Assessment of zebrafish embryo toxicity of environmentally relevant antibiotics

Antibiotics are essential drugs in modern medicine. After consumption antibiotics are excreted in unmetabolized form in the urine and reach our sewage treatment plants (STP). STP are not able to degrade all antibiotics leading to release of antibiotics into the aquatic environment. Aquatic animals are thus continuously exposed to antibiotics. This study involved assessment of the toxicity of eight antibiotics previously detected in the river Fyrisån, Uppsala, Sweden, in developing zebrafish (Danio rerio) embryos up to day six of age. The experiments included assessment of embryonal toxicity for the individual antibiotics as well as mixtures of all antibiotics. The mixtures were based on previously measured concentrations in river Fyrisån and tested in increasing concentrations up to 1000-times higher concentrations. In the toxicity assessment different lethal and sublethal endpoints were observed, such as early movements, heart rate, hatching time and length. These experiments were followed by behaviour study observing the swimming activity during alternating dark-light alternations. Lastly, a bioaccumulation study was performed on mixtures of antibiotics to determine if these antibiotics were bioaccumulative in zebrafish embryos. The results showed that these eight antibiotics, individually and as a mixture did not affect any of the endpoints. As for bioaccumulation, none of the eight antibiotics were bioaccumulating in zebrafish embryos. These antibiotics seem to be non-toxic during fish embryonal development. However, the results cannot determine the long-term effects of antibiotic exposure and thus further studies are needed to assess the potential toxicity of environmentally present antibiotics to fish.