Abundance and dynamics of antibiotic resistance genes and integrons in lake sediment microcosms

Abstract

Antibiotic resistance in bacteria causing disease is an ever growing threat to the world. Recently, environmental bacteria have become established as important both as sources of antibiotic resistance genes and in disseminating resistance genes. Low levels of antibiotics and other pharmaceuticals are regularly released into water environments via wastewater, and the concern is that such environmental contamination may serve to create hotspots for antibiotic resistance gene selection and dissemination. In this study, microcosms were created from water and sediments gathered from a lake in Sweden only lightly affected by human activities. The microcosms were exposed to a mixture of antibiotics of varying environmentally relevant concentrations (i.e., concentrations commonly encountered in wastewaters) in order to investigate the effect of low levels of antibiotics on antibiotic resistance gene abundances and dynamics in a previously uncontaminated environment. Antibiotic concentrations were measured using liquid chromatography-tandem mass spectrometry. Abundances of seven antibiotic resistance genes and the class 1 integron integrase gene, intI1, were quantified using real-time PCR. Resistance genes sulI and ermB were quantified in the microcosm sediments with mean abundances 5 and 15 gene copies/10(6) 16S rRNA gene copies, respectively. Class 1 integrons were determined in the sediments with a mean concentration of 3.8 × 10(4) copies/106 16S rRNA gene copies. The antibiotic treatment had no observable effect on antibiotic resistance gene or integron abundances.

Figure 1. Antibiotics quantified in the water phase over 100 days in microcosms for three microcosms (1x, 10x and 1000x antibiotic concentration).

Error bars indicate the standard error of the mean. The reduction (relative decrease of the initial concentration as compared to day ‘100’) is presented in parenthesis after each line. Sampling points without dots indicate that the antibiotic concentration was below the limit of quantification except for the quantification of oxytetracycline at day ‘28’ in the 1000x microcosm which could not be performed due to sample loss.

Figure 2. Mean by day of intI1 and 16S rRNA gene concentrations measured in the microcosm sediments.

Different coloured bars denote the microcosm which had different starting concentrations of antibiotics added. Error bars denote the standard error of the mean.