Date Published: October 11, 2018
Publisher: Public Library of Science
Author(s): Elodie Lesne, Loic Coutte, Luis Solans, Stephanie Slupek, Anne-Sophie Debrie, Véronique Dhennin, Philippe Froguel, David Hot, Camille Locht, Rudy Antoine, Françoise Jacob-Dubuisson, Eliane N. Miyaji.
The whooping cough agent Bordetella pertussis coordinately regulates the expression of its virulence factors with the two-component system BvgAS. In laboratory conditions, specific chemical modulators are used to trigger phenotypic modulation of B. pertussis from its default virulent Bvg+ phase to avirulent Bvg- or intermediate Bvgi phases, in which no virulence factors or only a subset of them are produced, respectively. Whether phenotypic modulation occurs in the host remains unknown. In this work, recombinant B. pertussis strains harboring BvgS variants were tested in a mouse model of infection and analyzed using transcriptomic approaches. Recombinant BP-BvgΔ65, which is in the Bvgi phase by default and can be up-modulated to the Bvg+ phase in vitro, could colonize the mouse nose but was rapidly cleared from the lungs, while Bvg+-phase strains colonized both organs for up to four weeks. These results indicated that phenotypic modulation, which might have restored the full virulence capability of BP-BvgΔ65, does not occur in mice or is temporally or spatially restricted and has no effect in those conditions. Transcriptomic analyses of this and other recombinant Bvgi and Bvg+-phase strains revealed that two distinct ranges of virulence gene expression allow colonization of the mouse nose and lungs, respectively. We also showed that a recombinant strain expressing moderately lower levels of the virulence genes than its wild type parent was as efficient at colonizing both organs. Altogether, genetic modifications of BvgS generate a range of phenotypic phases, which are useful tools to decipher host-pathogen interactions.
Bordetella pertussis is the agent of an acute respiratory disease, whooping cough. Despite the current global vaccination coverage of approximately 86% of the population, this bacterium still causes 16 million cases and 200,000 deaths per year . To colonize the human respiratory tract, B. pertussis produces a number of virulence factors, notably adhesins and toxins, whose expression is regulated by a two-component system called BvgAS . BvgS is a sensor-kinase protein that auto-phosphorylates and transfers the phosphoryl group via a complex cascade of phosphorylation to BvgA, the response regulator. BvgA acts as a canonical transcriptional activator in its phosphorylated form .
We genetically engineered a chimera, BvgSΔ65, in which the linker X of a BvgS homolog replaces the region encompassing the linker 1, the PAS domain and the linker 2 of BvgS, between the transmembrane and the DHp domains . In standard growth conditions, recombinant BP-BvgSΔ65 is in the Bvgi phase, and modulation causes an increase of BvgSΔ65 kinase activity, contrary to its effect on wt BvgS. We thus used the inverted regulation properties of this chimera as a tool to investigate host-pathogen interactions in an animal model of infection. Additional strains harboring other engineered versions of BvgS were also included in mouse colonization experiments. Two distinct groups of strains were thus revealed. BP-BvgSΔ65 and BPSMSS1, which are in the Bvgi phase, were able to colonize and to survive in the mice noses but were rapidly cleared from the lungs, while BP-BvgSΔ65 SS1 could colonize and survive in those two compartments, similar to the Bvg+-phase control strain BPSM. There thus appears to be distinct virulence thresholds for the colonization of the mouse nose, which both Bvgi- and Bvg+-phase bacteria can colonize, and the mouse lungs, which only Bvg+-phase bacteria can. Our transcriptomic analyses showed that those Bvgi- and Bvg+-phase strains populate distinct ranges of phenotypes, based on gene expression levels in the various strains. Such recombinant strains are useful tools to finely dissect the interactions of B. pertussis with its host.