Why a common hospital infection is so hard to wipe out


The tough bacterium P. aeruginosa uses an osmotic defence system to survive water attacks, writes Andrew Masterson.


An illustration of a Pseudomonas aeruginosa bacterium.
KATERYNA KON/SCIENCE PHOTO LIBRARY

As slimming stories go, it’s pretty spectacular. A bacterium called Pseudomonas aeruginosa can shed 20% of its weight in just 28 milliseconds.

This unusual ability to super-slim, research shows, is one of the key reasons the species is one of the most resilient in the bacterial world.

Form a human perspective, P. aeruginosa’s toughness is bad news. The species is resistant to most antibiotics and is the cause of many severe hospital-acquired xinfections. P. aeruginosa is seriously nasty – but a team led by Sergei Sukharev of Maryland University in the US has shown that it is also seriously well adapted.

The bacterium thrives in a wide variety of environments, including freshwater, soil and – significantly – the moist surfaces of surgical instruments. What’s more, sudden changes in its living arrangements, especially influxes of water, don’t worry it all.

It was this ability to survive inundation that intrigued Sukharev and his colleagues, because it appeared to defy physics.

Broadly comparable bacteria will swell up and burst when hit by a raindrop. This is because of a process called osmosis. The rain dilutes the media outside each bacterium, causing a pressure disparity. Water rushes into the bacterium through its semi-permeable cell wall, with fatal results.

Previous research demonstrated that another particularly hardy species of bacteria – E.coli – is able to partially mediate this type of “osmotic downshock” by pushing small solute molecules, called osmolytes, out through the cell wall, thus reducing the pressure gradient.

E.coli manages this evacuation through two channels – the second of which functions as an emergency measure, kicking in when the first is overwhelmed, pushing out larger osmolytes at a faster rate.

Sukharev’s team discovered that P. aeruginosa also uses a pair of osmolyte chutes to reduce the effects of osmosis, but also boasts two other adaptations.

First, its cell wall is less permeable to water, which means it takes longer for the inrushing moisture to build up pressure. Second, its osmolyte release is so concentrated and rapid that it can clear out one-fifth of its starting weight in a flash, thereby providing more room at lower pressure, greatly reducing the destructive power of the osmotic shock.

“These results move us one step closer to a mechanistic understanding of the physiological response to osmotic downshocks,” says Sukharev.

The study is published in The Journal of General Physiology.

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Andrew Masterson is an author and journalist based in Melbourne, Australia.
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