Bioavailability of Soil-Sorbed Tetracycline to Bacteria for Expression of Antibiotic Resistance.

The objective of this study is to determine the bioavailability of soil-sorbed tetracycline to bacteria for the expression of antibiotic resistance. Tetracyclines are a class of antimicrobials that have been extensively used to improve human and veterinary health. A large portion of tetracyclines used have been released to the environment, and could be strongly associated with soil minerals or organic fractions. This raises concerns whether the soil-sorbed tetracycline is still bioavailable to invoke the expression of antibiotic resistance genes in bacteria. In this study, we used E.coli MC4100/pTGM whole-cell biosensor as an effective tool to determine the bioavailability of smectite clay-sorbed tetracycline under varying conditions. The activities of the biosensor responded in a linear manner to the tetracycline intracellular concentrations which were measured by liquid chromatography coupled to tandem mass spectrometer. In clay suspension (0.01 g clay in 25 mL of solution), tetracycline sorbed by the clay could be desorbed to aqueous phase, and become bioavailable to the whole-cell biosensor. Under this setting, tetracycline in aqueous phase was the primary source available for uptake by bacteria, and triggered the expression antibiotic resistance. The bioavailability of clay-sorbed tetracycline was minimal to bacteria. The result of scanning electron microscope showed that most of the bacteria resided in the aqueous phase and formed large bacterial aggregates, whereas only few cells were attached to clay surfaces. When increasing the contacts of bacteria with clay particles, it is apparent that clay-sorbed tetracycline became bioavailable. The specific reason for such observation is not yet clear.