control
L. rhamnosus GG was cultured on MRS agar media and incubated at 37 for 24-48 hours. Two to four pure colonies with similar morphologies for each strain were selected and inoculated in 5 mL of MRS medium. The cultures were incubated in a shaking incubator at 37 and 110 rpm for 16 hours until the mid-log phase (OD600 ~ 0.5-1). The cultures were then transferred at a 1:100 ratio into 5 ml of MRS medium containing fluoride. Approximately 50 l of the homogenised bacterial suspension of the strain was inoculated into 4.95 ml of MRS medium, as well as 4.95 ml of high- and low-fluoride media. The high (2500 mg/l NaF) and low fluoride (9 mg/l NaF) medium was prepared with MRS media by adding an appropriate amount of NaF using the concentration formula C1V1 = C2V2. The control culture with no fluoride and the low fluoride culture were incubated in a shaking incubator under similar conditions of temperature and speed mentioned above for 24 hours (until the culture reached an OD600 of approximately 1.0), whereas high fluoride cultures were incubated for only 6 hours to achieve optimum OD and get some bacterial pellet for RNA extraction. The optical density of the medium was determined using UVVis Spectrophotometer at a wavelength of 600 nm, following the methods of Li et al (2020) and Dutta et al (2020) (Li et al., 2020, Dutta et al., 2020). After culturing for a specific time, the cells were collected by centrifugation for RNA extraction.