Montmorillonite reduces bioavailability of ciprofloxacin and exerts growth-inhibitory effects on human-associated bacteria

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, such as montmorillonite, have been observed to adsorb pharmaceuticals, including antibiotics. We investigated the influence of the mineral colloid montmorillonite on the bioavailability of ciprofloxacin (CIP) and its effect on bacterial growth. The bioavailability and efficacy of CIP in the presence of montmorillonite was compared to that of sulfamethoxazole, clindamycin and erythromycin. Montmorillonite reduced but did not completely prevent the growth-inhibitory effect of CIP in bacterial growth medium. The bacteria were able to grow at twice the CIP concentration in the presence of montmorillonite. The incomplete deactivation of CIP suggest that growth medium components might have decreased the adsorption of CIP to montmorillonite. In comparison, three more antibiotics were tested, but no reduction of their effect on bacterial growth due to the montmorillonite colloid was observed. Surprisingly, we also observed a strong inhibitory effect of the colloid on the growth of human-associated bacteria, such as S. aureus , but not on environmental bacteria, suggesting iron-induced stress responses and/or nutrient sequestration. We conclude that a selective potential of the highly active CIP in contaminated soils, which also contain nutrients that enable bacterial growth, cannot be ruled out.