Capsular Switching and ICE Transformation Occurred in Human Streptococcus agalactiae ST19 With High Pathogenicity to Fish

Group B Streptococcus cpsE Is Required for Serotype V Capsule Production and Aids in Biofilm Formation and Ascending Infection of the Reproductive Tract during Pregnancy

Group B Streptococcus (GBS) is an encapsulated Gram-positive pathogen that causes ascending infections of the reproductive tract during pregnancy. The capsule of this organism is a critical virulence factor that has been implicated in a variety of cellular processes to promote pathogenesis. Primarily comprised of carbohydrates, the GBS capsule and its synthesis is driven by the capsule polysaccharide synthesis (cps) operon. The cpsE gene within this operon encodes a putative glycosyltransferase that is responsible for the transfer of a Glc-1-P from UDP-Glc to an undecaprenyl lipid molecule. We hypothesized that the cpsE gene product is important for GBS virulence and ascending infection during pregnancy. Our work demonstrates that a GBS cpsE mutant secretes fewer carbohydrates, has a reduced capsule, and forms less biofilm than the wild-type parental strain. We show that, compared to the parental strain, the ΔcpsE deletion mutant is more readily taken up by human placental macrophages and has a significantly attenuated ability to invade and proliferate in the mouse reproductive tract. Taken together, these results demonstrate that the cpsE gene product is an important virulence factor that aids in GBS colonization and invasion of the gravid reproductive tract.

Genomic characteristics of Streptococcus agalactiae based on the pan-genome orthologous group analysis according to invasiveness and capsular genotype

OBJECTIVE

Following the construction of a bacterial pan-genome from the whole genome sequences on a web-based pipeline, all coding DNA sequences (CDSs) can be clustered into pan-genome orthologous groups (POGs), which is a similar approach to comparative genome hybridization on glass microscope slides. We aimed to clarify the genomic characteristics of Streptococcus agalactiae based on the POG analysis.

METHODS

Sixty-six S. agalactiae isolates obtained from invasive specimens (blood and cerebrospinal fluid) and non-invasive specimens (urine and vaginal discharge) between 2010 and 2017 in Korea were subjected to whole genome sequencing (WGS). Based on the WGS data, we conducted the POG analysis and constructed a phylogenetic tree along with capsular polysaccharide (CPS) genotyping. We compared the genomics of invasive vs. non-invasive isolates, as well as CPS III vs. non-CPS III genotypes.

RESULTS

Predicted pan- and core-genome sizes were 3416 and 1658 genes, respectively. We found four clusters consisting of CPS genotypes (III, VIII, Ib/VI, and Ia) in the phylogenetic tree. There were significant differences in two metabolic pathways specific to invasiveness, and in six metabolic pathways specific to CPS III type produced by CDSs.

CONCLUSION

Our observations reveal the pan- and core-genome sizes, four clusters of genomes distributed by CPS genotypes, and unique CDS features of S. agalactiae by comparative genomics in terms of invasiveness and CPS genotype.

Genomic Insights Into the Distribution and Evolution of Group B Streptococcus

Streptococcus agalactiae, also known as Group B Streptococcus (GBS), is a bacteria with truly protean biology. It infects a variety of hosts, among which the most commonly studied are humans, cattle, and fish. GBS holds a singular position in the history of bacterial genomics, as it was the substrate used to describe one of the first major conceptual advances of comparative genomics, the idea of the pan-genome. In this review, I describe a brief history of GBS and the major contributions of genomics to understanding its genome plasticity and evolution as well as its molecular epidemiology, focusing on the three hosts mentioned above. I also discuss one of the major recent paradigm shifts in our understanding of GBS evolution and disease burden: foodborne GBS can cause invasive infections in humans.