emm typing of invasive T28 group A streptococci, 1995–2004, Finland

Abstract

A total of 985 group A streptococcus (GAS) bacteraemia isolates collected in Finland during 1995–2004 were T-serotyped, and of these, 336 isolates of serotype T28 were subjected to further emm typing. The total number of isolates referred per year showed an increase within the study period, from 43 in 1995 to 130 in 2004. The annual incidence of invasive GAS (iGAS) bacteraemia showed a general increase during the study period, from 1.1 to 2.5 per 100 000 population. Serotype T28 remained among the most common serotypes, in addition to serotypes TB3264 and T1. The serotype T28 isolates were found to be distributed across six distinct emm types: emm28, emm77, emm53 (including subtypes 53.2 and 53.4), emm87, emm2 and emm4. The serotype distribution and the emm type distribution of serotype T28 fluctuated over time. Within the study period, the proportion of T28/emm28 isolates became the most prominent. During periods of low emm28 incidence, emm types 77 and 53 seemed to show a resurgence. emm typing revealed T28 isolates to be a genetically heterogeneous group harbouring a variety of distinct M proteins. This study confirms that T serotyping alone is not a sufficient method for epidemiological surveillance of iGAS.