EFFECTS OF THE VETERINARY ANTIMICROBIAL TYLOSIN ON ANAEROBIC DIGESTION

A laboratory-scale anaerobic sequencing batch reactor (ASBR) was operated for three consecutive periods with different influent tylosin concentrations (no tylosin, tylosin at 1.67 mg/L, and tylosin at 167 mg/L). The microbial community structure in the ASBR was determined using quantitative fluorescence in situ hybridization (FISH) and phylogenetic analyses of bacterial 16S ribosomal RNA gene clone libraries of biomass samples taken throughout these periods. Treatment performance deteriorated and accumulation of propionate and acetate occurred following the increase of the influent tylosin concentration to 167 mg/L. During the periods without tylosin addition and with an influent tylosin concentration of 1.67 mg/L, 16S rRNA gene sequences related to Syntrophobacter species were frequently detected. In addition, the relative abundance of Methanosaeta species was high. During the period with an influent tylosin concentration of 167 mg/L, Syntrophobacter-related 16S rRNA gene sequences were not detected and Methanosaeta levels decreased considerably. These results indicate that the accumulation of propionate and subsequent reactor failure observed after long-term exposure to tylosin are due to the inhibition of propionate-oxidizing syntrophic bacteria closely related to Syntrophobacter. Inhibition of Methanosaeta was attributed to high propionate concentrations and low pH in the ASBR.