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In activated sludge, the antibiotic resistance genes (ARGs) can be present either in the intracellular (iDNA) or extracellular DNA fraction (exDNA). Recent advances in the exDNA extraction methodology allow a better profiling of the pool of ARGs. However, little is known about how stress conditions modify the distribution of ARGs between both DNA fractions. Here, we performed two batch tests for analyzing the effects of two different stress conditions, namely nutrient starvation and high concentrations of sulfamethoxazole (1, 10 and 150 mg L-1) in activated sludge. We tracked by qPCR the resulting relative abundances of four target genes, namely the universal 16S rRNA gene, the class 1 integron-integrase geneintI1, and the sulfonamide resistance genessul1andsul2in both the iDNA and exDNA fractions. In the exDNA pool, unlike starvation, which provoked a decrease of 1-2 log10[copies] ng DNA-1in the concentration ofsul1andintI1, the presence of sulfamethoxazole did not influence the abundances ofsul1andsul2. However, high concentrations of sulfamethoxazole (150 mg L- 1) selected for microorganisms harboringsul1and, more remarkably,sul2genes in their iDNA during their exponential growth phase. The abundances ofintI1andsul1were positively correlated in the exDNA fraction (r>0.7), whereas no significant correlation (pHighlightsStarvation caused a decrease in the exDNA concentration of the activated sludge.A positive correlation in the abundances ofintI1andsul1was found in the exDNA fraction.High concentrations of SMX selected for antibiotic resistant microorganisms. |
