*2.4. Antibiotic Analyses in Plants*

Samples of the edible pakchoi portions (stem and leaf) were selected and freeze-dried for 24 h until moisture was no longer present [35]. Then, the freeze-dried samples were ground thoroughly, and the amount of antibiotics in the plant tissues were determined using ultrasonic extraction, solid-phase extraction, and liquid chromatography-mass spectrometry. The extraction method and clean-up procedure used was already described for the analysis of Chinese white cabbage, water spinach, and other crops [15]. Thereafter, the target compounds from treated samples were analyzed using an ultra-performance liquid chromatography-tandem mass spectrometer (Waters, Milford, MA, USA) equipped with an electrospray ionization source in multiple-reaction monitoring mode. Details of the quantitative analysis were described by Gros et al. [36].

#### *2.5. Enumeration of Total Cultivable Endophytic Bacteria (TCEB) and AREB*

To isolate endophytic bacteria, the edible portions of fresh pakchoi were immersed in 3% hydrogen peroxide for 30 min, followed by rinses with sterile deionized water (3 min × 3 times). Then, they were immersed in 70% ethanol for 1 min and rinsed as before [37]. Finally, surface-sterilized samples were dried using sterilized filter papers. To ensure the complete surface disinfection, 100 μL of the last wash water was spread on meat-peptone agar and cultivated at 30 ◦C for 3 days to check for colony growth [38]. Samples with no bacterial growth were considered successfully sterilized. For each experimental treatment, the disinfected vegetable was cut with a sterile scalpel into pieces and ground together with quartz sand in a sterile mortar. Then, 3 g of ground tissue was mixed with 10 mL of sterile water and the mixture was diluted to 10−3. Each 100 μL of diluted suspension was spread on meat-peptone agar and on corresponding antibiotic-containing agars (TC, CPL, and SMX at concentrations of 16, 64, and 76 mg·L<sup>−</sup>1, respectively) for cultivation at 28 ◦C for 3 days. Each sample was replicated three times. The colony-forming units (CFUs) of TCEB and AREB (endophytic bacteria resistant to TC, CPL, and SMX, respectively) were enumerated.
