Toxicity of five protein synthesis inhibiting antibiotics and their mixture to limnic bacterial communities

Antibiotics are common contaminants of aquatic environments, the protein synthesis inhibitors being one frequently detected class. Even though antibiotics target bacteria, little is known about the sensitivity of naturally occurring freshwater bacteria to these compounds. Limnic bacterial communities were therefore systematically exposed to five common protein synthesis inhibiting antibiotics, each representing a particular subgroup: streptomycin, chloramphenicol, fusidic acid, rifampicin and chlortetracycline. Full concentration–response curves and No Observed Effect Concentrations (NOECs) were determined for all antibiotics, using the 3Hleucine incorporation method. All test compounds were toxic to planktonic communities of limnic bacteria, with EC50s ranging from 0.138 μmol/L for chlortetracycline to 79.1 μmol/L for streptomycin. The order of toxicity was chlortetracycline > rifampicin > fusidic acid > chloramphenicol > streptomycin, based on the individual EC50 values. A comparison to reported chemical monitoring data shows that environmental concentrations of chlortetracycline are in a range that clearly inhibits the protein biosynthesis activity of planktonic bacterial communities. All compounds show exceptionally flat concentration–response relationship, for fusidic acid the ratio of EC50 to EC05 exceeds four orders of magnitude. This challenges the standard assessment factors of 10–100 for the extrapolation from high to low doses. Environmental exposure situations are often characterized by the presence of mixtures of antibiotics, e.g. in sewage effluents or river systems. Hence, also combined effects of the five antibiotics were determined, and compared to mixture toxicity predictions based on Concentration Addition and Independent Action. Concentration Addition slightly underestimated the observed EC50 by a factor of 1.5, independent on whether the prediction was based on single substance data that were recorded in parallel or whether historical data were used. Independent Action predicted higher mixture toxicity than Concentration Addition due to the flatness of the individual concentration–response curves. Implications of these findings for the environmental risk assessment of antibiotics and their mixtures are discussed.