Genome-wide gene expression analysis of Bordetella pertussis isolates associated with a resurgence in pertussis: elucidation of factors involved in the increased fitness of epidemic strains

Abstract

Bordetella pertussis (B. pertussis) is the causative agent of whooping cough, which is a highly contagious disease in the human respiratory tract. Despite vaccination since the 1950s, pertussis remains the most prevalent vaccine-preventable disease in developed countries. A recent resurgence pertussis is associated with the expansion of B. pertussis strains with a novel allele for the pertussis toxin (ptx) promoter ptxP3 in place of resident ptxP1 strains. The recent expansion of ptxP3 strains suggests that these strains carry mutations that have increased their fitness. Compared to the ptxP1 strains, ptxP3 strains produce more Ptx, which results in increased virulence and immune suppression. In this study, we investigated the contribution of gene expression changes of various genes on the increased fitness of the ptxP3 strains. Using genome-wide gene expression profiling, we show that several virulence genes had higher expression levels in the ptxP3 strains compared to the ptxP1 strains. We provide the first evidence that wildtype ptxP3 strains are better colonizers in an intranasal mouse infection model. This study shows that the ptxP3 mutation and the genetic background of ptxP3 strains affect fitness by contributing to the ability to colonize in a mouse infection model. These results show that the genetic background of ptxP3 strains with a higher expression of virulence genes contribute to increased fitness.

Figure 1. Functional categories of differentially expressed genes between the ptxP1 and ptxP3 strains.

The gene count (absolute number of genes) of differentially expressed genes per category (blue). And the gene count of differentially expressed genes relative to the total number of genes (red) present in the genomes of the analyzed strains.

Figure 2. Comparison of gene expression measurements by microarray hybridization and quantitative real-time PCR.

Log-transformed (in base 10) fold change values of the Q-PCR data (y-axis) were plotted against the log-transformed fold change values of microarray data (x-axis). The coefficient of determination (R 2) is given.

Figure 3. Role of the ptxP3 and ptxP1 mutations and the genetic background of ptxP1 and ptxP3 strains in colonizing the trachea (A) and lungs (B) in mice.

Mice were intranasally infected with the wildtype ptxP1 strain, the ptxP3 strain, isogenic strains carrying the ptxP3 allele in the ptxP1 genetic background (P1 gb:ptxP3) or the ptxP1 allele in the ptxP3 genetic background (P3 gb:ptxP1). CFUs were determined in the trachea and lungs four days post-infection. The mean is indicated by a thin line. P-values (uncorrected for multiple tests) were shown if greater than 0.05. The experiment was performed two times representative result is shown.