Invasive hypermucoid variant of group A Streptococcus is defective in growth and susceptible to DNA-damaging treatments

Hyaluronic acid capsule is one of the most important virulence factors of group A Streptococcus (GAS). Over-production of capsule has been thought to enhance GAS virulence during infections. However, although the increased of capsule expression associates with increased bacterial virulence and invasive ability, over-production of capsule has not often been observed among clinical isolates. In the present study, we identified two mucoid emm12 type isolates that can convert to the hypermucoid morphology under both in vitro and in vivo conditions. Consistent with previous studies, hypermucoid variants are more invasive in the mouse air-pouch infection model. However, one of the hypermucoid variants showed a growth-defective phenotype in regular broth culture conditions and is significantly more susceptible to various DNA-damaging treatments when compared with the mucoid variant. These properties of the hypermucoid variant may be adverse factors inhibiting its adaptation to the host environment during infections.