New research sheds light on bacterial firework display

Scientists at the Institute for Animal Health have revealed how a tropical bacterium is able invade cells, and ultimately trigger its escape using a homemade rocket. Their work is published in the November issue of Molecular Microbiology.

A group of scientists led by Dr Ed Galyov and Dr Mark Stevens have discovered that Burkholderia pseudomallei, which causes a disease called melioidosis, uses a special set of `effector` proteins to invade host cells. Their discovery has revealed surprising similarities with Salmonella, which use similar invasion tactics.

B. pseudomallei, along with Salmonella, Shigella, and Yersinia, has the ability to invade cells in the intestine and spread to deeper tissues. All of these species use specialised protein secretion systems known as type III secretion systems (TTSS). “The function of TTSSs is like a molecular syringe injecting a set of bacterial ‘effector’ proteins straight into target immune cells,” says Dr Galyov.

Some of these ‘effector’ proteins interact directly with host proteins, and this allows the bacteria to take over host cell processes for their own benefit. “But unlike Salmonella, we’ve discovered that B. pseudomallei uses the TTSS to break out of the phagocytic vacuole, where the bacteria live initially within the cell, and into the cytoplasm” says Dr Stevens.

Once in the cytoplasm Burkholderia multiply and are able to polymerise actin at one end of the cell. This propels the bacteria against the host cell membrane. Rocket-like protrusions from the cell surface can be seen, which may allow the bacteria to spread from cell to cell in a manner similar to Shigella and Listeria. “We have studied mutant Burkholderia strains that don’t have a working TTSS. These bacteria are unable to produce actin rockets, probably because they are unable to escape into the cytoplasm in the first place,” says Dr Stevens.

Dr Galyov explains, “What we’ve shown is that different bacteria use similar systems to take over host cell processes and spread disease. It follows that inhibitors that block these secretion process could be effective against a range of species.”