Microbial inhibition by pharmaceutical antibiotics in different soils--dose-response relations determined with the iron(III) reduction test.

Soil contamination from pharmaceuticals is an emerging problem, though quantitative data on their microbial effects are lacking. Thus, nine pharmaceutical antibiotics were tested for their effects on the microbial iron(III) reduction in six different topsoils. Complete dose-response curves were obtained and best-fit by sigmoidal Logit, Weibull, Box-Cox Logit, and Box-Cox Weibull equations (r2 0.73-1.00). The derived effective doses (ED [micromol/kg soil]) for the different antibiotics increased in the order (average ED50 in parentheses) chlortetracycline (53) < sulfadimethoxine (58) < oxytetracycline (170) < sulfadiazine (190) < sulfadimidine (270) = tetracycline (270) < sulfapyridine (430), though no effect was found for sulfanilamide and fenbendazole at doses up to 5,800 and 3,300 micromol/kg, respectively. Due to a strong soil adsorption, especially of the tetracyclines, the corresponding effective concentrations in the soil solution (EC50), derived from sorption experiments, were considerably smaller and ranged from 0.004 micromol/L (chlortetracycline) to 120 micromol/L (sulfapyridine). The effects of the antibiotics were governed by soil sorptive properties, especially the concentration of soil organic matter. The microbial inhibition was influenced indirectly by the soil pH, which affects the ionization status of the amphoteric antibiotics.