Tetracycline antibiotics have been detected in surface waters around the world and are known to promote antimicrobial resistance in environmental bacteria and cause adverse effects in aquatic organisms. This situation warrants investigation of the fate of tetracyclines in water treatment processes. Solution pH and Ca2+, Mg2+, and dissolved organic matter (DOM) content have been shown to influence the rate of tetracycline photolysis by solar irradiation. However, few studies have examined these phenomena under the monochromatic ultraviolet light commonly used for water disinfection. The objective of this study was to identify the influence of pH and Ca2+, Mg2+, and DOM concentrations on the photoproducts and residual antimicrobial activity of doxycycline, oxytetracycline, and minocycline for irradiation at 254 nm. These three tetracyclines were selected due to high consumption and unique structural components. Twenty-eight solution conditions were evaluated using a Box-Behnken experimental design. More than 10 transformation products were identified for each tetracycline. For doxycycline at pH 6.5, Mg2+ induced the formation of several dehydrogenated photoproducts that were not observed in the presence of Ca2+ or the absence of divalent metals. Interestingly, 10 mM Ca2+ increased the rate of doxycycline photolysis by up to 126% but exerted little influence on photoproduct distribution. The antimicrobial activity of irradiated solutions was measured as growth inhibition of Pseudomonas fluorescens. The measured potency equivalents of whole solutions containing parent tetracyclines and photoproducts suggested antagonistic interactions between doxycycline and Mg-specific photoproducts. In most solutions, the potency equivalents decreased in proportion to the parent tetracycline concentration, indicating that UV photolysis generally removes the antimicrobial activity of tetracyclines. While UV irradiation can convert tetracycline antibiotics into an array of photoproducts with diminished antimicrobial activity, the typical fluence in UV disinfection systems is inadequate for effective treatment of tetracyclines.

Effects of calcium, magnesium, and DOM on the photoproduct distribution and residual antimicrobial activity of tetracycline antibiotics irradiated by 254-nm light

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.