COMPARATIVE-ANALYSIS OF THE VIRULENCE CONTROL-SYSTEMS OF BORDETELLA-PERTUSSIS AND BORDETELLA-BRONCHISEPTICA

Bordetella pertussis and Bordetella bronchiseptica contain nearly iden tical BvgAS signal-transduction systems that mediate a biphasic transi tion between virulent (Bvg(+)) and avirulent (Bvg(-)) phases. In the B vg(+) phase, the two species express a similar set of adhesins and tox ins, and in both organisms the transition to the Bvg(-) phase occurs i n response to the same environmental signals (low temperature or the p resence of nicotinic acid or sulphate anion). These two species differ , however, with regard to Bvg(-)-phase phenotypes, host specificity, t he severity and course of the diseases they cause, and also potentiall y in their routes of transmission. To investigate the contribution of the virulence-control system to these phenotypic differences, we const ructed a chimeric B. bronchiseptica strain containing bvgAS from B. pe rtussis and compared it with wild-type B. bronchiseptica in vitro and in vivo. The chimeric strain was indistinguishable from the wild type in its ability to express Bvg(+)- and Bvg(-)-phase-specific factors. H owever, although the chimeric strain responded to the same signals as the wild type, it differed dramatically in sensitivity to these signal s; significantly more nicotinic acid or MgSO4 was required to modulate the chimeric strain compared with the wild-type strain. Despite this difference in signal sensitivity, the chimeric strain was indistinguis hable from the wild type in its ability to cause respiratory-tract inf ections in rats, indicating that the bvgAS loci of B. pertussis and B. bronchiseptica are functionally interchangeable in vivo. By exchangin g discrete fragments of bvgAS, we found that the periplasmic region of BvgS determines signal sensitivity.