Background: In the last years, the toxicity of the waters due to the presence of antibiotics
is well studied.
Introduction: The Microtox test, based on the measurement of bioluminescence from the bacterium
Vibrio fischeri, was applied on samples of river water spiked with several antibiotics to assess acute
toxicity. Thirteen antibiotics, presenting different mechanism of action, were selected as the most
widely used in anti-infective therapy.
Method: Toxicity was evaluated for drug concentrations distributed on six levels ranging from
1.0 - 50.0 μg/ml on one-component solutions and real samples spiked with drug mixtures. Bioluminescence was measured
after half-hour and 24 h of incubation. The results were reported as no observable effect concentration, lowest observable
effect concentration or half-effective concentration. The light output from the test samples was compared to that of a standard
phenol solution as a control.
Results: Chlortetracycline, Oxytetracycline, Ofloxacin, Norfloxacin, Oxolinic Acid, Nalidixic Acid and Streptomycin
were found to be very toxic after just 30 min of incubation at low concentration levels. Clindamycin, Neomycin, Ampicillin
and Amoxicillin induced bioluminescence decrease but below the toxicity limits permitted by law. In contrast, Erythromycin
and Sulphamethoxazole showed hormesis, a phenomenon increasing bioluminescence. The mixtures of antibiotics
showed toxicity amplified with additive or synergic activity.
Conclusion: The bioluminescence test was proved to be effective in measuring toxicity of drugs in environmental waters,
highlighting also the difficulties in predicting the toxicity in presence of several drugs or other pollutants.