Peptone water pw

Listeria monocytogenes was obtained from American Type Culture Collection (Manassas, VA, USA), with strains ATCC19113, ATCC19117, and ATCC15313, and used for the biofilm forming assays. The bacteria were cultured in tryptic soy broth (TSB, BD Difco, Franklin Lakes, NJ, USA) at 30 °C for 24 h, followed by another sub-culture at 18 h [49 (link)]. The cultures were centrifuged (11,000× g for 10 min) and washed two times with phosphate-buffered saline (PBS; Oxoid, Basigstoke, Winchester, UK). After that, peptone water (PW; Oxoid, Basingstoke, Winchester, UK) was added to the final bacterial solution to dilute it until it contained 10⁵ log CFU/mL of bacteria. Then, the strains were mixed together to make a mixed culture suspension for further experiments. Following a 48 h-incubation period at 30 °C, the final concentration was ascertained using the spread plate method on PALCAM agar (Oxoid, Basingstoke, Winchester, UK) using a PALCAM-selective supplement.

Ten milliliters of cell suspension in TSB was illuminated by 465, 520, and 625 nm LED for 24–30 h at the set temperature of 24.5 °C. Cell growth under LED illumination was monitored periodically by sampling at appropriate time intervals, diluting in 0.1% (wt/vol) peptone water (PW; Oxoid), and plating on tryptic soya agar (TSA; Oxoid). The cells grown under dark conditions (non-illuminated) at the set temperature of 25 °C served as a control in this study. The number of viable cells expressed as log CFU/ml was plotted against time. The growth curves were generated by fitting the data to the equation of Baranyi and Roberts (1994) using DMFit (https://browser.combase.cc/DMFit.aspx) and the growth parameters, namely, lag phase duration (LPD), specific growth rate (GR), doubling time (DT) and maximum population density (MPD), were calculated. Based on the growth curve, the time for the early stationary phase of cells under each LED illumination was also determined and the cells were collected for tolerance to each stress condition and RNA-seq analysis.

Vibrio parahaemolyticus was collected from American Type Culture Collection (Manassas, VA, USA) strain (ATCC 27969) and used for the biofilm-forming assays. The bacteria were cultured in tryptic soy broth (TSB, BD Difco, Franklin Lakes, NJ, USA) with 2.5% NaCl at 30 °C for 24 h followed by another sub-culture at 18 h [55 (link)]. Briefly, stock solutions of the bacteria strains (cell density: 10⁸–10⁹ CFU/mL) were stored in phosphate-buffered saline (PBS; Oxoid, Basingstoke, UK) containing 30% glycerol in a deep freezer at −80 °C. First, 100 μL of bacteria was inoculated into 10 mL of tryptic soy broth (TSB; BD Difco, Detroit, MI, USA) and cultured at 30 °C and 200 rpm in a shaking incubator (Vision Scientific, VS-8480, Seoul, Korea). After 24 h, 100 μL was taken from the culture medium and inoculated in 10 mL of fresh TSB, then placed in the shaking incubator under the same conditions as the previous day. The culture was centrifuge (11,000× g for 10 min) and washed two times with phosphate-buffered saline (PBS; Oxoid, Basigstoke, England). After that, peptone water (PW; Oxoid, Basingstoke, England) was added to the final bacterial solution to reach the 10⁵ log CFU/mL of bacteria. The formation of biofilms on surfaces of SS and HG was then accomplished using these inoculums (10⁵ CFU/mL).