Studies expose bacterial secret for infecting body

Bacteria that cause a range of diseases from dysentery to plague use a needle-like "sword" to attack cells, and some have evolved a shield to protect it from the immune system, several teams of researchers reported on Thursday.

Their findings could eventually lead to new treatments, although in the case of dysentery this could be especially difficult, the researchers report in Friday's issue of the journal Science.

"This is the first description of bacteria able to use this 'sword and shield' approach, showing how dysentery is able to infect the body so effectively," said Dr. Christoph Tang of Imperial College London, who led the study on dysentery.

Working with a team at Institut Pasteur in Paris, they used an electron microscope to study the tiny structures of a "molecular syringe" called an injectisome that shigella bacteria use to get into the cells that line the intestine.

These cells become inflamed, which in turn causes the cramps and bloody diarrhea of dysentery.

Tang's team found that the immune system can recognize the "needle" and attack the germ. So bacteria also have a "shield" on the surface, in the form of a complex molecule called a lipopolysaccharide, that hides the needles from patrolling antibodies.

These structures shorten themselves while the needle injects toxic proteins into the cell - just enough to let the needle reach the cell, while protecting it.

"This discovery greatly expands our understanding of how bacteria are sometimes able to evolve although it is unlikely to result in new treatments or vaccines for dysentery," Tang said in a statement.

"In this case, the dysentery bacteria has evolved into a highly effective and dangerous infection." Shigella dysentery kills up to a million people a year globally.

In a second study, Guy Cornelis of the Universitat Basel in Basel, Switzerland and colleagues studied a cousin of the plague bacteria, Yersinia enterocolitica, which also uses an injectisome.

In this case the needle has evolved to a certain length so that it can stick out enough from the bacterium to poke into the targeted cells.

"It is thus likely that needle length has evolved to match specific structures at the bacterial and host cell surfaces," they wrote in their report.