Effect of Azadirachta indica Paste on Lactobacillus rhamnosus in Dentinal Tubules and Its Cytotoxicity for Dental Pulp Stem Cells

Abstract

Objectives: Azadirachta indica var. siamensis Valeton, also known as neem, possesses various beneficial biological properties, including antibacterial activity. Neem paste (NZ-paste) was developed to enhance its application. This study aimed to investigate the antibacterial activity of NZ-paste against Lactobacillus rhamnosus (L. rhamnosus), its penetration ability, and its cytotoxicity for human dental pulp stem cells (hDPSCs). Materials and Methods: The active compounds of the neem extract were identified using high-performance liquid chromatography (HPLC). The rheological and physicochemical characteristics of 20mg/g NZ-paste were evaluated by pH measurement, penetration testing, and rheometer analysis. The antibacterial activity of NZ-paste against L. rhamnosus was assessed using agar disc diffusion and live/dead BacLight® assays in dentin, and confirmed by confocal laser stereomicroscopy (CLSM). The methyl thiazolyl tetrazolium assay was used to determine its cytotoxicity for hDPSCs. Results: HPLC analysis identified azadirachtin, quercetin, nimbin, and nimbidin as the main bioactive compounds. NZ-paste had a smooth, fluid-like texture, was water-soluble, and exhibited a pH of 7.09. Although its inhibition zone was comparable to that of Ca(OH)₂, NZ-paste demonstrated superior antibacterial efficacy in the dentinal tubule invasion model, as indicated by the lowest live/dead ratio confirmed by CLSM. NZ-paste at concentrations up to 10mg/mL did not affect hDPSC viability. Conclusion: The novel NZ-paste exhibits antibacterial activity against L. rhamnosus, with suitable penetration ability and favorable biocompatibility with hDPSCs at low concentrations.