Changes in antibacterial activity of triclosan and sulfa drugs due to photochemical transformations

Sulfa drugs and triclosan represent two classes of antibacterials that have been found in natural waters and for which photodegradation is anticipated to be a significant loss process. Parent antibacterial compounds and the products of photolysis reactions were compared for three sulfa drugs and triclosan to determine the extent to which photolysis affects their antibacterial potency on Escherichia coli DH5α. Sulfathiazole (median effective concentration [EC50] = 20.0 μM), sulfamethoxazole (EC50 = 12.3 μM), and sulfachloropyridazine (EC50 = 6.9 μM) inhibited bacterial growth but did not affect respiratory activity. Photolysis products of these sulfa drugs did not retain any measurable ability to inhibit growth. Triclosan inhibited both the growth (EC50 = 0.24 μM) and respiratory activity of E. coli DH5α. Triclosan photolysis products also exhibited no measurable effect on growth or respiratory activity. These experiments indicate that the products of triclosan and sulfa drug photolysis are unlikely to possess antibacterial activity in natural waters. The rapid screening method used for these two classes of compounds will be useful for helping to identify photolabile antibacterial compounds, for which photoproducts could require further investigation.