Loss of Dihydroxyacid Dehydratase Induces Auxotrophy in Bacillus anthracis

Anthrax disease is caused by infection with the bacteria Bacillus anthracis which, if left untreated, can result in fatal bacteremia and toxemia. Current treatment for infection requires prolonged administration of antibiotics. Despite this, inhalational and gastrointestinal anthrax still result in lethal disease. By identifying key metabolic steps that B. anthracis uses to grow in host-like environments, new targets for antibacterial strategies can be identified. Here, we report that the ilvD gene, which encodes dihydroxyacid dehydratase in the putative pathway for synthesizing branched chain amino acids, is necessary for B. anthracis to synthesize isoleucine de novo in an otherwise limiting microenvironment. We observed that ΔilvD B. anthracis cannot grow in media lacking isoleucine, but growth is restored when exogenous isoleucine is added. In addition, ΔilvD bacilli are unable to utilize human hemoglobin or serum albumin to overcome isoleucine auxotrophy, but can when provided with the murine forms. This species-specific effect is due to the lack of isoleucine in human hemoglobin. Furthermore, even when supplemented with physiological levels of human serum albumin, apotransferrin, fibrinogen, and IgG, the ilvD knockout strain grew poorly relative to nonsupplemented wild type. In addition, comparisons upon infecting humanized mice suggest that murine hemoglobin is a key source of isoleucine for both WT and ΔilvD bacilli. Further growth comparisons in murine and human blood show that the auxotrophy is detrimental for growth in human blood, not murine. This report identifies ilvD as necessary for isoleucine production in B. anthracis, and that it plays a key role in allowing the bacilli to effectively grow in isoleucine poor hosts. IMPORTANCE Anthrax disease, caused by B. anthracis, can cause lethal bacteremia and toxemia, even following treatment with antibiotics. This report identifies the ilvD gene, which encodes a dihydroxyacid dehydratase, as necessary for B. anthracis to synthesize the amino acid isoleucine in a nutrient-limiting environment, such as its mammalian host. The use of this strain further demonstrated a unique species-dependent utilization of hemoglobin as an exogenous source of extracellular isoleucine. By identifying mechanisms that B. anthracis uses to grow in host-like environments, new targets for therapeutic intervention are revealed.