Selection and emergence of pertussis toxin promoter ptxP3 allele in the evolution of Bordetella pertussis.

Evolutionary studies using single nucleotide polymorphisms (SNPs) have separated Bordetella pertussis isolates into six major clusters, with recent isolates forming cluster I. The expansion of cluster I isolates was characterised by changes in genes encoding antigenic components in acellular vaccines, including pertactin (Prn). Here, we determined the initial emergence of the pertussis toxin promoter allele, ptxP3, from an evolutionary perspective. This allele was previously shown in a study from the Netherlands to be associated with increased pertussis toxin production as a result of a single base mutation in the ptxP. The ptxP region of 313 worldwide isolates was sequenced, including 208 isolates from Australia collected over a 40 year period. Eight alleles were identified, of which only two predominated: ptxP1 and ptxP3. One novel allele was also found. ptxP3 was only found in SNP cluster I of B. pertussis and its emergence is concurrent with the change to the non-vaccine prn2 allele. Our results suggest that the globally distributed cluster I of B. pertussis has the ability to evade vaccine induced selection pressure.

Analysis of Bordetella pertussis pertactin and pertussis toxin types from Queensland, Australia, 1999-2003.

BACKGROUND: In Australia two acellular Bordetella pertussis vaccines have replaced the use of a whole cell vaccine. Both of the licensed acellular vaccines contain the following three components; pertussis toxoid, pertussis filamentous haemagglutinin and the 69 kDa pertactin adhesin. One vaccine also contains pertussis fimbriae 2 and 3. Various researchers have postulated that herd immunity due to high levels of pertussis vaccination might be influencing the makeup of endemic B. pertussis populations by selective pressure for strains possessing variants of these genes, in particular the pertactin gene type. Some publications have suggested that B. pertussis variants may be contributing to a reduced efficacy of the existing vaccines and a concomitant re-emergence of pertussis within vaccinated populations. This study was conducted to survey the pertactin and pertussis toxin subunit 1 types from B. pertussis isolates in Queensland, Australia following the introduction of acellular vaccines. METHODS: Forty-six B. pertussis isolates recovered from Queensland patients between 1999 and 2003 were examined by both DNA sequencing and LightCycler real time PCR to determine their pertactin and pertussis toxin subunit 1 genotypes. RESULTS: Pertactin typing showed that 38 isolates possessed the prn1 allele, 3 possessed the prn2 allele and 5 possessed the prn3 allele. All forty-six isolates possessed the pertussis toxin ptxS1A genotype. Amongst the circulating B. pertussis population in Queensland, 82.5% of the recovered clinical isolates therefore possessed the prn1/ptxS1A genotype. CONCLUSION: The results of this study compared to historical research on Queensland isolates suggest that B. pertussis pertactin and pertussis toxin variants are not becoming more prevalent in Queensland since the introduction of the acellular vaccines. Current prevalences of pertactin variants are significantly different to that described in a number of other countries with high vaccine coverage. Relative paucity of recovered isolates compared to notified infections, due primarily to non culture based pertussis diagnostics is however a confounding factor in the assessment of variant prevalence.

Temporal trends in circulating Bordetella pertussis strains in Australia.

Australia experienced a resurgence of pertussis in the 1990s despite improved vaccine coverage. Although much of the increase was attributable to increased detection of cases in older persons with waning immunity by serology, vaccine changes or alterations in circulating Bordetella pertussis strains may also have contributed. We determined the frequency of variants of B. pertussis pertactin (prn), and pertussis toxin subunit 1 (ptxS1) genes, restriction fragment length polymorphism (RFLP) types and fimbrial serotypes prevalent in Australia prior to, and during the 1990s. Ampoules of the whole-cell vaccine in use prior to 1999 and 84 B. pertussis isolates stored between 1967 and 1998 by laboratories around Australia were analysed. One pertactin allele, Prn3, not detected before 1985, was found in 24 out of 57 (42%) isolates between 1989 and 1998 (P<0.0001). PtxS1A was found in all isolates. IS1002 type 29, found in 17 out of 31 (55%) isolates tested, was the predominant RFLP type. The only difference in fimbrial serotype distribution between the time-periods was an increase in serotype 3 (P=0.054). The whole-cell vaccine contained only the alleles prn1 and ptxS1A. Antigenic shift in B. pertussis may have contributed to the re-emergence of pertussis in Australia. Monitoring these trends will be important as acellular vaccines are introduced and changes are made to pertussis vaccine schedules.