**1. Introduction**

Antibiotics are discharged from animals (and humans) and enter wastewater treatment plants (WWTPs) through the sewage system [1]. In general, antibiotics are not easy for wastewater treatment facilities to remove [2]. The release and persistence of antibiotics in the environments may lead to an increase in antibiotic-resistant bacteria [3]. Tetracycline antibiotics (TCs), such as tetracycline (TC), oxytetracycline (OTC), and chlortetracycline (CTC), are broad-spectrum antibacterial drugs that inhibit bacterial protein synthesis [4]. Sulfonamide antibiotics (SAs) act against most Gram-positive and many Gram-negative bacteria. SAs act as competitive inhibitors of *p*-aminobenzoic acid in the folic acid synthetic pathway to inhibit the growth of bacteria [5]. Three sulfonamides, sulfamethoxazole (SMX), sulfadimethoxine (SDM), and sulfamethazine (SMZ), are found in sludge in many WWTPs and rivers [6–8]. Amoxicillin (AMO) is a β-lactam family antibiotic drug. This class of antibiotics disrupts bacterial cell walls during bacterial growth [9]. AMO in the environment may lead to an increase in antibiotic-resistant bacteria [10].

Several methods for antibiotic removal from wastewater sludge have been proposed. These approaches include chemical oxidation methods such as UV- and solar-based procedures (UV/H2O2, solar/H2O2), ozonation, photocatalysis by Fe<sup>2</sup><sup>+</sup> or Fe<sup>3</sup>+/H2O2 and TiO2 photocatalysis [11]. Antibiotic degradation by fungal extracellular enzymes extracted from spent mushroom composts has also been reported [12]. Thermal hydrolysis/anaerobic digestion [13] and the use of aerobic granular sludge treated with manganese oxides [14,15] have also been proposed. Nevertheless, many antibiotics continue to be detected in wastewater sludge [16–18]. The use of microorganisms to eliminate antibiotics is a promising strategy [19–21]. The aerobic degradation of antibiotics has been observed in pure bacterial cultures and microbial consortiums. *Rhodococcus rhodochrous* and *Aspergillus niger* degrade pharmaceuticals via cometabolism [22]. *Stenotrophomonas maltophilia* DT1 biotransforms tetracycline [23]. *Klebsiella* sp. SQY5 degrades tetracycline (TEC) [24]. Sulfamethoxazole is biodegraded by individual and mixed bacteria [25,26]. Antibiotic sulfanilamide biodegradation is performed by acclimated microbial populations [27]. Chlortetracycline can be used as the sole carbon and nitrogen source by the acclimated microbiota [28].

The aim of this study was to examine the degradation of antibiotics in sludge by antibiotic-degrading bacteria under aerobic and anaerobic conditions. The microbial communities involved in the aerobic and anaerobic degradation of antibiotics in sludge are revealed.

#### **2. Materials and Methods**

#### *2.1. Chemicals*

SMX, SDM, SMZ, TC, OTC, CTC, AMO (Table S1) of 99.0% purity and all other chemicals were purchased from Sigma Chemical Co. (St. Louis, MO, USA). Solvents were purchased from Mallinckrodt (Paris, KY, USA).
