4.3. Formaldehyde Concentration Measurement

At the indicated time points, 140 μL of culture were collected and centrifuged for 10 min at 3300× g, 4 °C. Formaldehyde concentration in culture supernatants was determined by Nash assay [35]. 125 μL of culture supernatant was mixed with 125 μL of Nash reagent (2 M of ammonium acetate, 20 mM of acetylacetone, and 50 mM of acetic acid) in 96-well plate (Brandplates, Wertheim, Germany), and incubated at 37 °C for 1 h. The absorption at 412 nm was measured for each sample using plate reader (Infinite M200 pro, Tecan, Männedorf, Switzerland). A standard curve showed that the assay was linear at the range from 1 mM to 0 mM formaldehyde in M9.