It has been known since the 1950s that certain bacteria (which stain in a way which means they are described as Gram-negative bacteria), even if killed, will cause the blood of the horseshoe crab to turn into a semi-solid mass. This happens because the animal's blood cells, mobile cells called amoebocytes, contain granules with a clotting factor known as coagulogen; this is released outside the cell when bacterial by-products (called endotoxin) is encountered. The resulting coagulation is thought to contain bacterial infections in the animal's semi-closed circulatory system.
The common assumption has been that the same phenomenon is not seen with the blood of humans. However, a new study at the University of Chicago, National Institute of Allergy and Infectious Diseases, and Institut Pasteur in Paris, has shown that bacteria can directly cause human blood and plasma to clot under certain circumstances. The key to clot formation is the location of the bacteria, rather than the total number of bacteria or their level of concentration. In other words, for those bacteria that can activate coagulation factors, coagulation occurs only when a cluster of bacteria forms.
The discovery will improve scientists' understanding of coagulation during bacterial infections and may lead to new clinical methods for treating serious medical conditions such as sepsis and anthrax.
For more information, see: pharmaceutivcal microbiology