Abstract
Many anthropogenic compounds, such as antibiotics, are found at trace levels (-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 -1 but significant inhibition (P -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.
Highlights
Antibiotics are compounds that inhibit the growth of susceptible microbes
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
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
Summary
Antibiotics are compounds that inhibit the growth of susceptible microbes. The vast majority of antibiotics in clinical use are derived from organisms ubiquitous in the natural environment [1]. With antibiotics having been detected in wastewater effluents and aquatic systems in many countries around the world [5] [6] at concentrations of ηg to low μg∙L−1, the two methods above are inadequate at measuring the effects on microbial populations. We theorized that the [3H] thymidine incorporation into DNA method may be relevant and sensitive in measuring the effects of DNA inhibiting antibiotics. We report the effects of ciprofloxacin, one of the most prescribed of the quinolone antibiotics in the world [7], an antibiotic that inhibits DNA production in susceptible microbes, on planktonic bacteria in river and wetland waters as measured by the incorporation of [3H] thymidine into DNA. WHO warns that the levels of bacterial resistance to antibiotics have reached to the levels where medicinal treatment for even common infections is becoming limited [11]
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