L-serine-O-sulfate alters cellular ultrastructure and mitigates the capacity of biofilm formation in Streptococcus mutans UA159 via interfering with glutamate racemase.

Dental caries, primarily caused by Streptococcus mutans (S. mutans), remains a significant global health challenge. Conventional treatments often disrupt commensal oral flora, necessitating targeted antimicrobial strategies. This study investigated L‑serine-O-sulfate (LSOS), a substrate analog of glutamate racemase (MurI), as a potential agent for interfering with S. mutans UA159 virulence. Computational docking predicted LSOS binding to MurI, while circular dichroism spectroscopy confirmed LSOS-induced structural perturbations in recombinant MurI. LSOS exhibited concentration-dependent bactericidal effects, with 5.0 mM completely suppressing growth and 2.5 mM significantly extending doubling time (11.37 hrs vs. 5.68 hrs in controls). Transmission electron microscopy revealed progressive ultrastructural damage, characterized by membrane blebs and cell wall disintegration. Biofilm formation was severely impaired, with 63 % reduction in biomass and significant disruption of extracellular matrix integrity. Microarray-based gene expression analysis identified 119 differentially expressed genes, predominantly downregulated (111/119), affecting translation machinery, metabolic pathways, and transmembrane transport. Biosafety evaluation in L929 fibroblasts showed reduced proliferation (67.59 % of control at 2.5 mM after 48 hrs) with both G1-phase reduction and S-phase cell cycle accumulation. Caenorhabditis elegans demonstrated uncompromised survival and early development at concentrations <10 mM, with developmental toxicity emerging only at higher doses (≥20 mM). These findings establish LSOS as a promising anti-virulence agent targeting MurI in S. mutans UA159, with favorable biosafety profiles that warrant further investigation for dental caries prevention and treatment.