Virulence factors of Bordetella pertussis

Co-detection of Bordetella pertussis and other respiratory organisms in children hospitalised with lower respiratory tract infection

Multiple potential pathogens are frequently co-detected among children with lower respiratory tract infection (LRTI). Evidence indicates that Bordetella pertussis has an important role in the aetiology of LRTI. We aimed to study the association between B. pertussis and other respiratory pathogens in children hospitalised with severe LRTI, and to assess clinical relevance of co-detection. Nasopharyngeal (NP) swabs and induced sputa (IS) were tested with a B. pertussis specific PCR; additionally, IS was tested for other pathogens using a multiplex PCR. We included 454 children, median age 8 months (IQR 4-18), 31 (7%) of whom tested positive for B. pertussis. Children with B. pertussis had more bacterial pathogens detected (3 versus 2; P < 0.001). While B. pertussis showed no association with most pathogens, it was independently associated with Chlamydia pneumoniae, Mycoplasma pneumoniae and parainfluenza viruses with adjusted risk ratios of 4.01 (1.03-15.64), 4.17 (1.42-12.27) and 2.13 (1.03-4.55), respectively. There was a consistent increased risk of severe disease with B. pertussis. Patterns indicated even higher risks when B. pertussis was co-detected with any of the three organisms although not statistically significant. Improving vaccine coverage against B. pertussis would impact not only the incidence of pertussis but also that of severe LRTI generally.

Characterization of the bvgR locus of Bordetella pertussis

Bordetella pertussis, the causative agent of whooping cough, produces a wide array of factors that are associated with its ability to cause disease. The expression and regulation of these virulence factors is dependent upon the bvg locus (originally designated the vir locus), which encodes two proteins: BvgA, a 23-kDa cytoplasmic protein, and BvgS, a 135-kDa transmembrane protein. It is proposed that BvgS responds to environmental signals and interacts with BvgA, a transcriptional regulator which upon modification by BvgS binds to specific promoters and activates transcription. An additional class of genes is repressed by the bvg locus. Expression of this class, the bvg-repressed genes (vrgs [for vir-repressed genes]), is reduced under conditions in which expression of the aforementioned bvg-activated virulence factors is maximal; this repression is dependent upon the presence of an intact bvgAS locus. We have previously identified a locus required for regulation of all of the known bvg-repressed genes in B. pertussis. This locus, designated bvgR, maps to a location immediately downstream of bvgAS. We have undertaken deletion and complementation studies, as well as sequence analysis, in order to identify the bvgR open reading frame and identify the cis-acting sequences required for regulated expression of bvgR. Studies utilizing transcriptional fusions of bvgR to the gene encoding alkaline phosphatase have demonstrated that bvgR is activated at the level of transcription and that this activation is dependent upon an intact bvgAS locus.

Identification of a locus required for the regulation of bvg-repressed genes in Bordetella pertussis

In Bordetella pertussis, the coordinate regulation of virulence factor expression is controlled by the products of the bvgAS locus. In the presence of modulating signals such as MgSO4, nicotinic acid, or reduced temperature, the expression of bvg-activated genes is reduced while the expression of bvg-repressed genes is induced. One model for the regulation of bvg-repressed genes predicts the existence of a repressor protein encoded by a bvg-activated gene. Once activated, the product of this bvg-activated gene would bind to and repress transcription from the bvg-repressed genes. We isolated five genetically independent transposon insertion mutants of B. pertussis that have a phenotype consistent with the knockout of a putative bvg-regulated repressor. These mutants constitutively expressed a vrg6-phoA transcriptional fusion but demonstrated normal bvgAS function. Genomic mapping and DNA sequence analysis of the sites of transposon insertion demonstrated that these mutants define a locus downstream of bvgAS. Introduction of an in-frame, 12-bp insertion within this locus also conferred the mutant phenotype, confirming that the phenotype seen in the transposon mutants is the result of disruption of a distinct gene, which we have designated bvgR, and is not a consequence of polar effects on bvgAS.

Global regulatory mechanisms affect virulence gene expression in Bordetella pertussis

The influence was investigated of DNA gyrase-inhibiting drugs on the expression of various genes of Bordetella pertussis. We show that the promoters of the virulence regulatory bvg locus and of several bvg-regulated virulence factors, such as the fha, ptx, cya, fim2 and vrg6 loci are very sensitive to the action of novobiocin and coumermycin A, as reflected by transcriptional differences in gene expression. Inhibition of DNA gyrase by the drugs led to a strong decrease in transcription of these genes. Interestingly, one gene belonging to the bvg virulence regulon behaved differently: the promoter of the prn locus, coding for the outer membrane protein pertactin, involved in bacterial adhesion to eukaryotic cells, was induced after inhibition of DNA gyrase. The expression of other genes not belonging to the bvg regulon, such as those encoding porin (POR) and superoxide dismutase (SodB), were not, or only weakly, affected by the drugs. This demonstrates that with respect to drug-induced changes in DNA supercoiling there exist different types of promoters in B. pertussis. In an attempt to identify additional regulatory mechanisms that may modulate virulence gene expression, we investigated the effect of various environmental stimuli on the stability of the bvg-regulated vrg6 and the bvg-independent sodB transcripts. We found that some signals transduced via by the BvgS sensor protein, such as variations in the growth temperature or the presence of nicotinic acid, exerted a strong effect on the half life of these transcripts, whereas another modulating agent, MgSO4, did not have any influence.(ABSTRACT TRUNCATED AT 250 WORDS)

