2.10. FTIR analysis

FTIR spectroscopy was intensively applied to investigate the action mode of antimicrobial techniques and to monitor the structural changes in bacterial cell membranes under different stress [24], [25]. After different treatments, bacterial suspensions were obtained by centrifugation (8000 × g, 10 min, 4℃), double rinses, and resuspension with aseptic PBS buffer. Samples were subjected successively to overnight refrigeration at −20℃, two days of freezing at −80℃, vacuum freeze-drying, and then stored in desiccator until use. The FTIR analyses were performed by the Nicolet iN10 FT-IR spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). The absorption spectra were recorded within the range of 4000–400 cm⁻¹. Background obtained from the pure potassium bromide (KBr) disc was automatically subtracted from each sample spectrum, since the embedding of dried samples into the KBr pastille prior to analysis. For every single sample, average spectrum was acquired from three independent replicates, followed by the smoothing, normalization, and baseline-correction processing.