Bacillus anthracis anthrolysin O and three phospholipases C are functionally redundant in a murine model of inhalation anthrax

Although traditionally considered to be an extracellular pathogen, Bacillus anthracis has a brief intracellular step to initiate anthrax. At the onset of infection, B. anthracis must withstand the bactericidal activities of the macrophage. Recently, three phospholipases C (PLCs) were shown to contribute to macrophage-associated growth of B. anthracis by presumably aiding in the escape of the bacterium from phagocytic vacuoles following phagocytosis. However, in the absence of all three PLCs, vegetative bacilli were still observed growing in association with the macrophage, albeit to a lesser extent, implicating that additional factors are involved in this process. In this study, the contributions of the previously identified cholesterol-dependent cytolysin anthrolysin O (ALO) to B. anthracis pathogenesis were investigated following challenges of bone marrow-derived macrophages and intratracheal inoculations of mice. Disruption of ALO alone yielded no differences in virulence in mice. However, combinatorial deletions of ALO with the three PLCs resulted in attenuation in both tissue culture and murine challenges, suggesting that these toxins may have overlapping roles in anthrax pathogenesis.