Analytical methods of cell growth and fermentation products

Every 12 hours, three parallel samples (100 µL) were taken to monitor the bacteria density, consumption of glucose and xylose, and L-ornithine production. The OD600 value was detected to assess the cell growth by a microplate reader (BioTek Instruments, Winooski, VT, USA) after adding 0.125 mol/L HCl to dissolve CaCO3 (Zhang et al., 2019). The samples were centrifuged to obtain fermentation supernatant. The glucose concentration was analyzed by SBA-40E biosensor analyzer (Institute of Biology, Shandong Academy of Sciences, Jinan, Shandong, China). The level of xylose consumption was determined by high-performance liquid chromatography (HPLC) (Chen, Zhu, & Xia, 2014; Yim et al., 2013). The content of L-ornithine was determined by ninhydrin colorimetry (B. Zhang, M. Yu, Y. Zhou, et al., 2018). All experiments had triple parallels, and data are repressed as mean and standard deviation (SD).