Frequent Transmission of Streptococcus pneumoniae Serotype 35B and 35D, Clonal Complex 558 Lineage, across Continents and the Formation of Multiple Clades in Japan

Streptococcus pneumoniae is a common bacterial pathogen that causes infections in children worldwide, even after administration of the pneumococcal conjugate vaccine. S. pneumoniae serotype 35B, especially the clonal complex 558 (CC558) lineage, has emerged globally following implementation of the 13-valent pneumococcal conjugate vaccine. Serotype 35B strains are also associated with multidrug resistance to both β-lactams and non-β-lactam drugs. In addition, a novel serotype, 35D, which is closely related to 35B and differs in polysaccharide structure, was recently reported. However, the genetic relationship among globally disseminating serotype 35B and D (35B/D) strains remains unknown. To investigate the molecular epidemiology of global serotype 35B/D strains, we conducted a genomic analysis of serotype 35B/D strains from various continents, including those from the Japanese national surveillance collection. A total of 87 isolates were identified as serotype 35B/D in the Japanese surveillance collection (n = 1,358). All the isolates were assigned to either CC558 or CC2755. Serotype 35D isolates were interspersed with serotype 35B isolates. Phylogenetic analysis revealed the formation of multiple clusters by the Japanese serotype 35B/D-CC558 isolates among the foreign isolates, which suggested multiple events of introduction of the clone into Japan. The global 35B/D-CC558 strains were found to share specific penicillin-binding protein profiles, pbp1a-4, pbp2b-7, and pbp2x-7, associated with penicillin, cephalosporin, and carbapenem nonsusceptibility. Moreover, 88.5% of the Japanese 35B/D-CC558 and 35B/D-CC2755 isolates were found to harbor the Tn916-like integrative and conjugative elements Tn2009, Tn2010, and Tn6002, associated with multidrug resistance to macrolides and tetracyclines. The results of this study imply that serotype 35B/D-CC558 strains could be frequently transmitted intercontinentally.

Discovery of Novel Pneumococcal Serotype 35D, a Natural WciG-Deficient Variant of Serotype 35B

Pneumococcus (Streptococcus pneumoniae) remains a significant cause of morbidity and mortality, especially among those at the extremes of age. Its capsular polysaccharide is essential for systemic virulence. Over 90 serologically distinct pneumococcal capsular polysaccharides (serotypes) are recognized, but they are unequal in prevalence. Because antibodies against the capsule are protective, polysaccharide conjugate vaccines, which are constructed against the most prevalent serotypes, have caused great reductions in pneumococcal disease caused by these serotypes. In response, however, the relative prevalences of serotypes have shifted. Certain previously rare serotypes, such as serotype 35B, are increasing in prevalence. Serotype 35B is thus a likely future vaccine candidate, but due to their previous rarity, serotype 35B strains have not been scrutinized for underlying heterogeneity. We studied putative serotype 35B clinical isolates to assess the uniformity of their serological reactions. While most isolates exhibited the accepted serology of serotype 35B, one isolate failed to bind to critical serotyping reagents. We determined that the genetic basis for this aberrant serology was the presence of inactivating mutations in the O-acetyltransferase gene wciG Complementation studies in a wciG deletion strain verified that the mutant WciG was nonfunctional, and the serology of the mutant could be restored through complementation with a construct encoding a functional WciG. Nuclear magnetic resonance studies confirmed that the capsule of the WciG-deficient isolate lacked O-acetylation but was otherwise identical to serotype 35B. As this isolate expresses a unique serology with unique biochemistry and a stable genetic basis, we named its novel capsule serotype 35D.