Molecular epidemiology of nonencapsulated Streptococcus pneumoniae among Japanese children with acute otitis media

The introduction of pneumococcal conjugate vaccine may change the epidemiology of Streptococcus pneumoniae. The increased prevalence of non-vaccine serotypes as the cause of pneumococcal diseases has already reported in the United States and Europe. However, little attention has been focused on the S. pneumoniae. In this study, nonencapsulated S. pneumoniae were identified in 15 isolates (6.4%) out of 236 pneumococcal strains obtained from the nasopharynges of children with acute otitis media (AOM), in 3 isolates (14.3%) out of 21 strains from acute rhinosinusitis, and in 2 isolates (12.5%) out of 16 nasopharyngeal carriage strains obtained from normal healthy children. Among the 20 nonencapsulated S. pneumoniae isolates, 15 (75.0%) isolates had the pspK gene. Seven sequence types (STs) were identified: ST7502 (5 strains), ST1106 (2 strains), ST7803 (2 strains), ST7786 (1 strain), ST6741 (1 strain), ST7496 (1 strain), and ST8642 (1 strain). Because nonencapsulated S. pneumoniae strains are not targeted by the current available pneumococcal vaccines, these strains will gradually become more common in nasopharyngeal carriage. The increase in colonization and dissemination of these strains would increase the risk of AOM and other systemic pneumococcal diseases against which current vaccines cannot provide protection. Nonencapsulated S. pneumoniae may thus become more prevalent as human pathogen.