1. Introduction
Antibiotics are widely used in public health and animal husbandry in the treatment and prevention of bacterial infections. The US Food and Drug Administration (FDA) reported that 14.6 million kilograms of annual domestic sales of antimicrobial drugs were used in food-producing animal husbandry in 2014 (FDA. 2014). Particularly, tetracyclines accounted for 71% of these sales. In many countries, the use of antibiotics both for animal disease treatment and growth promotion is unsupervised, which often leads to abuse of antibiotics. Most of veterinary antibiotics were poorly absorbed within the gastrointestinal tract of livestock, causing the presence of biologically active antibiotics in animal waste. It was not unusual to detect high concentrations 19 of antibiotic residues in animal manures (Qiao et al., 2012; Pan et al., 2011;Nordenholt et al., 2016). In an agricultural field at Hannover, the tetracycline (TTC) level in the dried manure from the soil surface was 4 mg/kg (Gerd Hamscher, 2002). Similarly, Martinez analyzed pig manure and turkey dung samples from livestock farms in Austria, Vienna.
