This study describes early intracellular events occurring during the establ
ishment phase of Bacillus anthracis infections. Anthrax infections are init
iated by dormant endospores gaining access to the mammalian host and becomi
ng engulfed by regional macrophages (M phi). During systemic anthrax, late
stage events include vegetative growth in the blood to very high titres and
the synthesis of the anthrax exotoxin complex, which causes disease sympto
ms and death. Experiments focus on the early events occurring during the fi
rst few hours of the B. anthracis infectious cycle, from endospore germinat
ion up to and including release of the vegetative cell from phagocytes. We
found that newly vegetative bacilli escape from the phagocytic vesicles of
cultured M phi and replicate within the cytoplasm of these cells. Release f
rom the M phi occurs 4-6 h after endospore phagocytosis, timing that correl
ates with anthrax infection of test animals. Genetic analysis from this stu
dy indicates that the toxin plasmid pXO1 is required for release from the M
phi, whereas the capsule plasmid pXO2 is not. The transactivator atxA, loc
ated on pXO1, is also found to be essential for release, but the toxin gene
s themselves are not required. This suggests that M phi release of anthrax
bacilli is atxA regulated. The putative 'escape' genes may be located on th
e chromosome and/or on pXO1.