Sulfonamide disinfection byproducts exhibited severe toxicity to human commensal bacteria.

Many antibiotic disinfection byproducts have been detected but their toxicity has not been evaluated adequately. In this report, the chlorination reaction kinetics of five common sulfamides (SAs), reaction intermediates and their toxicity were investigated. Chlorination of sulfapyridine (SPD), sulfamethazine (SMT), sulfathiazole (STZ), and sulfisoxazole (SIZ) followed the second-order kinetics, and were degraded completely within 10 min. A large number of reaction intermediates were deteced by LC-MS, among which a total of 16 intermediates were detected for the first time. Toxicity of the five SAs chlorination solutions was evaluated separately by examining their effects on the growth rate of S. salivarius K12, a commensal bacterium in the human digestive system. After 30 min chlorination, solutions of SMT, STZ and sulfadiazine (SDZ) each exhibited severe toxicity by inhibiting the bacteria growth completely, whereas the inhibition was only 50 % and 20  % by SIZ and SPD respectively. Based on the comparison between toxicity test results and mass spectra, three SA chlorination intermediates, m/z 187.2 (C10H10N4), m/z 287.2 (C9H7N3O4S2) and m/z 215 (C7H10N4O2S/C12H14N4) were proposed to be the primary toxicants in the chlorination products. Our study demonstrated the power of combined approach of chemical analysis and toxicity testing in identifying toxic disinfection byproducts, and highlighted the ne ed for more research on the toxicity evaluation and risk assessment of antibiotic disinfection byproducts.