Identifying large-scale recombination and capsular switching events in Streptococcus agalactiae strains causing disease in adults in the United Kingdom between 2014 and 2015. Capsule switch in GBS

Cases of invasive Group B Streptococcal infections in adult population in the UK has steadily increasing over last 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 cases) isolated from adults and submitted to the national reference laboratory at Public Health England between January 2014 and December 2015. We have reported determined that capsular serotypes III (26.8%), Ia (26.2%) and V (14.9%) were most commonly causing disease, 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.