Immunolocalization of Streptococcus mutans sr gene product in recombinant Escherichia coli

A direct post-embedding immunogold method was used to determine the subcellular localization of the expression product of Streptococcus mutans sr gene in Escherichia coli cells. Immunolabelling was principally localized in the cytoplasm and was absent in the periplasmic space and extracellular compartment. We observed no difference in the localization of the labelling between bacteria originated from solid or liquid cultures. No difference in saliva binding could be observed between recombinant E. coli or control E. coli cells. These results showed that the cloned SR protein was not accessible at the surface of the host cell.