Susceptibility of irradiated mice to Bacillus anthracis Sterne by the intratracheal route of infection

The susceptibility of sublethally irradiated mice to pulmonary infection wi th Bacillus anthracis was investigated in a mouse model. Female B6D2F1/J mi ce were challenged intratracheally with 4.3 x 10(6), 3.7 x 10(7) and 4.4 x 10(8) cfu of B. anthracis Sterne spores 4 days after Co-60 gamma -radiation at a dose of 0, 1, 2, 3, 4, 5, 6 or 7 Gy, Bacterial cultures were obtained from lung, spleen homogenates and heart blood. A biphasic mode of mortalit y was observed, with a constant response of up to 3 or 4 Gy. (up to 18% mor tality), after which a sharp increase in mortality occurred (up to 100%), W hen irradiation was delayed beyond 15 days after inoculation, the susceptib ility to B, anthracis infection and subsequent mortality disappeared. B. an thracis was recovered fi om the organs and blood of up to 89% of the animal s. However, organisms of enteric origin were also isolated mixed with B. an thracis from up to 36% of the animals exposed to 3, 5 or 7 Gy, Inoculation of B. anthracis Delta -Sterne-1 that lacks lethal toxin and oedema toxin al so induced infection with B. anthracis, but not translocation of enteric mi cro-organisms. The synergic adverse effect of exposure to gamma -radiation followed by intratracheal challenge with B. anthracis was observed above 4 Gy. The lethal toxin of B. anthracis may enhance the emergence of polymicro bial infection with B. anthracis and enteric micro-organisms.