A phase variant of Bordetella pertussis with a mutation in a new locus involved in the regulation of pertussis toxin and adenylate cyclase toxin expression

A novel nonhemolytic phase variant of Bordetella pertussis was characterized. This strain is strongly impaired in the transcription of the pertussis and adenylate cyclase toxins, whereas other known virulence-related factors such as the filamentous hemagglutinin, the fimbriae, and the outer membrane protein pertactin are expressed and regulated normally. Complementation and allelic exchange experiments demonstrated that the mutation is localized neither in the bvg locus involved in virulence regulation nor in the genes responsible for synthesis and transport of the toxins pertussis and adenylate cyclase. Instead, the mutation impairing transcription of at least the two toxin genes is located in a new genetic locus, which acts together with the BvgA/S two-component regulatory system on the expression of a subset of virulence genes. Further analysis suggested that most presumably the mutation affects a sequence-specific DNA-binding protein which contributes to transcriptional activation. The mutant was nonlethal in a murine respiratory model, which corresponds well with the lack of expression of the toxins. However, the clearing rate of this mutant from the lungs of mice was much lower than that of a bvg mutant, suggesting that factors other than the toxins may play a role in the persistence of the bacteria in the respiratory tract of mice.

Repressor binding to a regulatory site in the DNA coding sequence is sufficient to confer transcriptional regulation of the vir-repressed genes (vrg genes) in Bordetella pertussis

Five TnphoA fusions to vir-repressed genes (vrg genes) have been identified in the respiratory pathogen Bordetella pertussis. A comparison of vrg DNA sequences suggests a consensus DNA element within the coding regions of four of five vrg genes. To determine the role of this DNA sequence in vrg regulation, a nucleotide substitution mutation in the conserved region of vrg-6 was isolated. This mutant showed constitutively high levels of expression in the absence of antigenic modulators, MgSO4 and nicotinic acid, suggesting that this DNA element may be a control site for vrg repression. Moreover, Northern (RNA) analysis and transcriptional fusion analysis suggest that vrg genes are regulated at the transcriptional level. To determine whether sequences in the coding region were sufficient to respond to antigenic modulation, a vrg-6::TnphoA promoter deletion plasmid that contained a heterologous promoter driving the expression of vrg-6 coding sequences from the vrg-6 translation start site to the TnphoA fusion junction was constructed. This heterologous construct responded to modulators in a vir-dependent fashion, indicating that sequences upstream of the coding sequence are not required for antigenic modulation. Southwestern (DNA-protein) analysis and mutational studies suggest that the vrg consensus DNA sequence is specifically recognized by a 34-kDa vir-activated gene (vag) product, whose binding results in down-regulation of vrg transcript levels. We conclude, at least for the vrg::TnphoA fusion strains, that a site on the DNA that corresponds to a consensus sequence located in the vrg coding region is sufficient to confer the transcriptional regulation (repression) of vrg genes when B. pertussis strains are grown under nonmodulating conditions.

A vir-repressed gene of Bordetella pertussis is required for virulence

Coordinate regulation of gene expression in Bordetella pertussis is controlled by the products of the vir locus, BvgA and BvgS. In the presence of modulating signals such as MgSO4 and nicotinic acid, expression of vir-activated genes (vag) is reduced, while expression of vir-repressed genes (vrg) is maximal. We have cloned one of these vir-repressed genes, vrg-6, in Escherichia coli. DNA sequencing has shown that vrg-6 is contained on a single EcoRI restriction endonuclease fragment and is predicted to code for a protein of 105 amino acids with a molecular weight of 11,441. The predicted protein product appears to have two domains, one consisting of seven hydrophobic proline-rich pentameric repeats and the other consisting of five alkaline trimeric repeats. Southern blot analysis has revealed vrg-6-homologous sequences in the chromosomes of Bordetella bronchiseptica and Bordetella parapertussis, but, unlike Bordetella pertussis, these species do not express vrg-6-homologous RNA when grown under modulating conditions. In order to assess the role of vrg gene products in B. pertussis pathogenesis, two 18323 derivatives which harbor TnphoA insertions in vrg genes were analyzed in a mouse model of respiratory infection. Strain SK6, which carries a vrg-6::TnphoA mutation, failed to induce lymphocytosis and was significantly less able to colonize lungs and trachea than its parent strain 18323 or than SK18, which harbors a TnphoA fusion in the vrg-18 locus. This is the first evidence that a vir-repressed gene may play an important role in the virulence of B. pertussis and the pathogenesis of whooping cough.