Impact of tetracycline-clay interactions on bacterial growth

Highlights: • Antimicrobial activity (AMA) of tetracycline (TC) was drastically reduced in the presence of different clays. • The reduction in AMA was attributed to significant removal of TC from solution by the clays. • The reduction in AMA was more obvious in the presence of MMT due to its higher cation exchange capacity. • The reduction in AMA may contribute to elevated TC resistance of both TC sensitive and resistant bacteria. -- Abstract: Antibiotics are extremely effective against bacterial infections due to their selective toxicity for bacteria rather than the host. Extensive use and misuse of antibiotics resulted in significant increases in antibiotic levels in aquatic and soil environments. Bacteria exposed to antibiotics with low concentrations may develop antibiotic resistance. In this study a swelling 2:1 clay mineral montmorillonite (MMT) and a non-swelling 1:1 clay mineral kaolinite were premixed with tetracycline (TC) of varying concentrations. The gram-negative bacteria Escherichia coli (E. coli) and Salmonella enterica (S. enterica) of both TC sensitive and TC resistant strains were tested for their growth in the presence TC-loaded clay minerals of different amounts and under different physico-chemical conditions. The antimicrobial activity of TC was significantly decreased in the presence of MMT. In the absence of MMT, no bacteria growth was found at a TC concentration 0.25 mg/mL and above. On the contrast, in the presence of MMT, 50% growth was still found for a TC resistant E. coli at a TC concentration of 5 mg/g. The influence of kaolinite was to a lesser degree. These results suggest that antimicrobial agents present in clayey soils could be responsible for possible mutation of bacteria of high antibiotic resistance.