Virulence, phenotype and genotype characteristics of invasive group B Streptococcus isolates obtained from Swedish pregnant women and neonates

Ann Clin Microbiol Antimicrob. 2022 Oct 13;21(1):43. doi: 10.1186/s12941-022-00534-2.


Group B streptococci (GBS) are bacteria that can cause preterm birth and invasive neonatal disease. Heterogeneous expression of virulence factors enables GBS to exist as both commensal bacteria and to become highly invasive. A molecular epidemiological study comparing GBS bacterial traits, genotype and host characteristics may indicate whether it is possible to predict the risk of perinatal invasive GBS disease and more accurately target intrapartum antibiotic prophylaxis. A total of 229 invasive GBS isolates from Swedish pregnant women or neonates were assessed for virulence and phenotypic traits: hemolysis zone, hemolytic pigment (Granada agar), Streptococcus B Carrot Broth (SBCB) assay, CAMP factor, and hyaluronidase activity. Genes regulating hemolytic pigment synthesis (covR/covS, abx1, stk1, stp1) were sequenced. Of the virulence factors and phenotypes assessed, a Granada pigment or SBCB score ≥ 2 captured more than 90% of EOD isolates with excellent inter-rater reliability. High enzyme activity of hyaluronidase was observed in 16% (36/229) of the invasive GBS isolates and notably, in one case of stillbirth. Hyaluronidase activity was also significantly higher in GBS isolates obtained from pregnant/postpartum individuals versus the stillbirth or neonatal invasive isolates (p < 0.001). Sequencing analysis found that abx1 (g.T106I), stk1 (g.T211N), stp1 (g.K469R) and covS (g.V343M) variants were present significantly more often in the higher (Granada pigment score ≥ 2) versus lower pigmented isolates (p < 0.001, each variant). Among the 203 higher Granada pigment scoring isolates, 22 (10.8%) isolates had 3 of the four sequence variants and 10 (4.9%) had 2 of the four sequence variants. Although heterogeneity in GBS virulence factor expression was observed, the vast majority were more highly pigmented and contained several common sequence variants in genes regulating pigment synthesis. High activity of hyaluronidase may increase risk for stillbirth and invasive disease in pregnant or postpartum individuals. Our findings suggest that testing for GBS pigmentation and hyaluronidase may, albeit imperfectly, identify pregnant people at risk for invasive disease and represent a step towards a personalized medical approach for the administration of intrapartum antibiotic prophylaxis.

Keywords: Early-onset disease; Group B Streptococcus; Late-onset disease; Neonate; Pregnancy; Preterm birth; Preterm labor; Streptococcus agalactiae.

MeSH terms

  • Agar / metabolism
  • Agar / therapeutic use
  • Anti-Bacterial Agents / therapeutic use
  • Female
  • Genotype
  • Humans
  • Hyaluronoglucosaminidase / genetics
  • Hyaluronoglucosaminidase / metabolism
  • Hyaluronoglucosaminidase / therapeutic use
  • Infant, Newborn
  • Phenotype
  • Pregnancy
  • Pregnant Women
  • Premature Birth* / drug therapy
  • Reproducibility of Results
  • Stillbirth
  • Streptococcal Infections* / microbiology
  • Streptococcus agalactiae
  • Sweden / epidemiology
  • Virulence / genetics
  • Virulence Factors / genetics
  • Virulence Factors / metabolism


  • Anti-Bacterial Agents
  • Virulence Factors
  • Agar
  • Hyaluronoglucosaminidase