Effect of a veterinary antibiotic and manure amendment on the structure and function of soil microbial community

This thesis was aimed at studying the sorption of the antibiotic tetracycline (Tc), at two different concentrations (100 and 500 mg kg-1), on two soils (a clay soil and a sandy soil). In particular, the effects on the microbial structural and functional diversity and on the resistance level were investigated in microcosms on three consecutive dates (2, 7, 60 d) since single treatment with manure and additional Tc. Microbial counts, PFLA, CLPP, FDA analysis were concordant in pointing out a short time detrimental effect on the microbial structure and function after the Tc treatment at the higher rate. The DGGE fingerprinting investigation on 16S-rRNA gene fragments and resistance gene level were based on a separate experiment with three consecutive manure and Tc additions to soil. A single treatment was effective to modify the whole bacterial community of clay soil, whereas additional treatments were necessary in the sandy soil. Manure increased the abundance of mobile genetic elements (intI1,trfA,korB) andsulgenes related to Tc resistance in soil. The genestetA,tetQ,tetWwere affected mostly by manure than Tc addition. Tc showed a transient effect on microbial structure after single treatment, whereas manure increased the bacterial diversity and resistance in soil after repeated treatments. The results were consistent with the bioavailability of Tc in soil.