Spread of a highly mucoid Streptococcus pyogenes emm3/ST15 clone

Background: Hyaluronic acid capsule plays a key role in Streptococcus pyogenes virulence. Circulation of mucoid or highly encapsulated strains has been related to rheumatic fever epidemics and invasive disease in several countries. In 2009, an outbreak of mucoid S. pyogenes isolates was detected in northern Spain. The aim of the study was to describe clinical and molecular characteristics of mucoid strains causing this outbreak and to compare them with a sample of non-mucoid S. pyogenes isolates obtained during the same period of time. Methods: All S. pyogenes isolates with a mucoid colony morphology (n = 132), 10% of non-mucoid (n = 144) and all invasive S. pyogenes isolates (n = 7) obtained in 2009 were included. Characterization was performed by T-agglutination, emm typing, pulsed field gel electrophoresis and multilocus sequence typing. Results: One clone characterized as emm3.1/ST15 comprised 98.5% (n = 130) of all mucoid isolates. Subjects of all ages were affected. Main clinical manifestations were pharyngitis and scarlet fever, but this clone also caused invasive disease: two cases of streptococcal toxic shock syndrome, one arthritis, and one celullitis with a fatal outcome. Mucoid isolates were more prone to cause invasive disease than non-mucoid isolates (p = 0.001). Conclusions: Although no acute rheumatic fever cases were detected, the most worrisome characteristics of this clone were the success for causing invasive disease and the merge of two virulent features: the serotype, emm3, and capsule hyper-production, expressed as a mucoid morphology.