Capsule synthesis by Bacillus anthracis is required for dissemination in murine inhalation anthrax

Abstract

Bacillus anthracis, the agent of anthrax, produces a poly-Dglutamic acid capsule that has been implicated in virulence. Many strains missing pXO2 (96 kb), which harbors the capsule biosynthetic operon capBCAD, but carrying pXO1 (182 kb) that harbors the anthrax toxin genes, are attenuated in animal models. Also, noncapsulated strains are readily phagocytosed by macrophage cell lines, whereas capsulated strains are resistant to phagocytosis. We show that a strain carrying both virulence plasmids but deleted specifically for capBCAD is highly attenuated in a mouse model for inhalation anthrax. The parent strain and capsule mutant initiated germination in the lungs, but the capsule mutant did not disseminate to the spleen. A mutant harboring capBCAD but deleted for the cap regulators acpA and acpB was also significantly attenuated, in agreement with the capsule-negative phenotype during in vitro growth. Surprisingly, an acpB mutant, but not an acpA mutant, displayed an elevated LD 50 and reduced ability to disseminate, indicating that acpA and acpB are not true functional homologs and that acpB may play a larger role in virulence than originally suspected.

Pre- and postexposure protection against virulent anthrax infection in mice by humanized monoclonal antibodies to Bacillus anthracis capsule

“…The biosynthetic operon capBCADE encoding the capsule is present on the plasmid pXO2 (3)(4)(5)(6). Strains that lack pXO2 are highly attenuated (7)(8)(9), and such strains have been used as vaccines to prevent anthrax in domesticated animals for more than 50 y (10). In a mouse model of pulmonary anthrax, encapsulation was shown to be essential for dissemination from the lungs and for persistence and survival of the bacterium in the host (7).…”

“…Strains that lack pXO2 are highly attenuated (7)(8)(9), and such strains have been used as vaccines to prevent anthrax in domesticated animals for more than 50 y (10). In a mouse model of pulmonary anthrax, encapsulation was shown to be essential for dissemination from the lungs and for persistence and survival of the bacterium in the host (7). Virulence appears to be associated with antiphagocytic properties of the capsule (5,11,12).…”

Treatment of Experimental Anthrax with Recombinant Capsule Depolymerase

“…The B. anthracis capsule (pX02) and toxin (pX01) plasmids encode factors that enhance survival in the host, likely in part by subverting the bactericidal activity of neutrophils. The capsule is essential for virulence (3,10,18,21), and its antiphagocytic property is a primary mechanism of immune cell evasion utilized by B. anthracis (21,22,26). The anthrax lethal toxin (PA plus LF) and edema toxin (PA plus EF) are known to inhibit some neutrophil functions (1,8,12,21,34,43), but in contrast to a deleterious effect of lethal toxin on macrophage survival from some animals (13,14), neither affects human neutrophil viability (8).…”

Constant domains influence binding of mouse–human chimeric antibodies to the capsular polypeptide ofBacillus anthracis

“…4,5 Like many microbial capsules, PGA has a high molecular weight, is resistant to degradation, and presents a significant barrier to the mammalian immune system. [6][7][8][9] Previous studies identified the capsule as a potential therapeutic target in both active and passive immunization strategies. [10][11][12] Capsule-binding antibodies increase opsonization and phagocytosis of vegetative bacteria.…”