Properties of surface-associated aggregated growth of Bordetella bronchiseptica

Bordetella bronchiseptica is a Gram-negative bacterial pathogen that colonises the upper respiratory tract of most mammals. Since common laboratory animals are natural hosts of B. bronchiseptica, this organism is an effective model system for studying bacterial pathogenesis in an actual in vivo host-pathogen setting. A majority of its virulence factors are transcriptionally regulated by a two-component system, BvgAS (Bordetella virulence genes). In the virulent phase (Bvg +), toxins such as adenylate cyclase haemolysin bifunctional toxin (CyaA) and adhesins such as filamentous haemagglutinin are expressed. In the avirulent phase (Bvg-), these virulence factors are not expressed. There is at least one distinct intermediate phase (Bvgi) when toxins are down-regulated but the adhesins are highly expressed. In the Bvg i phase, we discovered that B. bronchiseptica displays surface-associated aggregated growth. Adhesin filamentous haemagglutinin (FHA) was found to be required for the surface-associated aggregated growth. In addition, expression of adenylate cyclase toxin (CyaA) was shown to inhibit FHA adhesion function by direct protein-protein interaction. Furthermore, during surface-associated aggregated growth, B. bronchiseptica secretes and deposits complex carbohydrate matrix of which xylose is the major component. Upon inspecting the nasal cavity of infected animals using immunofluorescence microscopy, FHA deficient mutant shows a defect in colonisation of the non-ciliated nasal olfactory epithelia. Wild-type bacteria, on the other hand, colonise both the ciliated nasal respiratory epithelia and the ciliated olfactory epithelia. In addition, we have some evidence indicating the colonising bacteria in the nasal cavity appear to be growing in the Bvgi phase. Surface-associated aggregated growth in vitro and the nasal cavity colonisation studies may serve as important biological implications regarding the persistent infectious life cycle of the pathogen