PHENOTYPIC CHARACTERIZATION OF A NOVEL NEXT-GENERATION BACILLUS ANTHRACIS LIVE ATTENUATED SPORE VACCINE BASED ON THE htrA- (HIGH TEMPERATURE REQUIREMENT A) STERNE STRAIN

Anthrax is a lethal disease caused by the gram-positive spore-producing bacterium Bacillus anthracis. Live attenuated vaccines, such as the nonencapsulated Sterne strain, do not meet the safety standards mandated for human use in the Western world and are approved for veterinary purposes only. We demonstrate that disrupting the htrA gene, encoding the chaperone/protease HtrA (High Temperature Requirement A), in the Bacillus anthracis Sterne strain results in 10-10⁴-fold virulence attenuation in guinea pigs, rabbits and mice. Accordingly, htrA disruption was implemented for the development of a Sterne-derived safe live vaccine compatible with human use. The novel B. anthracis SterneDhtrA strain was phenotypically characterized with respect to growth, germination, sporulation, heat and oxidative stress sensitivity. The novel strain secretes functional anthrax toxins as demonstrated by specific toxin functional assays, yet is significantly less virulent than the Sterne vaccine strain. In spite of this attenuation, immunization with SterneDhtrA spores elicits immune responses which target toxaemia and bacteremia resulting in protection from subcutaneous or respiratory lethal challenge with a virulent strain in guinea pigs and rabbits.