Abstract
The number of invasive Streptococcus agalactiae (GBS) non-typeable (NT) isolates in Denmark received since 1999 has in general accounted for 10% of all invasive GBS isolates. We 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 phenotypic-based NT obtained from 2015 to 2017, as well as 10 reference isolates. Whole genome sequencing (WGS) was performed on all isolates and the data were analyzed 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%) isolates, of which the initial phenotyping problem for 13 isolates was found to be due to a problem with serotype Ia specific antiserum. Thirty-five isolates remained phenotypic non-typeable with a majority of genotype V isolates which do not express a capsular gene. From all the Danish invasive GBS isolates from 2015 to 2017, the 35 NT isolates were all detected in the age group above 21 years with bacteremia. The 35 NT isolates belonged to six different well-known human pathogenic clonal complexes. The CDC recommended sequences for capsule genotyping were the most optimal for serotype prediction, because of the sequence simplicity and clear cutoff values. However we recommend to also use other capsular sequences for the NT isolates, if they cannot be genotyped by the CDC method.
Highlights
Streptococcus agalactiae is a well-known pathogen primarily causing infections in newborns and the elderly (Brigtsen et al, 2015; Ballard et al, 2016)
There were no NT isolates detected from early-onset disease (EOD) and late-onset disease (LOD) or from age groups below 22 years of age (Table 1)
Serotype distribution of invasive GBS isolates often include an “eleventh” category designated as GBS non-typeable isolates (Slotved and Hoffmann, 2017)
Summary
Streptococcus agalactiae (group B streptococcus, GBS) is a well-known pathogen primarily causing infections in newborns and the elderly (Brigtsen et al, 2015; Ballard et al, 2016). Molecular techniques for genotyping of GBS isolates are increasingly being used, based on PCR assays and whole genome sequencing (WGS) (Streptococcus Laboratory, CDC, https://www.cdc.gov/streplab/ groupb-strep/index.html accessed 03-08-2021) (Brigtsen et al, 2015; Sheppard et al, 2016). It is well-known that a certain percentage of invasive GBS isolates are non-typeable (NT) by phenotypic methods, either because they do not express their capsule or the capsule is hitherto uncharacterized capsule (Rosini et al, 2015; Alhhazmi and Tyrrell, 2018). Since 1999, NT isolates in Denmark have generally accounted for 10% of all invasive GBS isolates received (Lambertsen et al, 2014; Lambertsen et al, 2010), occasionally the percentage of NT has reached 17% of all invasive GBS cases (Slotved and Hoffmann, 2020)
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