Analysis of Swedish Bordetella pertussis isolates with three typing methods: characterization of an epidemic lineage

Three Bordetella pertussis typing methods, pulsed-field gel electrophoresis (PFGE), multi-locus sequence typing (MLST), and multi-locus variable number tandem repeat analysis (MLVA) were compared using a collection of Swedish strains. Of the three typing methods used, PFGE was found to be the most discriminatory. MLVA and MLST were less discriminatory, but may be valuable for strain discrimination when culture is not possible as they are based on PCR. The combination of MLVA/MLST was found to be equally discriminatory as PFGE and should therefore also be considered. The relationship between predominant lineages in Sweden and the Netherlands, characterized by the PFGE type BpSR11 and the allele for the pertussis toxin promoter ptxP3, respectively, was investigated. Linkage was found between the PFGE type BpSR11 and ptxP3 in that all BpSR11 strains carried ptxP3. On the other hand ptxP3 was found in several other PFGE-types. The presence of the ptxP3 allele in different genetic backgrounds may indicate horizontal gene transfer within B. pertussis or homoplasy. Alternatively, this observation may be due to convergence of PFGE types.