Abstract
Background Streptococcus dysgalactiae subspecies equisimilis (SDSE) is an emerging global pathogen that can colonize and infect humans. Although most SDSE isolates possess the Lancefield group G carbohydrate, a significant minority have the group C carbohydrate. Isolates are further sub-typed on the basis of differences within the emm gene. To gain a better understanding of their molecular epidemiology and evolutionary relationships, multilocus sequence typing (MLST) analysis was performed on SDSE isolates collected from Australia, Europe and North America.Methodology/Principal FindingsThe 178 SDSE isolates, representing 37 emm types, segregate into 80 distinct sequence types (STs) that form 17 clonal complexes (CCs). Eight STs recovered from all three continents account for >50% of the isolates. Thus, a small number of STs are highly prevalent and have a wide geographic distribution. Both ST and CC strongly correlate with group carbohydrate. In contrast, eleven STs were associated with >1 emm type, suggestive of recombinational replacements involving the emm gene; furthermore, 35% of the emm types are associated with genetically distant STs. Data also reveal a history of extensive inter- and intra-species recombination involving the housekeeping genes used for MLST. Sequence analysis of single locus variants identified through goeBURST indicates that genetic change mediated by recombination occurred ∼4.4 times more frequently than by point mutation.Conclusions/SignificanceA few genetic lineages with an intercontinental distribution dominate among SDSE causing infections in humans. The distinction between group C and G isolates reflects recent evolution, and no long-term genetic isolation between them was found. Lateral gene transfer and recombination involving housekeeping genes and the emm gene are important mechanisms driving genetic variability in the SDSE population.
Highlights
Most streptococci displaying b-hemolysis fall within the pyogenic branch of the 16S rRNA-based taxonomy, and are pathogens or commensals of mammalian hosts [1,2]
Molecular typing multilocus sequence typing (MLST) was used to characterize 117 Streptococcus dysgalactiae subspecies equisimilis (SDSE) isolates collected from three continents (Table S1)
Most of the isolates selected for MLST were derived from large independent collections and were chosen, in part, based on prior knowledge of their emm type and geographic site of isolation, with the goal being the compilation of a genetically diverse data set
Summary
Most streptococci displaying b-hemolysis fall within the pyogenic branch of the 16S rRNA-based taxonomy, and are pathogens or commensals of mammalian hosts [1,2]. Two species within the pyogenic branch - Streptococcus dysgalactiae subspecies equisimilis (SDSE) and Streptococcus pyogenes (group A streptococcus, GAS) - colonize and/or infect the respiratory tract and skin of the human host [2]. Among the GAS and SDSE populations, differences in the sequences of individual emm genes are widely used for intra-species strain typing. MLST has been used extensively to study the b-hemolytic GAS [20,21,22] and S. agalactiae [23,24] populations, and was recently used to investigate genetic relationships amongst 61 geographically restricted SDSE isolates [25]. An intercontinental collection of SDSE isolates is characterized by MLST and emm typing, and the geographic distribution of the identified clones and their genetic relationships are defined
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