Hyaluronic acid capsule is a virulence factor for mucoid group A streptococci.

TLDR

Mucoid group A Streptococcus strains, which produce abundant hyaluronic acid capsules, have been linked to recent outbreaks of acute rheumatic fever. The study aimed to determine the role of the hyaluronic acid capsule in virulence. An acapsular mutant was generated from a mucoid strain by transposon mutagenesis to assess capsule function. The capsule was essential for growth in fresh human blood, resistance to phagocytic killing, and virulence in mice, as the acapsular mutant failed to grow, was phagocytosis‑sensitive, and exhibited a 100‑fold reduction in virulence, while M protein expression remained unchanged.

Abstract

Mucoid strains of group A Streptococcus have been associated with recent outbreaks of acute rheumatic fever. The mucoid colony morphology of these strains is a result of abundant production of capsular polysaccharide, which is composed of hyaluronic acid. To study the role of the hyaluronic acid capsule in virulence, we derived an acapsular mutant from a mucoid strain of group A Streptococcus by transposon mutagenesis. M protein expression was not altered in the mutant strain. The mucoid wild-type strain grew in fresh human blood and was resistant to phagocytic killing in vitro. In contrast, the acapsular mutant failed to grow in fresh human blood and was sensitive to phagocytic killing in vitro. Loss of capsule was associated with a 100-fold reduction in virulence of the organisms in mice. We conclude that the hyaluronic acid capsule protects mucoid group A streptococci from phagocytosis and has an important role in virulence.