Molecular epidemiology of disease-causing Streptococcus agalactiae in adults in England and Wales 2014-2015.

Streptococcus agalactiae (Group B Streptococcus, GBS) is an important cause of infections in adults with estimated blood infection rates of 3.9/100,000 per population for England and Wales combined. Capsular serotypes are the main target for vaccine development for GBS, however, vaccination may exert the selective pressure for virulent genotypes to switch capsules and escape vaccine coverage. A total of 194 GBS isolates causing invasive and non-invasive infections among adults in England and Wales between January 2014 and August 2015 were serotyped and whole genome sequenced. Genomic analysis determined MLST and presence of virulence factors such as surface protein, hvgA, and pilus island genes. GBS serotype III (26.8%), Ia (26.2%) and V (14.9%) were found to be most prevalent. A total of 15.9% (n=31) GBS strains were hvgA positive. Five major clonal groups CC1, CC8, CC17, CC19 and CC23 accounted for 94.8% of all isolates. An alpha or alpha-like protein gene was present in all 194 isolates, and specific associations between genes and serotypes, such as serotype Ib and II/bca, serotype III/rib, and serotype V/alp3 were observed. Multiple recombination events leading to potential capsular switch in two serotype Ib and II /ST1 isolates each from serotype V and one serotype IV/ CC17 lineage from serotype III were observed. Recombination events may result in vaccine serotype switching and or the acquisition of genes involved in pathogenicity. Therefore, continued surveillance of GBS genomics, further recombination events and determination of the underlying mechanisms remains of the utmost importance for development of GBS therapeutics.