Changes in some biological properties of Streptococcus salivarius isolates from infected rats

Isolation of colonies with altered colonial morphology was frequently observed following infection of specific pathogen-free rats fed a caries-inducing high sucrose diet with two human strains of Streptococcus salivarius. These isolates produced unique rough colonies on mitis salivarius agar, in contrast to the smooth, mucoid colonial morphology of the parental strain. The rough variants were characterized by a marked increase in the synthesis of polysaccharide from sucrose, especially cell-associated water-insoluble glucan, whereas the smooth parental strains mainly produced cell-free fructan. Glucose-grown cells of rough variants but not smooth parental strains were aggregated upon addition of sucrose. Such variants were not detected during laboratory transfers of the human strains of S. salivarius nor by mutagenesis of the cultures.