Studies on the polysaccharide elaborated by Streptococcus salivarius and associated enzymes

Abstract

Strains of Streptococcus salivarius (ATCC 13419, NCTC 8606 and 51) yielded, when cultured in a sucrose broth, extracel lular polysaccharides that proved to be D-fructans possessing -linkages. Sedimentation and gel-fiItration results suggested average molecular weights to lie in the range 106 to 25x 106. Methylation studies indicated these polymers to be branched molecules, degrees of branching being 9.2-12.3%, and in addition to possess either 2-6-linked fructo-furanose or 2-5-1 linked fructopyranose residues with branching in either case occurring at C-1. This ambiguity was resolved by showing the position of the linkage to reducing D-fructose groups in a disaccharide preparation obtained from a partial acid hydrolysate of the fructan of strain 51 to be C-6, thereby demonstrating that the fructan possesses 2-6-linked fructofuranose residues and complying therefore with the definition given for a Ievan. The extracellular levanase induced by culturing S. salivarius strain 51 in a levan containing medium was isolated and partially purified. It was shown to completely degrade levan in an exo-manner, D-fructose being liberated. A comparative study with the invertase of Candida utiIis demonstrated that the latter enzyme possesses a far wider affinity for B-fructofuranosido substrates such as levan, inulin, sucrose and methyl B-fructofuranoside. The ability of the levanase to degrade oligosaccharides obtained from inulin was investigated. A study of the lengths of branches in strain 51 levan was made by reducing-end linkage analysis in oligosaccharide preparations obtained from a partial acid hydrolysate of the levan. Chemical ionisation mass spectrometry was shown to facilitate the assignment of structure to the hexitol derivatives obtained. The exo levanase was used as a probe in an attempted investigation of the type-structure of the levan elaborated by S. salivarius strain 51, the evidence obtained suggesting the structure to be of the tree-1 ike type.