Hi-Res Images Reveal How a “SuperBug” Hides from Antibiotics

“The force” is not just for Jedi knights.

Bacteria have developed their own “force” to hide from our antibiotics, and they are increasingly using this strategy to chip away at the effectiveness of polymyxins, our last line of defense against some “superbug” infections.

Biologists at Columbia are now peering inside these bacteria with super high-resolution imaging techniques and have found places where drugs could disrupt the bugs’ defense and restore their susceptibility to these powerful antibiotics.

To evade detection by polymyxin antibiotics, bugs like E. coli, Salmonella, and Klebsiella pneumoniae--all gram-negative bacteria--are known to alter their electrostatic charge.

“Polymyxins find bacteria via electrostatic attraction,” says Vasileios Petrou, PhD, a postdoc in the lab of Filippo Mancia, PhD, assistant professor of physiology & cellular biophysics. “Polymyxins are positively charged, so they are attracted to negatively charged parts of the bacteria.”

Bacteria become resistant to polymyxins by placing a cap, made from a sugar molecule, over the negative charge. This trick alters the electrostatic forces between the bacteria and antibiotics.

“It’s like the bacteria become invisible to polymyxins,” Dr. Mancia says. “The antibiotics can’t stick to the bacteria or kill them.”

An enzyme called ArnT in the membrane of these bacteria is responsible for the capping. First, ArnT grabs a sugar from a lipid, then the sugar is planted on the negative charge.

The Columbia researchers were able to visualize the precise details of this process by using X-ray crystallography to reveal the location of each individual atom in the ArnT enzyme before and after it grabs the sugar [see video above].

These images reveal places where the enzyme could be disabled. “To grab the sugar, the ArnT enzyme must first bind to the lipid that carries it, and this binding happens in a large 'pocket' in the enzyme’s side,” says Jérémie Vendome, PhD, a research associate scientist in the lab of Barry Hönig.

Filling the pocket with a drug could prevent the binding. “Essentially, that would sensitize the bacteria to the antibiotic again,” Dr, Petrou says.

Dr. Vendome is now using computerized techniques to virtually screen millions of potential drug candidates to detect those that fit in the pocket. Hits generated from the virtual screening will be tested with polymyxins to see if the combination can eliminate antibiotic-resistant bacteria.

“We are not pharma, but we can do some initial development in the lab,” Dr. Mancia says. “We hope that this work will lead to the development of a co-drug that will allow us to extend the lives of already available antibiotics.”