Inactivation of glutamate racemase (MurI) eliminates virulence in Streptococcus mutans

Inhibition of enzymes required for bacterial cell wall synthesis is often lethal or leads to virulence defects. Glutamate racemase (MurI), an essential enzyme in peptidoglycan biosynthesis, has been an attractive target for therapeutic interventions. Streptococcus mutans, one of the many etiological factors of dental caries, possesses a series of virulence factors associated with cariogenicity. However, little is known regarding the mechanism by which MurI influences pathogenesis of S. mutans. In this work, a stable mutant of S. mutans deficient in glutamate racemase (S. mutans FW1718) was constructed to investigate the impact of murI inactivation on cariogenic virulence in S. mutans UA159. Microscopy revealed that the murI mutant exhibited an enlarged cell size, longer cell chains, diminished cell⬜cell aggregation, and altered cell surface ultrastructure compared with the wild-type. Characterization of this mutant revealed that murI deficiency weakened acidogenicity, aciduricity, and biofilm formation ability of S. mutans (P < 0.05). Real-time quantitative polymerase chain reaction (qRT-PCR) analysis demonstrated that the deletion of murI reduced the expression of the acidogenesis-related gene ldh by 44-fold (P < 0.0001). The expression levels of the gene coding for surface protein antigen P (spaP) and the acid-tolerance related gene (atpD) were down-regulated by 99% (P < 0.0001). Expression of comE, comD, gtfB and gtfC, genes related to biofilm formation, were down-regulated 8-, 43-, 85- and 298-fold in the murI mutant compared with the wild-type (P < 0.0001), respectively. Taken together, the current study provides the first evidence that MurI deficiency adversely affects S. mutans virulence properties, making MurI a potential target for controlling dental caries.