Effect of sulfonamide antibiotics on microbial diversity and activity in a Californian Mollic Haploxeralf

Up to 90% of antibiotics that are fed to livestock are excreted unaltered or as metabolites and thus are present in manure. By application of manure as fertilizer, veterinary antibiotics can reach soil and groundwater. The aim of this study is to determine the effect of three commonly used (and simultaneously applied) sulfonamide antibiotics on both function and structural diversity of soil microorganisms. To this end, the activity of the enzymes urease and dehydrogenase was determined, and the composition of phospholipid fatty acids (PLFA) was analyzed. Soil and manure were sampled at a dairy farm located in the Northern San Joaquin Valley, California, USA. Soil (700 g) was amended with either mineral water only (W-treatments), liquid manure (M-treatments), or with glucose solution (G-treatments). Each of these soil treatments was mixed with a cocktail of three sulfonamides: sulfadimethoxine (SDT), sulfamethoxazole (SMX), and sulfamethazine (SMZ) at five total concentration levels ranging from 0 (control) to 900 A mu g g (dm) (-1) . After 24, 48, 96, 168, 264, 384, and 504 h, UA and DHA were determined; PLFA composition in selected samples was analyzed at t = 168 h and 504 h of incubation. In the G-treatments, urease activity decreased with higher sulfonamide concentrations; no effect was observed when no glucose was added (W-treatments). While a dose-response relationship was observed for urease activity after 168 h, a similar inhibition was measured after 380 h at all sulfonamide concentrations. Sulfonamides also reduced dehydrogenase activity in the G-treatments, but results are less conclusive than for urease. With increasing sulfonamide concentration, microbial and bacterial biomass decreased in the G-treatments compared to the control at 168 h. Sulfonamides caused a relative community shift towards gram-negative bacteria and towards an increased proportion of fungal biomass. Strong inhibition of urease by manure (M-treatments) was observed even without the addition of sulfonamides. Sulfonamides clearly affected both the function and structural diversity of the soil microbial community over at least 16 days. The soil microbial community was affected by sulfonamides even at a relatively low concentration, although this soil receives regular input of manure that contains several antibiotics. Further research is needed addressing both long-term effects and lower sulfonamide concentrations under dynamic boundary conditions.