Population biology of 225 serogroup 6 Streptococcus pneumoniae isolates collected in China

Background: In this study, we defined the population biology of serogroup 6 Streptococcus pneumoniae collected in China and their antibiotic resistance profiles. Methods: The serotypes of 225 S. pneumoniae strains isolated between 1997 and 2011 were identified with the Quellung reaction and serotype-specific PCR. All isolated pneumococci were tested for their sensitivity to 11 kinds of antibiotics with the E-test method or disc diffusion. The sequence types (STs) were analyzed with multilocus sequence typing (MLST). Results: The frequencies of serotypes and subtypes 6A, 6B-I, 6B-II, 6C, and 6D among the 225 isolates were 46.7% (105/225), 19.6% (44/225), 25.8% (58/225), 6.2% (14/225), and 1.8% (4/225), respectively. Serotype 6E was not found in the serotype 6A isolates, and neither serotype 6F nor 6G was identified in any isolate. MLST analysis revealed 58 STs. The most common STs were ST982 (23.1%), ST90 (14.7%), ST4542 (7.6%), and ST2912 (4.9%). The rates of clonal complex 90 (CC90) and CC386 among the oral-penicillin-nonsusceptible isolates decreased over the years, whereas the rates of CC855 and CC3173 increased. The four CCs had similar penicillin MIC distributions, with a maximum MIC of 2 mu g/ml. Conclusions: This study identified the serotypes/subtypes and CCs/STs of group 6 S. pneumoniae present in China. No salient antibiotic-resistant clones were isolated among the serogroup 6 S. pneumoniae.