Emm gene polymorphism among temporally clustered group A streptococcal isolates in western Norway

Nineteen group A streptococcal isolates obtained in western Norway from patients with invasive disease during a period of high morbidity and mortality were examined for clonality and emm gene polymorphism. These isolates belonged to the prevalent serotypes during the outbreak, namely T1, T3 or T6. Restriction fragment length polymorphism and sequencing of the emm genes were used to compare these isolates with 14 isolates of the same serotype but from non-invasive infections. The restriction analysis did not identify specific invasive clones. The emm genes in three of the four T3 isolates from invasive disease had nucleotide substitutions inducing a charge difference in the N-terminal part of the M protein. The 4 T6 isolates had a longer emm amplicon when compared to 15 isolates from superficial infections and also showed nucleotide substitutions that could induce conformational changes in the hypervariable end of the M protein. Restriction analysis of the emm amplicon of the T6 isolates in order to estimate the number of A- and C-repeats is described. The emm gene sequence served as an epidemiological marker within the serotypes T3 and T6, but the significance of the emm polymorphism displayed by the isolates from invasive disease is uncertain at this stage.