Molecular characterisation of isolates of invasive non-typeable Group B Streptococcus in Denmark from 2015 to 2017

The number of invasive Streptococcus agalactiae (GBS) non-typeable (NT) isolates in Denmark received since 1999 has in generally accounted for 10% of all invasive GBS isolates, despite we have no GBS vaccine. As only limited data exists, we here present data on 55 clinical NT isolates based on clinical manifestations, clonal relationship, antimicrobial resistance (AMR) determinants and virulence factors. The GBS isolates included in this study were phenotypically-based NT obtained from 2015 – 2017, as well as ten reference isolates. Whole genome sequencing (WGS) was performed on all isolates and the data were analysed for the presence of both species specific genes, capsular genes (genotype), and other relevant genes. We furthermore compared different procedures for detection of serotype specific capsular genes. Overall we were able to genotype 54 of the 55 isolates. After retesting the isolates a phenotype was detected for 20 (36 percent) isolates. 35 isolates remained phenotypically non-typeable with a majority of genotype V isolates which do not express a capsular gene. The 35 NT isolates was only detected in the age group above 21 years with bacteremia. The 35 NT isolates belonged to 6 different well-known human pathogenic clonal complexes. The CDC recommended sequences for genotyping seemed as the optimal method for genotyping, because of the sequence simplicity and clear cutoff values. However we recommend to also use other capsular sequences for the NT isolates, which cannot be genotyped by the CDC method.