New images reveal how antibiotics shatter bacterial defenses

Composite image of E. coli exposed to the polymyxin antibiotic – the images show the changes to the outer layer of armor over time. From left to right: bacterium untreated by the antibiotic; bacterium after 15 minutes; after 30 minutes; after 60 minutes; after 90 minutes. The white scale bar is 250 nanometers across. Credit: Carolina Borrelli, Edward Douglas et al. / Nature Microbiology 

Researchers from University College London have revealed how polymyxins, crucial last-resort antibiotics, break down bacterial armor by forcing cells to overproduce and shed it. Astonishingly, the drugs only kill bacteria when they’re active, leaving dormant cells untouched. This discovery could explain recurring infections and inspire strategies to wake bacteria up before treatment. 

Polymyxins were discovered more than 80 years ago and are used as a last-resort treatment for infections caused by "Gram negative" bacteria. These bacteria have an outer surface layer that acts like armor and prevents certain antibiotics from penetrating the cell. Polymyxins are known to target this outer layer, but how they disrupt it and then kill bacteria is still not understood.

In the new study, the research team revealed in high-resolution images and biochemical experiments how the antibiotic Polymyxin B rapidly caused bumps and bulges to break out on the surface of an E. coli bacterial cell.

These protrusions, which appeared within minutes, were followed by the bacterium rapidly shedding its outer armor.

The antibiotic, the researchers concluded, had triggered the cell to produce and shed its armor. The more the cell tried to make new armor, the more it lost the armor it was making, at such a rate that it left gaps in its defenses, allowing the antibiotic to enter the cell and kill it.

However, the team found that this process - protrusions, fast production and shedding of armor, and cell death - only occurred when the cell was active. In dormant (sleeping) bacteria, armor production is switched off, making the antibiotic ineffective.

Co-senior author Dr Andrew Edwards, from Imperial, said: "For decades we've assumed that antibiotics that target bacterial armor were able to kill the microbes in any state, whether they're actively replicating or they were dormant. But this isn't the case. Through capturing these incredible images of single cells, we've been able to show that this class of antibiotics only work with help from the bacterium, and if the cells go into a hibernation-like state, the drugs no longer work -- which is very surprising."

Becoming dormant allows bacteria to survive unfavorable conditions such as a lack of food. They can stay dormant for many years and "wake up" when conditions become more favorable. This can allow bacteria to survive against antibiotics, for instance, and reawaken to cause recurrent infections in the body.

The E. coli cells were imaged at the London Centre for Nanotechnology at UCL. A tiny needle, only a few nanometers wide, was run over the bacterial cell, "feeling" the shape to create an image (a technique called atomic force microscopy) at much higher resolution than would be possible using light.

The team compared how active (growing) and inactive E. coli cells responded to polymyxin B in the lab, finding that the antibiotic efficiently eliminated active cells but did not kill dormant cells.

They also tested the E. coli cells' response with and without access to sugar (a food source that wakes up dormant cells). When sugar was present, the antibiotic killed previously dormant cells, but only after a delay of 15 minutes - the time needed for the bacteria to consume the sugar and resume production of its outer armor.

In conditions where the antibiotic was effective, the researchers detected more armor being released from the bacteria. They also observed the bulges occurring across the surface of the cell.

In conditions where it was ineffective, the antibiotic bound itself to the outer membrane but caused little damage.

The research paper can be found here: 

1. Carolina Borrelli, Edward J. A. Douglas, Sophia M. A. Riley, Aikaterini Ellas Lemonidi, Gerald Larrouy-Maumus, Wen-Jung Lu, Boyan B. Bonev, Andrew M. Edwards, Bart W. Hoogenboom. Polymyxin B lethality requires energy-dependent outer membrane disruption. Nature Microbiology, 2025; DOI: 10.1038/s41564-025-02133-1

 

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (http://www.pharmamicroresources.com/)