E. coli's secret weapon: a temporary tunnel

Paul Basilio, MDLinx | June 14, 2017

Researchers at Cornell University have identified E. coli’s defense mechanism against antimicrobials and other poisonous agents. The findings were recently published in the Proceedings of the National Academy of Sciences. 

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In the periplasm—the space between the inner and outer membranes of a bacteria's cell wall—defensive proteins that detect a poison form a tunnel between pumps in the cell's inner and outer membranes to eject the intruders. Artist's conception by Ace George Santiago.

The team tagged an E. coli cell’s proteins with fluorescent beacons. They found that the bacterial cell opens a tunnel through its cell wall and effluxes—or “pumps out”—the intruding agent.

“Dynamic assembly of these tunnels has long been hypothesized,” said Peng Chen, PhD, Professor of Chemistry and Chemical Biology. “Now we see them.”

Results of the study offer a potential avenue to combat antibiotic-resistant bacteria through the use of a cocktail of drugs.

“One [step] is to inhibit the assembly of the tunnel, and the next is to kill the bacteria,” said Dr. Chen.

A strain of E. coli that was known to pump out copper atoms was chosen for the study. After some genetic engineering, the team added an additional DNA sequence that codes for a fluorescent molecule to the DNA that codes for a defensive protein.

Under a powerful microscope, they exposed a bacterial cell to an environment containing copper atoms. The cell was periodically hit with an infrared laser to induce fluorescence. After following the blinking lights, the team had a “movie” that showed where the tagged protein traveled in the cell. They further genetically engineered the various proteins to turn their metal-binding capability on and off, and observed the effects.

The key protein is known as CusB. It resides in the periplasm, which is the space between the inner and outer membranes that make up the bacteria’s cell wall. When CusB binds to an “intruder” that has passed through the porous outer membrane, it changes its shape so that it will attach 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 then locks the inner and outer membranes together, interfering with the periplasm’s normal functions and making it less flexible. Researchers note that the ability to assemble the tunnel only when needed gives the cell an advantage.

This tunnel mechanism may also explain how bacteria develop resistance to antibiotics by mutating their defensive proteins to recognize them. Similar mechanisms may be found in other species of bacteria, the researchers suggested.

The work was supported by the Army Research Office and the National Institutes of Health.

The Cornell researchers also collaborated with scientists at the University of Houston, the University of Arizona and the University of California, Los Angeles.

To read more about the study, click here.

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