Human Saliva as a Nitrogen Source for Oral Streptococci

Abstract

The ability of human saliva supernatant to support the <i>in vitro</i> growth of <i>Streptococcus mutans</i> and <i>Streptococcus sanguis</i> was examined. Saliva supernatant could not serve as a sole source of energy, vitamins, minerals or accessory growth factors, but could serve as a source of organic nitrogen needed by the organisms for growth when tested in defined medium containing all nutrients except a nitrogen source. The greatest growth-promoting capacity of the saliva was found to be associated with the protein subfraction (molecular weight > 10,000). Growth of the organisms on the protein subfraction was not affected by the intrinsic proteolytic activity of saliva indicating that the proteins and not digestion products were utilized. Analysis of saliva supernatant and its subfractions for cysteine and half-cystine revealed that saliva contains very low levels of freely available cysteine, but higher levels of half-cystine, particularly in the protein subfraction.