Long-term antibiotic exposure in soil is associated with changes in microbial community structure and prevalence of class 1 integrons.

Antimicrobial resistance is one of the most significant challenges facing the global medical community and can be attributed to the use and misuse of antibiotics. This includes use as growth promoters or for prophylaxis and treatment of bacterial infection in intensively farmed livestock from where antibiotics can enter the environment as residues in manure. We characterised the impact of the long-term application of a mixture of veterinary antibiotics alone (tylosin, sulfamethazine and chlortetracycline) on class 1 integron prevalence and soil microbiota composition. Class 1 integron prevalence increased significantly (p < 0.005) from 0.006% in control samples to 0.064% in the treated plots. Soil microbiota were analysed using 16S rRNA gene sequencing and revealed significant alterations in composition. Of the 19 significantly different (p < 0.05) OTUs identified, 16 were of the Class Proteobacteria and these decreased in abundance relative to the control plots. Only one OTU, of the Class Cyanobacteria, was shown to increase in abundance significantly; a curiosity given the established sensitivity of this Class to antibiotics. We hypothesise that the overrepresentation of Proteobacteria as OTUs that decreased significantly in relative abundance, coupled with the observations of an increase in integron prevalence, may represent a strong selective pressure on these taxa.This work was financed by the UK Ministry of Defence (D.W.C.), Agriculture and Agri-Food Canada (E.T), Natural Environment Research Council (grant number NE/E004482/1 L.Z.) and the European Regional Development Fund (W.H.G.)