Microplastics alter development, behavior, and innate immunity responses following bacterial infection during zebrafish embryo-larval development

Although the hazards of microplastics (MPs) have been explored, no complete data exists on the effect of MPs on the egg chorion. This study aims to evaluate the modification of immune responses, metabolism, and behavior of zebrafish larvae (Danio rerio) depending on the moment of exposure. Larvae were exposed to 5 μm polystyrene microbeads at a concentration of 0, 100, or 1000 μg/l, according to a specified times of exposure (0-4, 4-8, 0-8 days postfertilization (dpf)), followed by a bacterial challenge at 8 dpf. After every 4 and 8 dpf, swimming activity, gene expression related to oxidative stress and immune system responses were assessed. During embryonic development, larvae exposed to a concentration of 1000 μg/l MPs already showed a significantly reduced tail coiling frequency, yolk sac resorption and heartbeat. At 8 dpf, swimming activity was altered, even without ingestion and a few days after the end of MP exposure. Our results indicated a difference in immune system (nfkb, il1β) and apoptosis (casp3a, bcl2) related gene expression depending on the timing of MP exposure, which highlighted a contrasting sensitivity according to the exposure time in MP studies. This study brings new insight into how MPs might affect zebrafish larvae health and development even without ingestion.