| Summary: | <p>Abstract</p> <p>Background</p> <p>Hyaluronic acid capsule plays a key role in <it>Streptococcus pyogenes </it>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 <it>S. pyogenes </it>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 <it>S. pyogenes </it>isolates obtained during the same period of time.</p> <p>Methods</p> <p>All <it>S. pyogenes </it>isolates with a mucoid colony morphology (n = 132), 10% of non-mucoid (n = 144) and all invasive <it>S. pyogenes </it>isolates (n = 7) obtained in 2009 were included. Characterization was performed by T-agglutination, <it>emm </it>typing, pulsed field gel electrophoresis and multilocus sequence typing.</p> <p>Results</p> <p>One clone characterized as <it>emm</it>3.1/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).</p> <p>Conclusions</p> <p>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, <it>emm</it>3, and capsule hyper-production, expressed as a mucoid morphology.</p>
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