In some invertebrates, such as horseshoe crabs, the presence of bacteria can directly trigger blood clotting, which stops infection from spreading. But the initiation of vertebrate blood clotting was thought to require a more complex system of biochemical signalling.
Rustem Ismagilov of the University of Chicago in Illinois and his co-workers suggest otherwise. The team found that clusters of Bacillus anthracis, which causes anthrax, and Bacillus cereus initiated coagulation in mouse and human blood within minutes, yet dispersed bacteria of the same species did not. The bacteria directly reacted with clotting enzymes called coagulation factors and the blood clotted only when these factors reached a critical density — which happened when the bacteria formed clusters.