Detachment of Streptococcus mutans biofilm cells by an endogenous enzymatic activity

Previous studies have shown that Streptococcus mutans NG8 possesses an endogenous surface protein-releasing enzyme (SPRE) activity that liberates its own surface proteins (S. F. Lee, Infect. Immun. 60:40324039, 1992). The present study was initiated to investigate the possible role of the release of surface proteins by SPRE in the detachment of biofilm cells in vitro. Initially, the characteristics of surface protein release by the strain (S. mutans BM71) used in this study were shown to be the same as those previously described for S. mutans NG8. BM71 displayed characteristics identical to those of NG8 in terms of pH optima and inhibitor sensitivity for protein release. Monolayer biofilms of S. mutans BM71 were formed on hydroxylapatite rods in a modified chemostat. Detachment of the biofilm cells was measured by viable cell counts of bacteria liberated after incubation of the biofilms in buffers. Results showed that biofilm cells were detached in a pH-dependent manner with a maximum rate at pH 5 (P = 0.016) to 6 (P = 0.002), a range similar to that for optimal surface protein release. The detachment of the biofilm cells was found to be inhibited by ZnCl₂ (P = 0.002 to 0.023), which also inhibited surface protein release. Detachment was not inhibited significantly by CaCl₂ (P = 0.525 to 0.784), precluding an ionic effect on inhibition by ZnCl₂. The extent of detachment could be increased (P = 0.046) by the addition of an SPRE preparation from S. mutans but not heat- inactivated SPRE (P = 0.665) or SPRE in the presence of ZnCl₂ (P = 0.199). Detachment was also studied by using biofilms of resting (viable but not dividing) cells. Results similar to those for biofilms formed from growing cells were obtained, indicating that cells detached from biofilms were not daughter cells. The results presented above show that monolayer biofilm cells of S. mutans under conditions of minimal shear force have the ability to detach from a surface and suggest that this detachment was mediated by an endogenous SPRE activity.