Effects of tetracycline on the soil microflora: Function, diversity, resistance

Background. Tetracycline is a widely used antibiotic in animal production. Significant amounts of the substance reach the soil via feces, urine and manure application. As tetracycline is a persistent compound with antibacterial activity, its presence in soil may have undesired direct and indirect effects. These have been investigated so far focusing on effects on selected microbial functions. Objectives. The aim of the present study was to obtain comprehensive information on potential effects of tetracycline on the soil microflora under environmentally relevant conditions. The investigations included function and structure of the microbial biocoenosis and the distribution of resistance genes. Methods. Pig manure rich in tetracycline resistance genes was applied to a sandy soil. This soil as well as an unamended soil were additionally treated with several concentrations of tetracycline. The spiked soils were incubated in outdoor lysimeters for several month. Substrate induced respiration, PLFAs, ten selected resistance genes, and the concentrations of tetracycline were determined. Results. The test concentrations, through far exceeding environmental relevance, caused only small effects. An establishment of resistance could not be detected. Applied resistance genes were not detectable at the end of the study even in the presence of added tetracycline. Conclusion. Due to the high sorption capacity of the antibiotic, environmentally relevant concentrations of tetracycline do not seem to cause undesired effects on the soil microflora.