Pasteur heated anthrax bacteria to about 42–43°C to attenuate them.
The treatment was designed to weaken Bacillus anthracis without destroying it completely. Pasteur’s laboratory found that anthrax bacilli grown at roughly 42°C could no longer form spores effectively. Because spores helped the organism survive and retain its virulence, restricting spore formation made the culture suitable for developing an attenuated vaccine.
This work built on Pasteur’s earlier success weakening the organism that caused chicken cholera. Anthrax was more difficult because its bacterium formed durable spores, and early attempts to attenuate it by simply exposing cultures to air were unreliable. The temperature method became part of the research leading to Pasteur’s famous 1881 public vaccination trial at Pouilly-le-Fort.
A common mix-up involves 55°C. Henri Toussaint had experimented with heating anthrax bacilli to 55°C for 10 minutes, aiming to kill them. Pasteur’s approach instead depended on controlled weakening, not merely killing the bacteria. The famous trial involved vaccinated and unvaccinated sheep, goats, and cattle; the vaccinated animals survived exposure to virulent anthrax.