Test of a Novel Streptococcus pneumoniae Serotype 6C Type Specific Polyclonal Antiserum (Factor Antiserum 6d) and Characterisation of Serotype 6C Isolates in Denmark

Background: In 2007, Park et al. identified a novel serotype among Streptococcus pneumoniae serogroup 6 which they named serotype 6C. The aim of this study was to evaluate with the Neufeld test a novel S. pneumoniae serotype 6C type specific polyclonal antiserum. In addition, serotype 6C isolates found in Denmark in 2007 and 2008 as well as eight old original serotype 6A isolates were characterised. Methods: In this study, 181 clinical Streptococcus pneumoniae isolates from Denmark 2007 and 2008 were examined; 96 isolates had previously been typed as serotype 6A and 85 as serotype 6B. In addition, eight older isolates from 1952 to 1987, earlier serotyped as 6A, were examined. Serotype 6C isolates were identified by PCR and serotyping with the Neufeld test using the novel type specific polyclonal antiserum, factor antiserum 6 d, in addition to factor antisera 6b, 6b* (absorbed free for cross-reactions to serotype 6C) and 6c. All antisera are commercially available and antiserum 6b obtained from the supplier after 1 January 2009 is antiserum 6b*. All serotype 6C isolates were further characterised using multi-locus sequence typing. Results: When retesting all 96 original serotype 6A isolates by PCR and the Neufeld test, 29.6% (24 of 81) of the invasive isolates in Denmark from 2007 and 2008 were recognised as serotype 6C. In addition, three of eight old isolates originally serotyped as 6A were identified to be serotype 6C. The oldest serotype 6C isolate was from 1962. The serotype 6C isolates belonged to eleven different sequence types (ST) and nine clonal complexes (CC), ST1692 (CC395), ST386 (CC386) and ST481 (CC460) were the predominant types. Conclusions: We tested a novel polyclonal antiserum 6 d, as well as modified antiserum 6b*, provided a scheme for the serotyping of S. pneumoniae serogroup 6 using the Neufeld test and compared the serotyping method with PCR based methods. The two types of methods provided the same results. In future, it will, therefore, be possible to test also serotype 6C in accordance to the standard method for serotyping of S. pneumoniae recommended by WHO. Among all invasive isolates from Denmark 2007 and 2008, serotype 6C constituted 29.6% of the original serotype 6A isolates. The serotype 6C isolates were found to be diverse belonging to a number of different STs and CCs of which most have been observed in other countries previously. Serotype 6C is regarded as an old serotype being present among S. pneumoniae isolates in Denmark for at least 48 years. The genetic diversity of serotype 6C isolates and their genetic relationship to other serotypes suggested that serotype 6C strains may have arisen from several different independent recombination events involving different parental strains such as serotypes 6A, 6B, 23F and 4.