Matrix-Assisted Laser Desorption Ionization–Time of Flight and Comparative Genomic Analysis of M-18 Group A Streptococcus Strains Associated with an Acute Rheumatic Fever Outbreak in Northeast Italy in 2012 and 2013

Acute rheumatic fever (ARF) is a postsuppurative sequela caused byStreptococcus pyogenes infections affecting school-age children. We describe here the occurrence of an ARF outbreak that occurred in Bologna province, northeastern Italy, between November 2012 and May 2013. Molecular analysis revealed that ARF-related group AStreptococcus (GAS) strains belonged to the M-18 serotype, including subtypesemm18.29 andemm18.32 . All M-18 GAS strains shared the same antigenic profile, including SpeA, SpeB, SpeC, SpeL, SpeM, and SmeZ. Matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) analysis revealed that M-18 GAS strains grouped separately from other serotypes, suggesting a differentS. pyogenes lineage. Single nucleotide polymorphisms and phylogenetic analysis based on whole-genome sequencing showed thatemm18.29 andemm18.32 GAS strains clustered in two distinct groups, highlighting genetic variations between these subtypes. Comparative analysis revealed a similar genome architecture betweenemm18.29 andemm18.32 strains that differed from noninvasiveemm18.0 strains. The major sources of differences between M-18 genomes were attributable to the prophage elements. Prophage regions contained several virulence factors that could have contributed to the pathogenic potential ofemm18.29 andemm18.32 strains. Notably, phage ΦSPBO.1 carried erythrogenic toxin A gene (speA1 ) in six ARF-related M-18 GAS strains but not inemm18.0 strains. In addition, a phage-encoded hyaluronidase gene (hylP.2 ) presented different variants among M-18 GAS strains by showing internal deletions located in the α-helical and TSβH regions. In conclusion, our study yielded insights into the genome structure of M-18 GAS strains responsible for the ARF outbreak in Italy, thus expanding our knowledge of this serotype.