Elucidating the Potential for Transfer of Antibiotic Resistance Genes through Agro-Ecosystems.

Increases in the occurrence of antibiotic resistance (AR) in clinical pathogens raises concerns about how resistance is spread within and between human, environmental and agricultural eco-systems. Livestock manure applied to soil may increase the abundance of AR bacteria as a result of addition of manure-associated-bacteria containing AR genes and/or through selection and enrichment of AR bacteria that are part of the native soil flora. These AR genes are often associated with class 1 integrons which are found on mobile genetic elements (MGE) that are readily transferred between bacterial populations. In field studies, the abundance of AR genes for sulfonamide, tetracycline and erythromycin were found to be higher in soils with applied poultry, beef or swine manure than in grass or soils with no applied manure. The abundance of the class one integron integrase (intI1) gene in the same soils with applied poultry (2.7 ± 1.4 X 10⁷ copies intI1 gene g^-1), beef (2.1 ± 0.84 X 10¹⁰ copies intI1 gene g^-1) or swine (1.9 ± 1.0 X 10⁸ copies intI1 gene g^-1) manure was also higher than in grass or soils (0.1 to 4.5  X 10⁶ copies intI1 gene g^-1) with no applied manure. IncP-1 were detected by PCR-Southern blot hybridization but not by qPCR. The detection of plasmid-specific sequences and integrons does not demonstrate mobility, therefore, exogenous plasmid isolation will be necessary to confirm that these elements are transferable. However, these results suggest that there is potential for AR genes in these soil systems to be associated with MGE that can contribute to their spread to other eco-systems. In general, more data are needed to fill gaps in understanding how AR factors are transmitted through agro-ecosystems so better risk assessment tools and mitigation strategies can be developed.