Mystery Solved: Cellular Pumps Help E. Coli Beat Antibiotics

New research reveals how E. coli bacteria construct elaborate and effective tunnels to pump unwanted molecules like antibiotics and other toxins out of cells. The discovery could help us better understand how antibiotic resistance occurs and give us a leg-up to beat them at their own game.

By using special imaging techniques, scientists were able to watch a copper molecule — a toxic invader — open up a hole in the E. coli cell wall, interact with bacterial proteins and get pumped back out of the bacteria.

The research, led by Ace George Santiago and Peng Chen from Cornell University, was published the week of June 12 in the online version of the Proceedings of the National Academy of Sciences.

The Good and the Bad E. Coli

E. coli lives happily in our gut — it's an important part of a healthy gut microbe community called the microbiome — and most strains of the bacteria don't make us sick. But, some E. coli can be deadly, and their ability to combat the actions of antibiotics makes those infections medically challenging and dangerous.

A disease-causing E. coli infection usually results in a case of diarrhea familiar to those who've had traveler's diarrhea, but some strains like O157:H7 produce a potent Shiga toxin that can cause bloody diarrhea and kidney failure.

Antibiotic-resistance of E. coli is a big problem because it's a common pathogen in humans. A study of drug resistance in different E. coli bacteria found that 23.3% of E. coli in raw chicken, 20% of the bacteria in vegetable salad, 13.3% in raw meat, 10% in raw egg, and 6.7% in unpasteurized milk were resistant to one or more antibiotics.

The more antibiotics out there, the more likely that bacteria will develop resistance to it. The overuse of antibiotics (particularly in the agricultural industry), over-prescription of antibiotics by doctors, and incomplete courses of antibiotics taken by patients all contribute to the problem.

When in the presence of antibiotics, bacteria develop resistance through several processes. Bacteria multiply rapidly, and if a mutation occurs during that rapid proliferation that makes some of the bacteria able to fight back against an antibiotic present, those mutated bacteria will be the ones to survive and multiply. Antibiotic resistance genes can also be transferred from one bacteria to another by a linking up and information transfer process called conjugation.

Still another mechanism bacteria use to exclude antibiotics is through the use of an efflux pump to move the drugs out of the bacteria. It's this pump that Santiago and Chen's research team investigated.

A Tunnel with a Pump

To visualize the efflux pump process in E. coli, the researchers genetically modified the bacteria. A fluorescent tag was attached to CusB proteins, which span the inner and outer membranes of the bacterial cell wall. CusB proteins function to detect toxins, such as metals and antibiotics. The fluorescent tag allowed the scientists to track the protein when it was confronted by toxic copper atoms.

The team watched the glowing CusB bind to the copper that had moved through the outer cell membrane. When that happened, the CusB changed shape and attached itself between two other proteins in the inner and outer membranes to connect them in rigid links that formed the walls of microscopic tunnels in the cell wall. Once the copper atom reached the inner membrane, the protein there grabbed it and pushed it out the tunnel.

The tunnel is a highly-adapted mechanism and the bacteria only assemble it when needed to expel an intruder — in this case, the copper atom. The study authors believe this is the way E. coli pump out antibiotics before they can damage the bacteria.

Inhibiting the tunnel assembly, then hitting the E. coli with antibiotics might be the way to get resistant infections under control — our control, not the bacteria's.

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