Introduction

Bacillus anthracis, the aetiological agent of anthrax, is a Gram-positive, spore-forming, extracellular bacterium belonging to the Bacillus cereusgroup. The spore is both the form of persistence in the environment and the infecting form. Two toxins, and a poly-glutamic acid, antiphagocytic, capsule are the main virulence factors

Metabolic pathways, biodiversity and emergence of new strains

Specific sugar utilisation can be used to discriminate between the main groups found in France (A1 and B2). This allows a rapid initial orientation on the origin of the isolated strains, before further genomic analysis. Recent emergence in the USA and in Africa of B. cereus strains having acquired the virulence plasmids of B. anthracis, thus exhibiting similar pathogenicity as B. anthracis bona fide, raises the public health problem of differential diagnosis and therapy, and the fundamental research question of unraveling the mechanisms of emergence, adaptation to the acquisition of new genetic materials and impact of the combined virulence factors of both B. cereus and B. anthracis on the host-bacterial crosstalk during infection. These aspects are approached through a close collaboration with the RKI (Berlin, Germany).

Vegetative form surface

“Sortases” catalyze the covalent anchoring of proteins harboring an “LPXTG” motif. B. anthracispossesses three sortases and 10 “LPXTG” proteins.. The LPXTG proteins are anchored by the predicted sortase. Yet, overexpressed LPXTG proteins are found surface accessible even in absence of their sortase.

Regulation of virulence and persistence factors synthesis

No pXO1-encoded element but atxA, the pXO1-encoded master virulence regulator, is required for bicarbonate induction of a toxin gene transcription. Overexpressing AtxA in absence of bicarbonate does not increase toxin gene trascription. CodY, a transition state regulator that represses virulence in many pathogens, activates that of B. anthracis. CodY indirectly enhances toxin production, by increasing at a post-translational level AtxA accumulation, controls iron acquisition, but has no effect on capsule biosynthesis.

Animal models

In vivo real time analysis of bioluminescent B. anthracisdissemination during cutaneous, inhalational and gastrointestinal infection in mice has shown the respective role of capsule and toxins. This led to identification of previously undescribed portals of entry and target organs. We have shown a previously undescribed role for the capsule; it is an adhesin, mediating close interaction of the bacteria with vascular endothelium in vivo. This modifies the current view of B. anthracispathophysiology. Further studies with wild-type strains expressing both toxins and capsule have exemplified the previously undescribed central role of the edema toxin in virulence. Histopathological studies have shown a temporal differential expression of the lesions caused by each toxin.

Subversion of the innate immune defenses

We have characterised how B. anthracistoxins inhibits the production of effectors of the innate immune system, such as chemokines, cytokines and the bactericidal enzyme group IIA phospholipase A2 in key target cells (alveolar macrophages, bronchial epithelial cells, dendritic cells). Detailed analysis of the disruption of the cell signal transduction pathways have shown that the end-effects involve epigenetic mechanisms leading to impaired recruitment of transcription factors on their promoter. To understand the initial host control mechanisms and the bacterial evasion strategies, we have detailed how B. anthracis spores activate macrophages and induce IFNg secretion by cells of the innate immune defenses. We identified that communication between host cells and spore-activated macrophages was both cytokine- and contact-dependent, leading to recruitment of inflammatory cells in vivo. By altering spore-induced cytokine production, toxin-secreting B. anthracis prevent immune cell contacts, leading to successful bacterial colonization and spreading of infection.

Keywords: Anthrax, Bacillus anthracis, Bacillus cereus, Spore surface, capsule, toxins, sortases, poly-glutamate, in vivo bioluminescence, regulation, bicarbonate, immunity

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