A Unique Mechanism that Allows Bacteria to Infect
In the 1980s, researchers studying the disease-causing species of Yersinia, a group of gram-negative bacteria, found that large amounts of proteins were being produced and secreted by the bacteria. These proteins lacked a signal-sequence that two known secretion mechanisms require for transport across the double membrane of gram-negative bacteria. The proteins must then be secreted by a third type of system, which researchers called the type III system.

Recently, genes coding for the type III system are turning up in other gram-negative animal pathogens, and even in more distantly related plant pathogens. The genes seem to be more closely related to each other than do the bacteria. Furthermore, the genes are similar to those that code for bacterial flagella.

The function of these proteins is still under investigation, but it seems that some proteins are used to transfer other virulence proteins into nearby eukaryotic cells. Given the similarity of the type III genes to the genes that code for flagella, the transfer proteins may form a flagellum-like structure that shoots virulence proteins into the host cells. Once in the eukaryotic cells, the virulence proteins may determine the hosts response to the pathogens. In Yersinia, proteins secreted by the type III system are injected into macrophages and disrupt signals that tell the macrophages to engulf bacteria. Salmonella and Shigella use their type III proteins to enter the cytoplasm of eukaryotic cells and thus be protected from the immune system. The proteins secreted by E. coli alter the cytoskeleton of nearby intestinal eukaryotic cells, resulting in a bulge onto which the bacterial cells can tightly bind.

Currently, researchers are looking for a way to use this knowledge of the bacterial machinery to disarm the bacteria, possibly by causing the bacteria to release the virulence proteins before they are near eukaryotic cells. Others are studying the eukaryotic proteins and the process by which they are affected.

Source: "A shared strategy for virulence" by Marcia Barinaga, Science, May 31, 1996.