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Seasonal dynamics of tetracycline resistance genes and antibiotics in a British Columbia agricultural watershed Keen, Patricia Lynn


The hypothesis of this research was that antibiotic resistance genes and/or antibiotics resulting from the veterinary use could be transported through ecosystems and are the most important agricultural cause promoting antibiotic resistance development in environmental bacteria. The underlying premise of this hypothesis was that exposure of bacteria to sub-inhibitory concentrations of antibiotics and/or mobile genetic elements that encode for antibiotic resistance may affect bacterial ecology in natural ecosystems. This, in turn, could threaten the efficacy of human and veterinary medicine through increasing the probability of environmental exposure of pathogens to resistant bacteria species. Antibiotic residues were measured in environmental samples collected from a British Columbia agricultural watershed by electrospray ionization liquid chromatography tandem mass spectrometry (ESI LC MS/MS). Four tetracycline resistance genes, chosen from the ribosomal protection protein supergroup, were measured in the same samples by real time quantitative polymerase chain reaction analyses (qPCR). Chemical analyses by ESI LC MS/MS of water, soil and compost samples were highly variable and did not reliably confirm concentrations of antibiotics in the receiving environment under investigation indicating that these might be close to the detection limits. However, the four selected tetracycline resistance genes could be consistently measured by qPCR in the same samples and monitored over time. The seasonal trend of higher relative abundance of the four tetracycline resistance genes (compared to bacterial biomass as indicated by measurement of abundance of 16S rRNA genes) was measured in wetter winter months. Positive statistical correlations (p < 0.05) between instantaneous and 48 h stream discharge and the total of the four tetracycline resistance genes demonstrated that precipitation and hydrologic conditions influence the transport of antibiotic resistance genes in natural ecosystems. In the soil of a field fertilized by poultry compost, the relative abundance and the proportion of tetracycline resistance genes within the bacteria biomass (as represented by normalization to abundance of 16S rRNA genes) remained relatively constant over time. Mesocosm experiments showed that exposure to light, water quality and presence of periphyton biofilms were among the factors that influence the fate of antibiotic residues and tetracycline resistance genes in aquatic ecosystems.

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