Alternate response of Bacillus anthracis atxA and acpA to serum, HCO3- and CO2

Bacillus anthracis overcomes host immune responses by producing capsule and secreting toxins. Production of these virulence factors in response to entering the host environment was shown to be regulated by atxA, the major virulence regulator, known to be activated by HCO3- and CO2. While toxin production is regulated directly by atxA, capsule production is independently mediated by two regulators; acpA and acpB. In addition, it was demonstrated that acpA has at least two promotors, one of them shared with atxA. We used a genetic approach to study capsule and toxin production under different conditions. Unlike previous works utilizing NBY-HCO3- medium under CO2 enriched conditions, we used a sDMEM-based medium. Thus, toxin and capsule production can be induced in ambient or CO2 enriched atmosphere. Using this system, we could differentiate between induction by 10% NRS, 10% CO2 or 0.75% HCO3-. In response to high CO2, capsule production is induced by acpA in an atxA-independent manner, with little to no toxin (protective antigen PA) production. atxA is activated in response to serum independently of CO2, inducing toxin and capsule production in an acpA or acpB dependent manner. HCO3- was also found to activate atxA, but in non-physiological concentrations. Our findings may help explain the first stages of inhalational infection, in which spores germinating in dendritic cells require protection (by encapsulation) without affecting cell migration to the draining lymph-node by toxin secretion.