Prevalence and molecular characterisation of Streptococcus pneumoniae serotype 6C in Queensland, Australia

Streptococcus pneumoniae serotype 6C was first identified in 2007, although retrospective studies have since identified serotype 6C among stored isolates dating back to 1962. We investigated the incidence and genetic diversity of serotype 6C strains isolated from Queensland patients between 2001 and 2011. Isolates were identified by Quellung reaction and antimicrobial susceptibility testing was performed. The incidence of serotype 6C among serogroup 6 Queensland invasive pneumococcal disease increased from 6.8% (2001-2004) to 39% (2005-2010) of serogroup 6 isolates (P = 0). Genetic diversity of Queensland 6C isolates was high, with molecular analysis identifying 19 sequence types by multi-locus sequence typing, and 35 types by multi-locus variable-number tandem repeat analysis.