Effect of subculturing on expression of a cell-surface protein antigen by Streptococcus mutans

Two freshly isolated strains, Xc and Yc, of Streptococcus mutans serotype c from human dental plaque were subcultured 100 times in Brain Heart Infusion broth. The cell-surface hydrophobicity of strain Xc markedly decreased after subculturing 60 times, but that of strain Yc remained unaltered. Radioimmunoassay showed a close correlation between surface hydrophobicity and the amount of a cell-surface protein antigen (PAc) of Mr 190,000. One hydrophilic variant (strain Xc100L), one relatively hydrophobic variant (strain Xc100H), and two hydrophobic variants (strains Yc100H1 and Yc100H2) were isolated from the 100-fold subcultures of hydrophobic strains Xc and Yc, respectively. SDS-PAGE showed that the amount of cell-associated and cell-free PAc of strain Xc100L was smaller than that of strains Xc and Xc100H. Strain Yc100H2 produced larger amounts of cell-associated PAc than strains Yc and Yc100H1. Resting cells of hydrophilic strain Xc100L attached in smaller numbers to saliva-coated hydroxyapatite than did other hydrophobic strains. RNA dot-blot analysis demonstrated a significant decrease in PAc-specific mRNA in strain Xc100L, as compared with strains Xc and Xc100H. Neither rearrangement nor deletion in the structural gene (pac) for PAc of these strains was observed by Southern blot analysis. These findings suggest that a mechanism which regulates the transcription of the pac gene participates in the quantitative variation of PAc after repeated subculturing.