Reduced antibiotic effect of ciprofloxacin on bacteria in the presence of montmorillonite

Antibiotic residues exceeding selective concentrations for antibiotic-resistant bacteria have been detected in various environments, including manure, wastewater, and effluents from wastewater treatment plants. When these residues come into contact with soils, for instance, due to wastewater irrigation or fertilization with manure, they interact with soil constituents. Soil colloids (1-1000 nm), such as montmorillonite, have been observed to adsorb pharmaceuticals, including antibiotics. We investigated the effect of colloids on the bioavailability of ciprofloxacin and found, that added to bacterial growth medium, montmorillonite reduces, but does not completely prevent, the growth-inhibitory effect of the antibiotic. The bacteria were able to grow at up to roughly double the concentration of ciprofloxacin in the presence of montmorillonite. We show that the incomplete deactivation of ciprofloxacin was most probably caused by medium components that decreased the adsorption of ciprofloxacin to montmorillonite. We conclude that a selective potential of this highly active antibiotic in contaminated soils, which also contain nutrients enabling bacterial growth, cannot be ruled out.