Abstract
Long-term irrigation with untreated wastewater can lead to an accumulation of antibiotic substances and antibiotic resistance genes in soil. However, little is known so far about effects of wastewater, applied for decades, on the abundance of IncP-1 plasmids and class 1 integrons which may contribute to the accumulation and spread of resistance genes in the environment, and their correlation with heavy metal concentrations. Therefore, a chronosequence of soils that were irrigated with wastewater from 0 to 100 years was sampled in the Mezquital Valley in Mexico in the dry season. The total community DNA was extracted and the absolute and relative abundance (relative to 16S rRNA genes) of antibiotic resistance genes (tet(W), tet(Q), aadA), class 1 integrons (intI1), quaternary ammonium compound resistance genes (qacE+qacEΔ1) and IncP-1 plasmids (korB) were quantified by real-time PCR. Except for intI1 and qacE+qacEΔ1 the abundances of selected genes were below the detection limit in non-irrigated soil. Confirming the results of a previous study, the absolute abundance of 16S rRNA genes in the samples increased significantly over time (linear regression model, p < 0.05) suggesting an increase in bacterial biomass due to repeated irrigation with wastewater. Correspondingly, all tested antibiotic resistance genes as well as intI1 and korB significantly increased in abundance over the period of 100 years of irrigation. In parallel, concentrations of the heavy metals Zn, Cu, Pb, Ni, and Cr significantly increased. However, no significant positive correlations were observed between the relative abundance of selected genes and years of irrigation, indicating no enrichment in the soil bacterial community due to repeated wastewater irrigation or due to a potential co-selection by increasing concentrations of heavy metals.
Highlights
Wastewater irrigation is a widely used practice worldwide, especially in arid and semiarid regions, to alleviate water shortages in agriculture (Siebe and Cifuentes, 1995; Jimenez and Chávez, 2004; Elifantz et al, 2011; Frenk et al, 2014)
Wastewater typically contains a diverse mixture of pharmaceuticals, pathogenic bacteria, antibiotic resistant bacteria, resistance genes, and heavy metals, which can reach and affect the environment and might pose a risk for human health when wastewater is not properly treated or directly applied as fertilizer (Moura www.frontiersin.org et al, 2010; Bouki et al, 2013; Norton-Brandao et al, 2013; Rivera-Utrilla et al, 2013)
In the Mezquital Valley, results from a cross-sectional survey done 20 years ago revealed that intestinal helminth infections represent the highest risk associated with exposure to wastewater irrigation (Blumenthal et al, 1991; Cifuentes et al, 1991)
Summary
Wastewater irrigation is a widely used practice worldwide, especially in arid and semiarid regions, to alleviate water shortages in agriculture (Siebe and Cifuentes, 1995; Jimenez and Chávez, 2004; Elifantz et al, 2011; Frenk et al, 2014). More than 20 million ha of land are estimated to be irrigated with wastewater globally, and in developing countries the number of people consuming produce irrigated with poorly or nontreated water is increasing (Amoah et al, 2007; Raschid-Sally and Priyantha, 2008). Besides high concentrations of organic matter, wastewater typically contains large amounts of pollutants including detergents, heavy metals, pharmaceuticals including antibiotics, as well as pathogenic and antibiotic resistant bacteria carrying resistance determinants, class 1 integrons and mobile genetic elements (MGEs) (Baquero et al, 2008; Levantesi et al, 2010; Moura et al, 2010; Chávez et al, 2011; Malik and Aleem, 2011; Bruchmann et al, 2013; Rizzo et al, 2013; Manzetti and Ghisi, 2014). Previous studies showed an increase of pharmaceuticals
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