Serotype distribution and antimicrobial susceptibility of Streptococcus pneumoniae strains isolated in Japan after introduction of the routine immunization program

Pneumococcal vaccines have reduced the incidences of Streptococcus pneumoniae infections among children and adults, but a relative increase in the prevalence of non-vaccine serotypes has been reported. To follow the changing epidemiology of pneumococcal diseases, capsular serotyping and antimicrobial susceptibility testing was performed on 534 pneumococcal isolates obtained from a hospital in Japan after routine immunization was launched, between October 2014 and May 2016. Serotype distributions and antimicrobial susceptibilities were evaluated among the total patient population, and were compared by age and sample groups and by serotype group, respectively. Serotypes targeted by the 13-valent pneumococcal conjugate vaccine (PCV13) were identified in 14.6%, 44.5%, and 40.2% of the samples from the <5, 5-64, and ≥65 year age groups, respectively. The 23-valent pneumococcal polysaccharide vaccine serotypes (PPSV23) were identified in 42.4%, 68.2%, and 63.1% of the samples, respectively; whereas non-PCV13 serotypes or non-PPSV serotypes (NVT) comprised 46.8% of all isolates. Among NVT, strain 35B was the most frequently isolated, followed by 15A, particularly in sputum samples collected from children <5 years old. Meanwhile, serotype 3, which is targeted by the PCV13 and PPSV23, was the most prevalent among patients aged ≥65 and 5-64 years. Antimicrobial susceptibility testing revealed that 88.9% and 81.0% of serotype 35B was non-susceptible to penicillin and meropenem, respectively, and 89.4% of 15A was non-susceptible to penicillin. Our data suggest rapid effects of pneumococcal vaccines and progression of serotype replacement. Besides invasive potential, the increased prevalence of non-vaccine serotypes with highly non-susceptible to penicillin was a concern. Continuous monitoring of pneumococcal serotypes and antimicrobial susceptibility is necessary for developing optimal preventive strategies.