability (OMP), F-pili-related gene expression, and pili density. Exposure to n-TiO₂ + n-Ag mixtures under light, however, suppresses the effects of the individual ENMs on bacteriophage infection, despite high OMP, amplified up-regulation in F-pili and membrane protein expression, and augmented pili density. We propose that greater oxidative stress on the cell membrane induced by the photoexcited ENM mixtures in comparison to individual ENM exposure, as previously detailed, damages membrane proteins (e.g., TolA) vital to bacteriophage entry and dominates other mechanisms. Overall, our results provide mechanistic insight into the complex interactions among bacteria, bacteriophage, and ENMs, under environmentally relevant conditions, and further detail their potential ecological risks.