Roles of Streptococcus mutans dextranase anchored to the cell wall by sortase

Streptococcus mutans is a major contributor to dental caries. Its ability to adhere to surfaces and process dietary sugars is central to its pathogenicity. Prior research has shown that S. mutans produces dextranase (Dex), an enzyme that degrades dextran, a polysaccharide formed from sucrose. However, the mechanism by which Dex is anchored to the bacterial cell wall remained unclear. While it was already known that sortase enzymes mediate protein anchoring in Gram-positive bacteria, the specific role of sortase A (SrtA) in anchoring Dex had not been resolved. This gap motivated the construction of genetic mutants to investigate the function of SrtA and Dex in S. mutans. No prior work had resolved whether cell wall anchoring of Dex affects adhesion or nutrient utilization. This uncertainty drove the current study’s design.

The study aimed to determine how SrtA influences the localization of Dex in S. mutans and how that localization affects bacterial behavior. The researchers focused on the specific problem of whether cell wall-anchored Dex controls adhesion and glucan utilization. The motivation stemmed from the need to clarify the functional consequences of Dex localization. By constructing mutants lacking either SrtA or Dex, the authors sought to isolate the effects of each protein. The study also aimed to compare the biological properties of the srtA mutant with the wild type. The goal was to assess colony morphology, adhesion, and dextran fermentation. The authors proposed that SrtA anchors Dex to the cell wall, and that this anchoring affects extracellular glucan metabolism and adhesion. The study sought to confirm this hypothesis through genetic and phenotypic analyses.

The researchers constructed genetic mutants of S. mutans by insertional inactivation of the srtA and dex genes. They used Western blot analysis with a Dex antiserum to determine whether Dex was cell wall-associated or secreted. Colony morphology was assessed on Todd Hewitt agar supplemented with sucrose. Adherence to a smooth surface was tested under conditions with and without sucrose. Dextran fermentation was evaluated to determine whether the srtA mutant retained the ability to process dextran. The adhesion of the srtA mutant was also tested in the presence of exogenous dextranase. The study combined genetic manipulation with phenotypic assays to assess the role of SrtA and Dex. The approach allowed the authors to distinguish between cell wall-anchored and extracellular forms of Dex. The use of antiserum and agar-based assays provided direct evidence of protein localization and colony behavior.

Western blot analysis revealed that in the srtA mutant, Dex was secreted into the culture supernatant rather than remaining cell wall-associated. In contrast, the wild type retained cell wall-bound Dex. The srtA mutant retained the ability to ferment dextran, indicating that extracellular Dex could still function in glucan degradation. However, the colony morphology of the srtA mutant on sucrose-containing agar was significantly larger and ring-like compared to the wild type. The srtA mutant exhibited increased adhesion to a smooth surface when sucrose was present. This adhesion was reduced when exogenous dextranase was added. These findings suggest that SrtA is necessary for anchoring Dex to the cell wall. The results indicate that cell wall-anchored Dex influences adhesion and glucan utilization, while extracellular Dex functions primarily in glucan degradation.

The authors concluded that SrtA mediates the anchoring of Dex to the cell wall in S. mutans. They proposed that cell wall-anchored Dex controls both adhesion to surfaces and the utilization of extracellular glucan as a nutrient source. In contrast, extracellular Dex is only responsible for degrading extracellular glucan. The study found that the srtA mutant retained dextran fermentation but exhibited altered colony morphology and increased adhesion. The addition of exogenous dextranase reduced adhesion in the srtA mutant, supporting the role of extracellular Dex in glucan degradation. The authors emphasized that the localization of Dex—whether cell wall-bound or extracellular—determines its functional role. They suggested that cell wall anchoring of Dex is essential for regulating adhesion and nutrient utilization. These findings align with the hypothesis that SrtA is necessary for Dex anchoring and that this anchoring affects bacterial behavior.