The physics behind those bacterial grenades on your agar plate

Bacteria have an impact on the physics of bubble formation which can help them to spread, explosively

Nov 22, 2018

Poster image credit: Laurent Mekul from LabLimes

It’s late. You’re on your last set of dilutions for your spot colony counts (or Miles and Misra). You didn’t want to make thousands of plates so you’re really packing the spots in there. It’s ok though, you’re a pro. You’ve done this before and you know exactly how many spots you can get on a plate so you blaze through the dilutions like microbiology machine. 

But then, disaster strikes. As you eject the last dilution from the tip you see tiny bubbles forming on the surface of the drop. You just threw a bacterial grenade onto your geometrically perfect plate. All you can do is curse under your breath and put the plate in the incubator, hoping that the fallout won’t be as bad as last time. 

New research at MIT has come out to explain exactly what happens to bacteria trapped in a bubble like this. The idea was to relate it to the spread of pathogenic bacteria in liquids but I could not help but think of my Miles and Misra plates. It turns out that the presence of bacteria in a bubble affects its physical properties. First of all, bacteria are able to cling to the inside surface of the bubble for a long time, they don’t simply skid down the sides. While they are there, they influence the lifetime of the bubble. Contrary to what I thought, the presence of bacteria (E. coli in the study) in the bubbles made them persist for longer. This, in turn, led to thinning of the bubble wall and meant that when it finally popped, it exploded in a storm of light aerosols as opposed to collapsing heavily in on itself. 

The presence of bacteria in these bubbles quite dramatically affects their potential to spread. While the article was not strictly a microbiology paper in my view (I grimaced a little when I read that they diluted the bacteria in distilled water) they raised an interesting question, are bacteria evolved to spread in this way? Has this trait been selected for somewhere in the evolutionary history of the microorganisms? It’s an interesting question and for me, there is no clear answer but I look forward to someone taking this research further. 

Poulain, S. and Bourouiba, L.. (2018) Biosurfactants Change the Thinning of Contaminated Bubbles at Bacteria-Laden Water Interfaces. Phys. Rev. Lett. 10.1103/PhysRevLett.121.204502

As an aside, when I went to access the paper I was really impressed by the questions that APS asked to check that I wasn’t a robot:

Ben Libberton

Science Communicator, Freelance

I'm a freelance science communicator, formally a Postdoc in the biofilm field. I'm interested in how bacteria cause disease and look to technology to produce novel tools to study and ultimately prevent infection.

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