Tetracycline antibiotics have been widely detected in wastewater, animal manure, and surface water, emphasizing the need for improved treatment. A few studies have investigated the effects of individual environmental factors (e.g., pH, Ca2+, Mg2+, dissolved organic matter (DOM)) on tetracycline photodegradation and residual antimicrobial activity. However, because Ca2+ and Mg2+ can interact with both tetracyclines and DOM, mixture-based studies are needed to understand whether the potential influences of Ca2+ and Mg2+ on tetracycline degradation and antimicrobial activity are mitigated by DOM and vice versa. We hypothesized that Ca2+, Mg2+, and DOM concentrations would affect the phototransformation kinetics (254 nm) and residual antimicrobial activity of doxycycline, minocycline, and oxytetracycline. To test this hypothesis, experimental solutions that initially contained 10 mM of one tetracycline, 0-10 mM Ca2+, 0-10 mM Mg2+, and 0-10 mgC L-1 DOM were irradiated at 254 nm. Samples were periodically collected and analyzed for tetracycline concentration and antimicrobial activity. This presentation will focus on the antimicrobial activity results, which were recorded using growth inhibition assays with Pseudomonas fluorescens. The half-maximal concentrations for growth inhibition (IC50) were calculated for the parent antibiotics and serially diluted samples from the photodegradation experiments. Higher Ca2+ concentrations tended to mitigate the antimicrobial activity of parent tetracyclines. For example, the IC50 of doxycycline in the 10 mM Ca2+, 1 mM Mg2+, and 0 mgC L-1 DOM solution was 0.33 ± 0.07 µM, whereas the IC50 was 0.17 ± 0.06 µM in solutions with no Ca2+, Mg2+, or DOM. The water quality parameters also affected removal of residual antimicrobial activity during photolysis at 254 nm. Notably, Mg2+ and DOM facilitated faster removal of residual antimicrobial activity. The potency equivalents of irradiated samples were calculated to ascertain the antimicrobial activity contributions of transformation products formed in solutions with variable Ca2+, Mg2+, and DOM contents. For most conditions, the tetracyclines exhibited a stoichiometric decrease in antimicrobial activity, suggesting negligible contributions from the transformation products. Overall, these results inform the complex interactions of water quality parameters on tetracycline photolysis and removal of residual antimicrobial activity in engineered UV processes.

Complex environmental matrices affect the residual antimicrobial activity of tetracycline antibiotics during photolysis at 254 nm

ACS Spring Meeting 2021

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