2.6 Analytical Methods
The optical density at 600 nm (OD600) was measured in a 1.5-mL cuvette (1 cm light path length) with a spectrophotometer (UV-16-1, Shimazu). One unit of OD600 was equivalent to 0.534 g/L cell dry weight. Glucose in the fermentation broth samples was determined with a YSI 2300 glucose analyzer (Yellow Spring, OH). All metabolites including acetone, butanol, ethanol, acetic acid, and butyric acid in the fermentation broth samples were analyzed, after removing cells by centrifugation or microfiltration using 0.2-µm syringe filters and acidified with phosphoric acid (1% v/v), with a gas chromatograph (GC-2014, Shimadzu) following the method described elsewhere (Xu et al., 2015).