Potassium Release from Desiccated Salmonella

The potassium-sensitive benzofuran isophthalate probe (PBFI; Invitrogen, Carlsbad, CA, USA) was used to determine the release of potassium ions from desiccation-adapted Salmonella Tennessee as described previously [9 (link),11 (link)]. Briefly, aliquots (1 mL, each) of desiccation-adapted cells in saline (Figure 1) were harvested by centrifugation at 8000× g for 5 min, and cell pellets were resuspended in 5 mM HEPES buffer (Sigma Aldrich) containing 5 mM glucose (Fisher Scientific, Fair Lawn, NJ, USA). Portions of these cell suspensions (90 µL) were added to wells of a black 96-well microplate (Corning) followed by the addition of the PBFI probe at a final concentration of 2 µM. Then, 10 µL of carvacrol or thymol was added to each well to achieve final concentrations of 80 to 800 µg/mL of each compound, and polymyxin (10 µg/mL) served as a positive control. Concentrations of released potassium ions were measured as changes in fluorescence using a microplate reader (PerkinElmer, Wellesley, MA, USA) at excitation and emission wavelengths of 346 and 505 nm, respectively. Fluorescence measurements were captured every 100 s and normalized by subtracting the background noise.