Streptococcus agalactiae Strains with Chromosomal Deletions Evade Detection with Molecular Methods

Area of Science:

• Microbiology
• Molecular Diagnostics
• Genomics

Background:

• Effective surveillance of microbial populations is essential for evaluating molecular diagnostic test performance.
• Streptococcus agalactiae (group B Streptococcus [GBS]) is a significant pathogen, and its molecular detection is crucial.

Purpose of the Study:

• To characterize Streptococcus agalactiae (GBS) isolates with deletions in or near the hemolysin gene (cfb).
• To investigate the impact of these genetic variations on the performance of molecular diagnostic assays, specifically the Xpert GBS and GBS LB assays.

Main Methods:

• Whole-genome sequencing was performed on 15 GBS isolates exhibiting PCR-negative, culture-positive results.
• Isolates were collected from multiple geographic locations in the United States and Ireland.
• Prospective surveillance studies were conducted to determine the prevalence of GBS isolates with deletions.

Main Results:

• Four distinct chromosomal DNA deletions, ranging from 181 bp to 49 kb, were identified in the GBS isolates.
• The prevalence of GBS isolates with these deletions was less than 1% in three locations but reached 7% in a fourth.
• Multiple pulsed-field gel electrophoresis types were observed among the deleted isolates, with one type showing broad geographic distribution.

Conclusions:

• Chromosomal deletions affecting the cfb gene region can lead to false-negative results in molecular GBS diagnostic tests.
• The prevalence of these deletion-carrying GBS isolates varies geographically, with some strains showing widespread dissemination.
• Continuous surveillance is necessary to monitor the performance of molecular diagnostic tests and detect emerging resistance mechanisms or genetic variations in pathogens.