Bioavailability of Geosorbent-Sorbed Tetracycline to an Escherichia coli Bioreporter for Expression of Antibiotic Resistance Genes.

Tetracyclines are a class of antimicrobials used extensively in human and veterinary medicine, and in livestock production. A large portion of tetracycline administered to humans and animals is eventually disseminated to the environment, and resides primarily in soils/sediments. Little is known whether soil-sorbed tetracycline is still bioavailable to microbial populations for development and enrichment of antibiotic resistance. In this study, E. coli MC4100/pTGM whole-cell bioreporter was used as an effective tool to probe the bioavailability of tetracycline sorbed by five geosorbents, illite clay, smectite clay, activated carbon, iron oxide and Pahokee peat soil. In order to achieve more close contacts between bacteria and geosorbent surfaces, the solution volume was reduced while the geosorbent mass remained constant.  The experimental settings with reducing solution volume manifested the enhanced antibiotic resistance response from the bioreporter, indicating greater bioavailability of geosorbent-sorbed tetracycline in the systems with low solution to sorbent ratios. Meanwhile, tetracycline concentration in culture solution increased with reducing solution volume.  Tetracycline sorbed by activated carbon showed the lowest bioavailability among the five geosorbents, which could be due to strong sorption of tetracycline in the porous structures of activated carbon. In comparison to illite, smectite-sorbed tetracycline demonstrated higher bioavailability in the systems with more solution, but was less bioavailable with smaller solution volume. Tetracycline sorbed by Pahokee peat soil showed readily bioavailable plausibly due to enhanced desorption from dissolved organic matter. Tetracycline sorbed on positively charged iron oxide at neutral pH was also bioavailable to the bioreporter. Overall, increasing the contacts among bacteria and geosorbents enhance the bioavailability of sorbed tetracycline, hence increasing the exposure and risk to the surrounding microbial communities for development and enrichment of antibiotic resistance.