Streptococcus mutans has the capacity of inducing
dental caries. Thus, to develop a novel way of preventing dental
caries, a cell wall hydrolase-producing strain was isolated and
its characteristics were investigated. Among 200 alkalophilic
strains isolated from soil, 8 strains exhibited lytic activities
against Streptococcus mutans. However, strain YU5215 with
the highest cell wall hydrolase activity was selected for
further study. Strain YU5215 was identified as a novel strain
of Bacillus based on analyzing its 16S rDNA sequence
and Bergey’s Manual of Systematic Bacteriology, and thus
designated as Bacillus mutanolyticus YU5215. The optimal
conditions for the production of the cell wall hydrolase from
Bacillus mutanolyticus YU5215 consisted of glucose (0.8%),
yeast extract (1.2%), polypeptone (0.5%), K2HPO4 (0.1%),
MgSO4·7H2O (0.02%), and Na2CO3 (1.0%) at pH 10.0.
Bacillus mutanolyticus YU5215 was cultured at 30oC for
72 h to produce the cell wall hydrolase, which was then
purified by acetone precipitation and CM-agarose column
chromatography. The molecular weight of the lytic enzyme
was determined as 22,700 Da by SDS-PAGE. When the cell
wall peptidoglycan of Streptococcus mutans was digested
with the lytic enzyme, no increase in the reducing sugars was
observed, while the free amino acids increased, indicating
that the lytic enzyme had an endopeptidase-like property. The
amino terminus of the cell wall peptidoglycan digested by the
lytic enzyme was determined as a glutamic acid, while the
lytic site of the lytic enzyme in the Streptococcus mutans
peptidoglycan was identified as the peptide linkage of L-Ala
and D-Glu.