Bacteria-Mediated Effects of Antibiotics on <i>Daphnia</i> Nutrition

In polluted environments, contaminant effects may be manifested via both direct toxicity to the host and changes in its microbiota, affecting bacteria–host interactions. In this context, particularly relevant is exposure to antibiotics released into environment. We examined effects of the antibiotic trimethoprim on microbiota of Daphnia magna and concomitant changes in the host feeding. In daphnids exposed to 0.25 mg L^–1 trimethoprim for 24 h, the microbiota was strongly affected, with (1) up to 21-fold decrease in 16S rRNA gene abundance and (2) a shift from balanced communities dominated by <i>Curvibacter</i>, <i>Aquabacterium,</i> and <i>Limnohabitans</i> in controls to significantly lower diversity under dominance of <i>Pelomonas</i> in the exposed animals. Moreover, decreased feeding and digestion was observed in the animals exposed to 0.25–2 mg L^–1 trimethoprim for 48 h and then fed ¹⁴C-labeled algae. Whereas the proportion of intact algal cells in the guts increased with increased trimethoprim concentration, ingestion and incorporation rates as well as digestion and incorporation efficiencies decreased significantly. Thus, antibiotics may impact nontarget species via changes in their microbiota leading to compromised nutrition and, ultimately, growth. These bacteria-mediated effects in nontarget organisms may not be unique for antibiotics, but also relevant for environmental pollutants of various nature