Cases of invasive Group B Streptococcal infections in the adult UK population have steadily increased over recent years, with most common serotypes being V, III and Ia, but less is known of the genetic background of these strains. We have carried out in-depth analysis of whole genome sequences of 193 clinically important GBS isolates (186 were from invasive and 7 were from non-invasive infection) isolated from adults and submitted to the National Reference Laboratory at UK Health Security Agency between January 2014 and December 2015. We have determined that capsular serotypes III (26.8%), Ia (26.2%) and V (14.9%) were most commonly identified, with slight differences in gender and age distribution. Most isolates (n=185) grouped to 5 clonal complexes: CC1, CC8, CC17, CC19 and CC23 with common associations between specific serotypes and virulence genes. Additionally, we have identified large recombination events mediating potential capsular switching events between ST1 serotype V and serotypes Ib (n=2 isolates), II (n=2 isolates) and VI (n=2 isolates); ST19 serotype III and serotype V (n=5 isolates); CC17 serotype III and serotype IV (n=1 isolate).

The high genetic diversity of disease-causing isolates and multiple recombination events reported in this study, highlight the need for routine surveillance of the circulating disease-causing GBS strains. This information is crucial to better understand global spread of GBS serotypes and genotypes and will form the baseline information for any future GBS vaccine research in the UK and worldwide.

Impact statement
This study is the first study to report on in-depth genomic analysis of the disease-causing GBS in adult population in the UK. We describe the most common serotype-genotype combinations, including multi-locus sequence types (MLST) and major virulence gene combinations for the specific serotypes, found in our dataset. Importantly, we report on various potential capsular type switching caused by recombination events for certain GBS genotypes.

Data Summary
The raw sequencing reads from the 193 GBS isolates used in this study are available from the European Nucleotide Archive under the project accession number PRJEB18093 [24] and genome assemblies are available under project accession number PRJEB48944. Accession numbers for individual GBS isolates are provided in Table S1.