Genotype Characterization of Group B Streptococcus Isolated from Infants with Invasive Diseases in South Korea

Background: Group B streptococcus (GBS) is one of the leading causes of invasive infections in infants. This study aimed to investigate the genotypic diversity of GBS causing invasive infections in infants and to observe the prevalence of the highly virulent clone in South Korea. Methods: Invasive strains of GBS were collected prospectively from infants admitted at 4 hospitals during 1995-2015. Serotype and multilocus sequence typing were determined. All isolates underwent polymerase chain reaction amplification to detect the presence of the hypervirulent GBS adhesin (hvgA) gene. Antibiotic susceptibility testing was done by E-test, and erythromycin resistance genes were detected using polymerase chain reaction amplification. Results: Among 98 GBS isolates collected, 14 sequence types (STs) were found; ST1 (20.4%), ST17 (19.4%) and ST19 (18.4%) were the most prevalent. The dominant serotype capsule expressed by ST1 was serotype V, ST17 and ST19 were all serotype III and ST23 was serotype Ia. hvgA gene was detected in 19.4% (n = 19) of the isolates; all were ST17, serotype III. A significant temporal trend of serotype III isolates was observed; as ST17 increased (P = 0.001) in proportion, ST19 decreased (P = 0.009). Erythromycin resistance was found in 42.9% (42/98); dominant strains were ermB-positive ST1 serotype V (n = 18/20, 90%), ermB-positive ST17 serotype III (n = 10/19, 52.6%) and ermA-positive ST335 serotype III (n = 7/7, 100%). Conclusions: The predominant STs causing invasive infections in South Korea were ST1, ST19 and ST17. Among serotype III isolates, an increase in proportion of the hypervirulent ST17 strains was observed. Erythromycin resistance was significantly associated with ST1.