Sensitivity of Planktonic Aquatic Bacteria to Ciprofloxacin

Many anthropogenic compounds, such as antibiotics, are found at trace levels (<μg·L^-1) in aquatic and terrestrial systems. The effect of these compounds on the metabolism and function of microbes are difficult to assess because the assays used, such as the minimum inhibitory concentration (MIC) and the disk diffusion methods, lack the sensitivities to measure bacterial response to these very low levels of antibiotics on bacterial populations. Therefore, we theorized that the [³H] thymidine incorporation into DNA method might be sensitive in determining the effect of DNA inhibiting antibiotics on DNA production in planktonic bacteria in aquatic systems. Utilizing the ³H thymidine method, we measured the effects of ciprofloxacin on DNA production on planktonic bacteria in river and pond waters. Ciprofloxacin significantly (P < 0.02) inhibited river water bacteria at a concentration of 25 μg·L^-1 but significant inhibition (P < 0.01) occurred at 1000 μg·L^-1in pond water. The very low concentration required to inhibit DNA production in river water bacteria indicates that bacteria are extremely sensitive to antibiotics at very low concentrations. A likely reason for the differences in inhibition between the two waters is due to ciprofloxacin becoming bound, and possibly becoming biologically inactive, in the pond water due to higher dissolved organic carbon content. This work demonstrates that bacteria in some aquatic systems can be significantly impacted by low concentrations of anthropogenic antibiotics finding their way into these systems and that our assumptions as to the concentrations at which antibiotics affect microbes are highly underestimated.