Bacteria can turn bursting bubbles into microbial grenades

15 Nov 2018

Image: © rcfotostock/

Research into the physics of bubbles has found that, when they become laced with bacteria, bursting can turn them into a bioweapon.

Away from the harmless pursuit of blowing bubbles, bubbles in the wild can, in some cases, be the harbingers of disease. That’s according to a new study from MIT which found that when natural bubbles contaminated with bacteria burst, they act as tiny microbial grenades unleashing the microorganisms into the air.

The study, published to Physical Review Letters, also found that bacteria can affect the bubble’s longevity, with an infected bubble capable of lasting 10 times longer than an uncontaminated one.

Over the course of a few minutes, the cap of the infected bubble thins. The thinner the bubble, the higher the number of droplets it can launch into the air when the bubble inevitably bursts. By the researcher’s estimates, a single droplet can carry thousands of microorganisms, and each bubble emits hundreds of droplets.

What is ‘normal’ bubble behaviour?

Researchers Lydia Bourouiba and Stephanie Poulain co-authored the research, with the latter having spent the past several years meticulously generating, imaging and characterising clean, uncontaminated bubbles to determine what is ‘normal’ bubble behaviour.

Interestingly, this discovery came about by accident after the researchers, in the process of moving to a new lab, had left a beaker of water exposed. When used in later experiments, they found some unexpected results.

“The bubbles produced from this water lived much longer and had a peculiar thinning evolution compared to that of typical clean water bubbles,” said Poulain. The researchers’ initial belief that the water had been contaminated was later confirmed in tests.

In trying to explain why the contaminated bubbles last longer, the researchers filmed bursting bubbles of contaminated water as well as uncontaminated water using a high-speed imaging camera.

Slow-motion GIF of a bubble bursting and releasing microbes.

When an old bubble bursts at the water surface, its cap fragments into numerous small droplets. Image: Bourouiba/Poulain/MIT

Why this is important

After a series of experiments, Bourouiba suspected that the reason infected bubbles last longer and thin much faster after a period of time is because of what they secrete – a theory later proven correct.

“Bacteria are alive and, like anything alive, they make waste, and that waste typically is something that potentially could interact with the bubble’s interface,” she said.

Aside from lasting 10 times longer than standard bubbles, infected bubbles are also 10 times smaller and eject 10 times faster. This amounts to hundreds of droplets measuring only a few-dozen microns emitted at speeds of the order of 10 metres per second.

The discovery has major implications for climatology and meteorology as they can learn more about foam bubbles that burst at the surface of oceans during a hazardous oil spill, for example.

Colm Gorey was a senior journalist with Silicon Republic