Whole genome sequence (WGS) based strain characterization of invasive group B Streptococcus (iGBS)

Background: Group B Streptococcus (GBS) is a major neonatal pathogen and cause of invasive disease in non-pregnant adults. Invasive group B Streptococcus (iGBS) strain characterization data for non-pregnant adults are limited in middle- and low-income countries. Recently, invasive disease in humans associated with GBS serotype III sequence type 283 infection and raw fish consumption was reported from Southeast Asia.
Methods: iGBS blood isolates from population-based surveillance in Thailand were subjected to WGS and strain characterization. Serotypes, multilocus sequence types, antimicrobial resistance, and virulence determinants were determined using a validated bioinformatics pipeline. Single nucleotide polymorphism (SNP) analysis with a cutoff of ≤10 SNPs was used to define clusters.
Results: We identified 335 iGBS isolates, 12 from infants and 322 from adults, during 2007–2015. The predominant lineage was clonal complex (CC) 283(83.9%), followed by CC1(6.9%), CC12(3.0%), CC23(2.1%), CC17(1.2%), and CC19(0.9%). Serotype III accounted for 86.9% of isolates; 6.9% were V, 3.3% Ia, 1.2% II, 0.9% Ib, and 0.9% VI. Although the infant numbers were small, 58.3% were associated with serotype III/CC283. Based on SNP analysis of the serotype III/ST283 strains, most of the isolates clustered; the overall pairwise distance was 22 SNPs. Most isolates possessed Serine-rich repeat glycoprotein determinants srr1 or srr2 (99.1%). Alpha protein family genes were present in most of the isolates [99.1%; alpha(86.3%), alp2/3(5.4%), rib(4.8%), alp1(2.7%)]. At least 1 of the 3 distinct pilus backbone determinants were detected in all isolates; the combination of PI1_PI2A (94.3%) was most frequent. Presence of the hvgA virulence gene was restricted to serotype III/CC17 isolates.
Conclusions: The high prevalence of genomically related iGBS serotype III/CC283 strains in infants is suggestive of recent emergence of a fish-associated clade as a human pathogen in Thailand and warrants further genomic studies to understand the transmission of this lineage leading to invasive disease.