Modeling acute impact of sulfamethoxazole on the utilization of simple and complex substrates by fast growing microbial culture

BACKGROUND: This study involved model evaluation of the acute impact of sulfamethoxazole on utilization of peptone mixture and acetate by fast growing microbial cultures under aerobic conditions. These substrates were selected to represent complex and readily biodegradable organic carbon sources, respectively. Acclimated biomass was obtained from two fill/draw reactors sustained at a sludge age of 2days, one fed with peptone mixture and the other with acetate. Acute inhibition was tested in two parallel sets of batch reactors. Each reactor set was started with acclimated biomass seeding and pulse sulfamethoxazole dosing, including a control reactor without antibiotic addition. RESULTS: Model evaluation of the oxygen uptake rate, chemical oxygen demand and intracellular storage profiles indicated that sulfamethoxazole stopped substrate storage and accelerated endogenous respiration, but it did not affect microbial growth. The major inhibitory effect was on process stoichiometry, leading to partial substrate utilization. Enhanced endogenous respiration could be explained by higher maintenance energy required for possibly generating specific resistance mechanisms against the inhibitory effects of sulfamethoxazole. CONCLUSION: The impact of sulfamethoxazole was different depending upon the nature of the substrate. For peptone mixture, it was stoichiometric, leading to partial substrate utilization. For acetate, it exerted slight inhibition on microbial growth, with full substrate utilization.