Composite image of Escherichia coli exposed to the polymyxin antibiotic. The images show the changes to the outer layer of armour over time. From left to right: untreated bacterium; bacterium after 15 minutes; after 30 minutes; after 60 minutes; after 90 minutes. The white scale bar is 250 nanometres across. Credit: Carolina Borrelli, Edward Douglas et al. / Nature Microbiology
Researchers have used an imaging technique which relies on touch instead of light to show how a last-line antibiotic causes the outer armour of bacteria to bubble, distort and shed away.
The findings could help make polymycins – a last resort treatment for infections caused by gram-negative bacteria – more effective.
“Polymyxins are an important line of defence against Gram-negative bacteria, which cause many deadly drug-resistant infections. It is important we understand how they work,” says Professor Bart Hoogenboom of University College London, co-senior author of a paper presenting the results in Nature Microbiology.
Gram-negative bacteria, such as Escherichia coli, have an outer membrane which acts as a protective barrier to keep antibiotics out.
The team used atomic force microscopy to produce real-time images showing how both active and dormant E. coli responded to polymyxin B.
In this technique, the tip of a needle just nanometres wide is used to “feel” across the surface of the sample. The surface’s high and low points deflect the tip up and down, and this information is used to generate the 3-dimensional topography of the sample.
“For decades we’ve assumed that antibiotics that target bacterial armour were able to kill the microbes in any state, whether they’re actively replicating or they were dormant. But this isn’t the case,” says co-senior author Dr Andrew Edwards, from Imperial College London in the UK.
“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.”
They found the antibiotic caused gram-negative bacterial cells to produce more of their outer membrane and then shed it. This left gaps in their defences allowing antibiotic to enter and kill them.
However, this did not happen when the cells were in a dormant state. They do this to survive unfavourable conditions, such as a lack of food, and then “wake up” later. This allows them to survive antibiotics and cause recurrent infections.
Polymyxin B was able to kill the dormant cells but only 15 minutes after they were provided with a food source – sugar. This was just enough time to consume it and resume outer membrane production.
“Our next challenge is to use these findings to make the antibiotics more effective,” says Hoogenboom.
“One strategy might be to combine polymyxin treatment – counterintuitively – with treatments that promote armour production and/or wake up ‘sleeping’ bacteria so these cells can be eliminated too.
“Our work also shows we need to take into account what state bacteria are in when we are assessing the effectiveness of antibiotics.”