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    • Tetracyclines constitute one of the most important antibioti

    2023-02-20

    Tetracyclines constitute one of the most important antibiotic groups used as veterinary and human medicine and as feed additives in the agricultural sector (Li et al., 2011a, Simon, 2005). Annual worldwide production of tetracyclines is estimated to be in the thousands of tons (Michalova et al., 2004), which ranks second among antibiotics. Moreover, it is the most produced and frequently used antibiotic in China (Cheng, 2005, Xie et al., 2010). For instance, the estimated total amount of tetracyclines used in China in 2013 was approximately 12,000tons; human consumption and veterinary application accounted for 1770tons and 4430tons, respectively (Zhang et al., 2015). Sulfonamides are widely used in the therapeutic treatment of animals and humans, but they also pose environmental contamination risks. In 2013, the total amount of sulfonamides used in China was estimated to be approximately 7920tons, of which human consumption and veterinary application accounted for 905tons and 5970tons, respectively (Zhang et al., 2015). In addition, tetracycline and sulfamethazine are the most extensively used compounds of tetracyclines and sulfonamides, which are frequently found in the terrestrial and aquatic environment (Pan et al., 2014, Zhang et al., 2015). Tetracycline has been reported to persist in surface water and soil for more than one year (Zuccato et al., 2000). In recent years, the occurrence of Linezolid mg and antibiotic resistance genes (ARGs) in urban water has become an important issue for the protection of the terrestrial environment and public health, as domestic or fishpond wastewater are frequently used for agricultural activities (e.g., irrigation and cultivation). The widespread occurrence of antibiotics in wastewater and agricultural soils may exert selection pressure on environmental microorganisms, resulting in the proliferation of antibiotic resistance in microorganisms. ARGs are considered new contaminants and may pose potential health risks to humans worldwide (Zhu et al., 2013). ARGs in microorganisms could be transferred to indigenous environmental bacteria by horizontal gene transfer (Gillings et al., 2014). The usage of antibiotics in agricultural soil was recognized as an important factor in the generation and propagation of ARGs (Kristiansson et al., 2011). ARGs could migrate to deeper soil layers and possibly contaminate groundwater by vertical transport (Huang et al., 2013). Joy et al. (2014) reported that the abundance of ARGs in surface soils was in the orders of magnitude higher than that in subsoil after manure amendment. However, only a limited number of studies examined the occurrence of antibiotics and relative ARGs in wastewater-irrigated agricultural soils. Tamtam et al. (2011) determined antibiotic concentrations in soils after long-term wastewater irrigation, but they did not examine the ARGs in soils; Negreanu et al. (2012) assessed the long-term effect of wastewater irrigation on antibiotic resistance in soils, but they did not measure the concentrations of antibiotics in soils, nor did they explore the possible correlations between antibiotic concentrations and ARG abundances. Further study is therefore needed to better understand the impact of wastewater irrigation on the occurrence of antibiotics and ARGs, as well as their relationships in agricultural soils. In this study, soils from six agricultural sites with long-term wastewater irrigation in southern China were sampled and the levels of two typical antibiotics, tetracycline (TC) and sulfamethazine (SMZ), as well as eleven ARGs (tet A, tet B, tet C and tet E, which code for energy-dependent efflux proteins that export tetracyclines out of the cell; tet M, tet O and tet S, which code for ribosomal protection proteins; tet X, which encodes tetracycline-inactivating enzymes; and sul I, sul II and sul III, which code for dihydropteroate synthases) were quantified using LC-MS/MS and real-time qPCR. The objectives were (1) to assess the effects of long-term wastewater irrigation on the occurrence of antibiotics and relative ARGs in agricultural soils; (2) to verify the vertical distribution of antibiotics and ARG abundances at different soil depths (0–10 and 10–20cm); and (3) to investigate the relationships between antibiotic concentrations and ARG levels in wastewater and irrigated agricultural soils. This study is the first to investigate the effects of long-term wastewater irrigation on the occurrence of antibiotics and ARGs in agricultural soils in southern China.