EFFECTIVENESS OF DIFFERENT SODIUM FLUORIDE CONCENTRATIONS AGAINST LACTOBACILLUS CASEI IN THE INHIBITION ZONE: A NOVEL INOCULATION MICROBIOLOGICAL STUDY

Background: Lactobacillus Casei is a bacterium related to dental caries by producing acidification of the oral
environment and leading to teeth demineralization. It appears that these bacteria correlate with another pathogenic
microorganism, as evidenced by increased extracellular protein production. Sodium fluoride has been used as a
preventive treatment to prevent acidification and reduce the risk of caries disease. At the same time, the optimal sodium fluoride concentration for prevention and treatment remains unclear, which also allows for safety in dental products.
The present trial aims to find the best sodium fluoride concentration that is effective in eliminating Lactobacillus Casei.
Methods: To evaluate the inhibitory effect of sodium fluoride on Lactobacillus casei, we conducted a preliminary trial
using distilled water (0% sodium fluoride) to refine experimental procedures. This trial revealed contamination from
airborne dust, prompting adjustments such as working near a Bunsen burner flame to create a sterile environment and manually sterilizing all equipment with ethanol before and after use. Bacterial inoculation involved evenly spreading L. casei from yogurt onto MRS agar plates using sterile cotton swabs. Filter paper discs were prepared by dipping them into sodium fluoride solutions of varying concentrations (0.15%, 0.30%, 0.45%, and 0.60%) and placed on the agar.
Plates were incubated at 32.5°C for 72 hours. The radius of the inhibition zone around each disc was measured using a
digital caliper, and results were averaged for each concentration. Sodium fluoride solutions were prepared through serial dilutions, starting from 99% sodium fluoride powder mixed with distilled water.
Results: The results demonstrated a strong positive correlation (r = 0.942) between the sodium fluoride concentration
and the radius of the inhibition zone, as illustrated by the non-linear trend in the processed data. At 0% sodium fluoride, a small inhibition zone (mean 0.58 mm) was observed due to contamination, but with increasing concentrations, the inhibition zone expanded significantly, peaking at 3.30 mm for 0.60% sodium fluoride. Standard deviations were minimal for most concentrations, indicating homogeneity in the data, except for 0% and 0.45%, which showed greater variability. The statistical analysis confirmed the significance of the correlation (p = 0.016), rejecting the null hypothesis and supporting the alternative hypothesis that higher sodium fluoride concentrations inhibit bacterial growth more effectively. Error bars highlighted the precision of the data, with the lowest error observed at 0.30% sodium fluoride (0.11 mm).
Conclusion: The most effective concentration of sodium fluoride was 0.60%. This concentration obtained the widest
zone of inhibition against Lactobacillus Cassei, followed by 0.45% concentration.