Defense Secret of E. coli Revealed

Cornell University researchers have found out how E. coli bacteria defend themselves against antibiotics and other poisons. When undesirable molecules show up, the bacterial cell opens a tunnel through its cell wall and pumps out the intruders. According to Peng Chen, professor of chemistry and chemical biology, scientists have hypothesized on the assembly of these tunnels for a long time. Now the team has seen them.

Chen and colleagues selected an E. coli strain that pumps out copper atoms that would poison the bacteria. The researchers genetically engineered it, adding an additional DNA sequence that codes for a fluorescent molecule to its defense. Using a powerful microscope, they exposed a bacterial cell to copper atoms and periodically zapped the cell with an infrared laser to induce fluorescence. They then witnessed how the tagged protein traveled in the cell. The team genetically engineered various proteins to turn their metal-binding capability on and off, and observed the effects.

The key protein, known as CusB, lives in the periplasm. When CusB binds to an intruder that has passed through the porous outer membrane, it changes its shape and attaches itself between two related proteins in the inner and outer membranes to form a complex known as CusCBA that acts as a tunnel through the cell wall. The inner protein has a mechanism to grab the intruder and push it through. The tunnel locks the inner and outer membranes together, making the periplasm less flexible and interfering with its normal functions. This defense mechanism against toxic metals may also explain how bacteria develop resistance to antibiotics, by mutating their defensive proteins to recognize them.

For more details, read the Cornell Chronicle.