Specificity of the glucan-binding lectin of Streptococcus cricetus

Abstract

The specificity of the glucan-binding lectin (GBL) of Streptococcus cricetus AHT was determined. Examination of the kinetics of aggregation of cell suspensions with glucans containing various percentages of a-1,6, a-1,4, a-1,3, and a-1,2 anomeric linkages revealed that only glucans with at least 80% a-1,6 linkages promoted strong aggregation. Moreover, only linear glucans with molecular weights greater than 5 x 105 were capable of causing rapid aggregation of the bacteria. The lectin was observed to be present on S. cricetus strains, on Streptococcus sobrinus, and on several Streptococcus mutans strains. Preincubation of suspensions of S. cricetus AHT with glucan T10 (molecular weight of 10,000) before the addition of high-molecular-weight glucan resulted in competitive inhibition in a concentration-dependent manner. Inhibition was achieved also with isomaltopentaose, isomaltohexaose, and isomaltooctaose, but at higher concentrations than glucan T10. In contrast, no inhibition was observed with maltoheptaose, providing additional evidence for the specificity of GBL. Treatment of suspensions of S. cricetus AHT with trypsin before and after aggregation with high-molecular-weight glucan revealed a substantial level of protection of GBL when in a bound state. Collectively, these results indicated that GBL has an absolute affinity for glucans rich in at-1,6 linkages and possesses an active site which recognizes internal sequences and accommodates isomaltosaccharides of at least nine residues. This unusual specificity may contribute to the colonization of S. cricetus, S. sobninus, and S. mutans in glucan-containing plaque in the oral cavity.

On the relative importance of specific and non-specific approaches to oral microbial adhesion

“…Taking a specific approach, Drake et al [16] studied the absence or presence of glucan-binding lectins on mutans streptococcal strains by measuring macroscopic agglutination of ceils by glucan T2000 with light scattering. Their data are summarized in Table 2, expressed as an agglutination rate constant.…”

“…Their data are summarized in Table 2, expressed as an agglutination rate constant. Drake et al [16] argued that agglutination by glucan T2000 indicates the presence of a glucan-binding lectin (GBL) on the cells.…”

“…[61] and Drake et al [16] are identical, it is interesting to compare their data in order to illustrate that specificity and non-specificity can indeed be different expressions for the same phenomenon.…”

On mechanisms of oral microbial adhesion

“…'4 myriad of different interactions by which bacteria may adhere has been described in the literature (van Oss 1991), which, depending on the discipline of the investigators involved, range from highly specific, lectin-like interactions (Drake et al 1988a;Kishimoto et al 1991) to purely nonspecific electrostatic and van der Waals forces (Rutter and Vincent 1980;Busscher e f al. 1992a;van Loosdrecht et al 1987).…”

“…On the other hand, mutans streptococci like S. crzcetus, S . sobrtnus and S. mutans, reportedly possessing GBLs (Drake et al 1988a), were less negatively charged and had isoelectric points between pH 2 and 3. Since S .…”