The presence of tetracycline in cow manure changes the impact of repeated manure application on soil bacterial communities

The effect of tetracycline (Tc) and cow manure on soil bacterial community composition and antibiotic resistance gene (ARG) abundance in soil was investigated in the present microcosm study. Effects of repeated applications of cow manure spiked with Tc in two concentrations or without Tc on the bacterial communities of a clayey and a sandy soil with different history of anthropogenic pollution by sewer flooding were investigated. Soil samples were taken 60 days after each of three amendments. Denaturing gradient gel electrophoresis (DGGE) fingerprints of 16S rRNA gene amplicons from total community DNA revealed soil type-dependent changes in the bacterial community composition in response to manure and to Tc, which became more pronounced with repeated applications. Repeated manure amendments and Tc, in particular at high concentration, triggered the further increase of ARGs tet(A), tet(O), tet(Q), tet(W), sul1, and mobile genetic elements (MGEs) IncP-1ε plasmids and intI1, in a soil type-dependent manner. In the clay soil with no anthropogenic history, the ARG and MGE abundances were low or not detectable, while manure amendments caused pronounced increases in their relative abundance. In the sandy soil with a history of anthropogenic impact, ARGs and MGEs were already present at a higher level and strong increases were mainly observed for the relative abundances of sul2 and MGEs. Here, we show for the first time that effects of repeated cow manure applications might be dependent on soil type and foregoing anthropogenic soil pollution and that the presence of Tc could further increase the abundance of ARGs and MGEs.