Invasive Serotype 35B Pneumococci Including an Expanding Serotype Switch Lineage, United States, 2015-2016

Area of Science:

• Microbiology
• Genomics
• Epidemiology

Background:

• Invasive pneumococcal disease (IPD) caused by nonvaccine serotypes remains a public health concern.
• The prevalence of specific pneumococcal serotypes can shift following vaccine implementation.
• Serotype 35B has emerged as a cause of IPD, particularly in the post-vaccine era.

Purpose of the Study:

• To characterize the genetic makeup of nonvaccine serotype 35B pneumococcal strains causing IPD in the US.
• To investigate the evolutionary mechanisms, including capsular switching, contributing to the rise of 35B IPD.
• To assess the potential threat posed by emergent 35B strains in relation to vaccine development.

Main Methods:

• Whole-genome sequencing of 199 nonvaccine serotype 35B pneumococcal strains from 2015-2016.
• Analysis of historical IPD data from the Active Bacterial Core surveillance program.
• Comparative genomic analysis to identify capsular switch events and lineage tracking.

Main Results:

• Penicillin-nonsusceptible 35B IPD increased significantly after the introduction of pneumococcal conjugate vaccines (PCV7 and PCV13).
• The 35B/sequence type (ST) 558 lineage was identified as the primary driver of this increase.
• A capsular switch event involving the 35B/ST558 donor and the 9V/ST156 recipient lineage generated emergent 35B/ST156 strains.

Conclusions:

• The 35B/ST558 lineage and emergent 35B/ST156 strains represent a growing threat for invasive pneumococcal disease.
• Capsular switching events are a significant mechanism for the emergence of novel pathogenic pneumococcal strains.
• Next-generation pneumococcal vaccines should consider including protection against serotype 35B.