This study examined how two plasmids affect the virulence and immunogenicity of Bacillus anthracis, the bacterium causing anthrax. Researchers compared wild-type strains carrying 110 MDa and 60 MDa plasmids with derivative strains that had been cured of these plasmids. Strains lacking the 110 MDa plasmid showed approximately 100-fold reduced virulence in mice compared to parental strains. Additionally, guinea pigs immunized with plasmid-free derivatives of the non-encapsulated vaccine strain 34F2 provided no protection against challenge with the virulent strain 17JB, which carries both plasmids. In contrast, guinea pigs immunized with the original 34F2 strain (which retains the 110 MDa plasmid) showed complete resistance to the same challenge. The results demonstrate a strong correlation between the presence of the 110 MDa plasmid and both virulence and immunogenicity. The 110 MDa plasmid appears to mediate production of protective antigen, the key immunogenic factor. These findings suggest the 110 MDa plasmid could serve as a genetic marker for identifying effective anthrax vaccine strains.

Key findings

• Bacillus anthracis strains lacking the 110 MDa plasmid were >100-fold less virulent to mice than wild-type strains
• Loss of the 60 MDa plasmid also reduced virulence approximately 100-fold
• Guinea pigs immunized with 110 MDa plasmid-free vaccine derivatives lost all immunogenicity against anthrax challenge
• The 110 MDa plasmid mediates production of protective antigen, the critical virulence and immunogenicity factor
• The 110 MDa plasmid could serve as a genetic marker for identifying effective anthrax vaccine strains