Deciphering
Bordetella pertussis
epidemiology through culture-independent multiplex amplicon and metagenomic sequencing
Authors: Luu Laurence Don Wai , Rafique Raisa , Payne Michael , Octavia Sophie , Robson Jennifer , Sintchenko Vitali , Lan Ruiting 
Published: November 2024

Whooping cough (pertussis) has re-emerged despite high vaccine coverage in Australia and many other countries worldwide, partly attributable to genetic adaptation of the causative organism,
Bordetella pertussis,
to vaccines. Therefore, genomic surveillance has become essential to monitor circulating strains for these genetic changes. However, increasing uptake of PCR for the diagnosis of pertussis has affected the availability of cultured isolates for typing. In this study, we evaluated the use of targeted multiplex PCR (mPCR) amplicon sequencing and shotgun metagenomic sequencing for culture-independent typing of
B. pertussis
directly from respiratory swabs. We developed a nine-target mPCR amplicon assay that could accurately type major lineages [
ptxP3/
non-
ptxpP3
,
fim3A/B
,
fhaB3/
non-
fhaB3,
and epidemic lineages (ELs) 1–5] circulating in Australia. Validation using DNA from isolates and 178 residual specimens collected in 2010–2012 (
n
= 87) and 2019 (
n
= 91) showed that mPCR amplicon sequencing was highly sensitive with a limit of detection of 4.6 copies [IS
481
cycle threshold (Ct) 27.3]. Shotgun metagenomic sequencing was successful in genotyping
B. pertussis
in 84% of clinical specimens with PCR Ct < 24 and was concordant with mPCR typing results. The results revealed an expansion of EL4 strains from 2010 to 2012 to 2019 in Australia and identified unrecognized co-circulating cases of
Bordetella holmesii
. This study provides valuable insight into the circulating lineages in Australia prior to the COVID-19 pandemic during which border closure and other interventions reduced pertussis cases to an all-time low, and paves the way for the genomic surveillance of
B. pertussis
in the era of culture-independent PCR-based diagnosis.

IMPORTANCE

In this paper, we evaluated the use of targeted multiplex PCR (mPCR) amplicon sequencing and shotgun metagenomic sequencing for culture-independent typing of
Bordetella pertussis
directly in respiratory swabs. We first developed a novel targeted mPCR amplicon sequencing assay that can type major circulating lineages and validated its accuracy and sensitivity on 178 DNA extracts from clinical swabs. We also demonstrate the feasibility of using deep metagenomic sequencing for determining strain lineage and markers of virulence, vaccine adaptation, macrolide resistance, and co-infections. Our culture-independent typing methods applied to clinical specimens revealed the expansion of a major global epidemic lineage in Australia (termed EL4) just prior to the COVID-19 pandemic. It also detected cases of previously hidden co-infections from another
Bordetella
species called
Bordetella holmesii
. These findings offer valuable insight into the circulating pertussis lineages in Australia prior to the COVID-19 pandemic during which border closure and other interventions reduced pertussis cases to an all-time low. It also provides comparative data for future surveillance as pertussis resurgence after the COVID-19 pandemic has been reported this year in Australia and many other countries. Overall, our paper demonstrates the utility, sensitivity, and specificity of mPCR amplicon and metagenomic sequencing-based culture-independent typing of
B. pertussis
, which not only paves the way for culture-independent genomic surveillance of
B. pertussis
but also for other pathogens in the era of PCR-based diagnosis.