Lancefield group A Streptococcus dysgalactiae subsp. equisimilis bacteremia with genomic characterization of the gacA-gacL locus

Streptococcus dysgalactiae subsp. equisimilis (SDSE) typically expresses Lancefield group C or G antigens, whereas group A isolates are very uncommon. We report a clinical case of bacteremia caused by a Lancefield group A SDSE isolate with its genomic characteristics. A 73-year-old man was hospitalized for acute decompensated heart failure. His blood cultures yielded Lancefield group A β-hemolytic streptococcus, initially raising clinical suspicion of Streptococcus pyogenes bacteremia. However, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry identified the isolate as S. dysgalactiae, for which the patient was successfully treated with a 14-day course of antimicrobial therapy. Later, we performed a whole-genome sequencing that confirmed the isolate as SDSE, showing 99.21% average nucleotide identity to a deposited strain of SDSE SS1722. The isolate was assigned to sequence type 177 and emm subtype stG652.0. All 12 group A carbohydrate biosynthesis genes, gacA-gacL, were detected on a single scaffold in the same order as in S. pyogenes MGAS5005. The genes showed 77.1-99.9% nucleotide identity to the corresponding MGAS5005 sequences; notably, gacF-gacL showed identities of 99.4-99.9%, whereas gacA-gacE showed lower identities of 77.1-91.5%. Virulence-associated gene profiling detected emm, spg, ska, slo, sagA, scpA, and speG, whereas speA, speC, speL, speM, and ssa were not detected. This case demonstrates that Lancefield group A reactivity in β-hemolytic streptococci does not necessarily indicate S. pyogenes. When Lancefield grouping and species identification are discordant, genomic analysis can clarify both the taxonomic identity of the isolate and the genetic basis of its antigenic phenotype.