Streptococcus mutans, a cariogenic agent, has a glucan-binding protein gene,
gbpC, and
S. criceti possesses four
gbpC homologs, including
dblA and
dblB, as does
S. sobrinus. The
S. criceti dblB gene encodes a 1,717-amino-acid protein having two repetitive alanine-rich and proline-rich regions and an LPXTG motif, which is recognized by the sortase SrtA, near the C terminus. Reverse transcription-PCR analysis indicated no cotranscription of the
dblA and
dblB genes of
S. criceti. As we could not obtain a
dblB mutant of
S. criceti, the
dblB gene was characterized in
S. mutans strain GS-5, which has genetic mutations in both
gbpC and
spaP genes and shows an inability to agglutinate triggered by dextran. A dextran-induced agglutination assay showed that
S. mutans cells carrying
dblB agglutinated in the presence of dextran. A hydrophobicity assay showed that the cells containing
dblB were hydrophobic. A biofilm formation assay showed that the
dblB gene was associated with biofilm formation by cells cultivated in brain heart infusion broth supplemented with glucose and maltose, but not sucrose. Nucleotide sequence analysis of the
S. criceti strains studied revealed a frameshift mutation in the
srtA gene encoding sortase, but intact
dblA and
dblB genes were found in dextran-induced agglutination-negative strains, whereas intact
dblA,
dblB and
srtA genes were found in dextran-induced agglutination-positive strains. These results suggest the cell-surface localization of
dblA and
dblB gene products by SrtA and the responsibility of
dblB for dextran-induced agglutination, cell-surface hydrophobicity and biofilm formation in
S. criceti.
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