Enhancement of Bordetella parapertussis infection by Bordetella pertussis in mixed infection of the respiratory tract

Abstract

The epidemiological and pathogenic relationship between Bordetella pertussis and Bordetella parapertussis, the two causes of whooping cough (pertussis), is unclear. We hypothesized that B. pertussis, due to its immunosuppressive activities, might enhance B. parapertussis infection when the two species were present in a coinfection of the respiratory tract. The dynamics of this relationship were examined using the mouse intranasal inoculation model. Infection of the mouse respiratory tract by B. parapertussis was not only enhanced by the presence of B. pertussis, but B. parapertussis significantly outcompeted B. pertussis in this model. Staggered inoculation of the two organisms revealed that the advantage for B. parapertussis is established at an early stage of infection. Coadministration of PT enhanced B. parapertussis single infection, but had no effect on mixed infections. Mixed infection with a PT-deficient B. pertussis strain did not enhance B. parapertussis infection. Interestingly, the depletion of airway macrophages reversed the competitive relationship between these two organisms, but the depletion of neutrophils had no effect on mixed infection or B. parapertussis infection. We conclude that B. pertussis, through the action of PT, can enhance a B. parapertussis infection, possibly by an inhibitory effect on innate immunity.

Figure 1. Dynamics of mixed infection with B. pertussis and B. parapertussis in mice

A. Mean CFU recovered from the respiratory tract of Balb/c mice inoculated with a 1:1 mix (10⁶ total CFU) of B. pertussis (Bp mixed) and B. parapertussis (Bpp mixed), or with 5 × 10⁵ CFU of B. pertussis alone (Bp single) or B. parapertussis alone (Bpp single), on day 7 post-inoculation. Significant differences are shown by P value. B. Mean CFU recovered from the respiratory tract of Balb/c mice inoculated with a 1:1 mix (10⁶ total CFU) of B. pertussis (Bp) and B. parapertussis (Bpp) on the indicated day post-inoculation. * P<0.05.

Figure 2. Effect of input ratio of B. pertussis and B. parapertussis upon their competitive co-infection

Mean CFU recovered from the respiratory tract of Balb/c mice inoculated with the indicated ratio of B. pertussis (Bp) and B. parapertussis (Bpp) (10⁶ total CFU) on day 7 post-inoculation. The competitive index (CI) for the first 3 ratios is shown below (ND – not determined due to zero values). All CI values were significantly different from 1 (p<0.05).

Figure 3. Effect of PT co-administration on B. parapertussis infection and on mixed infection of B. pertussis and B. parapertussis

A. Mean CFU of B. parapertussis recovered from the respiratory tract of Balb/c mice on day 7 post-inoculation with 5 × 10⁵ CFU with co-administered PT or PBS. B. Mean CFU of B. pertussis (Bp) or B. parapertussis (Bpp) recovered from the respiratory tract of mice on day 7 post-inoculation with a 1:1 mix (10⁶ total CFU) with co-administered PT or PBS. * P<0.05.

Figure 4. B. pertussis ΔPT does not enhance B. parapertussis growth in mixed infection

Mean CFU of B. parapertussis (Bpp) or B. pertussis ΔPT recovered from the respiratory tract of Balb/c mice on day 7 post-inoculation with a 1:1 mix of the 2 strains (10⁶ total CFU) or with 5 × 10⁵ CFU B. parapertussis alone (Bpp single) or B. pertussis ΔPT alone (ΔPT single). Significant difference is shown by P value (NS – not significant).

Figure 5. Effect of resident airway macrophage depletion on B. parapertussis single infection or mixed infection with B. pertussis

A. Mean CFU of B. pertussis (Bp) or B. parapertussis (Bpp) recovered from the respiratory tract of Balb/c mice on day 4 post-inoculation with a 1:1 mix of the 2 strains (10⁶ total CFU). Mice were pre-treated 2 days prior to bacterial inoculation with intranasal administration of clodronate liposomes (CL) to deplete airway macrophages or PBS liposomes (PL) as a control. Significant difference is shown by P value (NS – not significant). B. Mean CFU of B. parapertussis recovered from the respiratory tract of Balb/c mice on day 4 post-bacterial inoculation (5 × 10⁵ CFU) in CL-treated and PL- treated mice.

Figure 6. Effect of neutrophil depletion on mixed infection with B. pertussis and B. parapertussis or on B. parapertussis single infection

A. Mean CFU of B. pertussis (Bp) or B. parapertussis (Bpp) recovered from the respiratory tract of Balb/c mice on day 4 post-inoculation with a 1:1 mix of the 2 strains (10⁶ total CFU). Mice were treated with i.p. injection of either neutrophil-depleting antibody (RB6) or control IgG on the day before bacterial inoculation and every other day subsequently. * P<0.05 (vs Bp). NS – not significant (RB6 vs IgG for either strain). B. Mean CFU of B. parapertussis recovered from the respiratory tract of Balb/c mice on day 4 post-inoculation (5 × 10⁵ CFU) in RB6-treated or control IgG-treated mice.