Bacteriological and molecular characterization of temperature- and CO ₂ -dependent Streptococcus pneumoniae serotype 24F ST162 isolated from Japanese children

Abstract

Streptococcus pneumoniae serotype 24F is one of the most prevalent non-vaccine serotypes that causes invasive pneumococcal disease (IPD) in many countries, including Japan. This study aimed to analyze the bacteriological and molecular characteristics of serotype 24F sequence type (ST) 162, which has been increasingly isolated from pediatric patients with IPD in Japan recently. The examination of growth conditions, sequencing of genes associated with the CO 2 dependence, and antimicrobial susceptibility testing at 35°C under 5% CO 2 were performed for 10 isolates obtained from Japanese children with IPD caused by serotype 24F ST162. All isolates failed to grow at 35°C in ambient air; however, they showed growth at 30°C in ambient air and at 35°C under 5% CO 2 . Sequencing of murF involved in cell wall synthesis indicated that all isolates had a single amino acid substitution, MurF A179V . Additionally, all isolates were sensitive to penicillin G and resistant to trimethoprim-sulfamethoxazole. Furthermore, 25 non-capnophilic strains were obtained from all CO 2 -dependent isolates, and their murF sequences were compared. Thirteen of the 25 non-capnophilic strains displayed a different amino acid substitution, MurF V179A , whereas the other 12 presented the previously described MurF A179V . This suggests that a proportion of the CO 2 -dependent phenotype of serotype 24F ST162 may have been conferred by MurF A179V ; however, the mechanism for the CO 2 dependence of these isolates warrants further investigation. The CO 2 -dependent serotype 24F ST162 pneumococcal isolates shared common characteristics in terms of growth patterns, molecular basis, and antimicrobial susceptibility; therefore, future epidemiological trends of this clone must be closely monitored. IMPORTANCE We characterized Streptococcus pneumoniae serotype 24F sequence type (ST) 162 isolated from Japanese children with invasive pneumococcal disease (IPD). Owing to its highly invasive nature, serotype 24F is expected to be isolated from clinically significant cases. Serotype 24F ST162 isolates tested in the present study did not grow at 35°C in ambient air. Therefore, antimicrobial susceptibility testing using the broth microdilution method, which is usually conducted in ambient air, cannot be performed, posing a clinical challenge. Clinical practitioners and laboratory personnel should be aware of the epidemiological, bacteriological, and molecular characteristics of serotype 24F ST162. We believe that our findings can help diagnose and treat IPD caused by serotype 24F ST162, a serotype expected to become problematic in the post-13 valent pneumococcal conjugate vaccine era.