A survey of serogroup 6 Streptococcus pneumoniae isolates from children in China has identified no antibiotic-resistant clones, leading the authors to speculate that the isolates may be under antibiotic selective pressure.
The findings are important because serogroup 6 is one of the most common causes of invasive pneumococcal disease in unvaccinated populations, say Yonghong Yang (Beijing Children’s Hospital Capital Medical University, China) and co-authors in BMC Infectious Diseases.
Yang et al investigated the population biology of 255 strains of serogroup 6 S. pneumoniae isolated from paediatric nasopharyngeal specimens collected between 1997 and 2011.
Serotypes were identified using the Quellung reaction and polymerase chain reaction; type 6A was the most prevalent, accounting for 46.7% of isolates, followed by 6B-II (25.8%), 6B-I (19.6%), 6C (6.2%) and 6D (1.8%).
Multilocus sequence typing revealed 58 distinct strains, the most common of which were ST982 (23.1%), ST90 (14.7%) and ST4542 (7.6%). The researchers identified eight new sequence types and a novel gki locus.
They then performed an eBURST analysis to visualise relationships among the isolates and to assign strains to a clonal complex (CC). This resulted in eight CCs and 26 singletons; notably, two of the most common strains, ST982 and ST2912, were singletons, the researchers remark.
Meanwhile, six CCs contained 10 or more isolates, and these, together with ST982 and ST2912, accounted for approximately 80% of all the isolates tested.
Finally, the team tested all isolates for susceptibility to 11 antibiotics. All the common CCs and STs were susceptible to these drugs, with the exception of CC90 isolates.
The team speculates that the absence of salient resistant clones in serogroup 6 contrasts with serotypes 19F and 23F and indicates that the former are under antibiotic selective pressure. “This speculation corresponds to the decreasing frequency of serogroup 6 among all S. pneumoniae isolates over time,” they remark.
The authors conclude: “Further long-term surveys of serogroup 6 S. pneumoniae are required to monitor the clonal prevalence and antimicrobial resistance in this important human pathogen.”