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

Abstract

Background: Group B Streptococcus (GBS) is one of the leading causes of invasive bacterial diseases in infants, and the disease risk is highest during the first three months of age. The purpose of this study was to investigate the genotypic diversity of GBS isolated from infants with invasive diseases in South Korea, and to observe the prevalence of the highly virulent clone. Method: Isolates from infants with invasive GBS diseases were collected prospectively from infants with invasive GBS diseases admitted at 4 referral hospitals from 1995 to 2015, in South Korea. Capsular serotype was determined by sequencing analysis, and multilocus sequence typing (MLST) was carried out. All GBS isolates were tested for the presence of the gene encoding the hypervirulent surface adhesin (hvgA) by PCR amplification. Antibiotic susceptibility testing was done by gradient diffusion E-test, and ermB and mefA genes were detected using PCR amplification. Results: Among the 98 GBS isolates collected, 16.3% of the isolates were early-onset disease (EOD), 69.4% were late-onset disease (LOD), and 14.3% were late late-onset disease (LLOD). Fourteen STs were found

ST1 (20.4%), ST17 (19.4%) and ST19 (18.4%) were the most prevalent. eBURST analysis revealed 2 clonal complexes (CCs) and 6 singletons. Serotype III (n=50, 51.0%) was the most prevalent, followed by V (n=18, 18.4%), Ia (n=15, 15.3%), and Ib (n=13, 13.3%). Isolates of the same ST usually expressed one dominant serotype capsule. The dominant serotype capsule expressed by ST1 was serotype V, ST17 and ST19 was serotype III, and ST23 was serotype Ia. The hvgA gene was detected in 19.4% (n=19) of the isolates

all ST17 and serotype III (P<0.001). A significant temporal trend of the isolates in serotype III was observed

as ST17 increased (P=0.001) in proportion, ST19 decreased (P=0.009). ST1 was a significant pathogen in EOD (43.8%) compared to LOD (16.2%) and LLOD (14.3%) (P=0.039), and was less inclined to cause meningitis (P=0.005) compared with other STs. Erythromycin resistance was significantly associated with ST1 than other STs (P<0.001), with 85% (n=17/20) of the isolates resistant. In contrast, only 7.1% (n=1/14) of ST23 were resistant to erythromycin. Conclusion: Genotype analysis helps in understanding the specific traits of strains that cause invasive diseases in infants. In particular, ST1 is an important strain causing EOD with a high rate of macrolide resistance, and manifests as diseases other than meningitis. The hypervirulent ST17 is prevalent in South Korea, and a trend in the increase in its proportion is seen together with the decrease in ST19.