Walking bacteria - and some weighty researcher cajones (repost)

[This post was originally published at webeasties.wordpress.com]

Most papers I read these days are long. Nature and Science papers tend to have 3-4 figures (Cell and Immunity papers can be twice that), tons of supplementary data and are at least a couple pages of dense, science-speak prose. I think I once read a paper (from like 20 years ago) that had a gene sequence as figure 1, a hand-drawn model for figure 2 and one figure of functional data, and I thought that was sparse.

So imagine my surprise when I stumbled on this new paper. One figure. Less than 500 words. And it's about bacteria that seem to get up on their legs (wait, bacteria have legs?!?!). Published in Science - one of the most prestigious science journals in the world. Anyone that is willing to submit a 1 figure paper (not to mention get it accepted) in Science with a sentence like

Bacteria stood upright and "walked" [...]

in the abstract is either extremely clever, or extremely ballsy (or a healthy combination of both).

Here's what they did: they took pictures of huge numbers of Pseudomonas aeruginosa at the surface of biofilms and then used computer software to analyze how individual bacteria were behaving. They noticed that a large number of bacteria near the surface appeared to lift up into a vertical orientation, then walk along the surface of the biofilm on little appendages called type-IV pilli (TFP). I've mentioned biofilms before, but the easiest way to think of them is as a bacterial community. Mostly we think of bacteria as single-celled individuals, but biofilm-forming bugs can achieve a measure of cooperation, and the formation of biofilms is a requirement for a lot of bacterial pathogens (like Pseudomonas) to actually cause disease.

The TFP's were always known to be used for locomotion, like the propellers of a boat. Indeed, when these bugs were in a horizontal orientation, they could crawl in straight lines for long distances. But in the standing orientation, the pilli seemed to act like legs for the bacteria to scuttle along at a faster rate, though they seemed less directional:

Each mechanism confers advantages for surface exploration[...] Crawling enabled directional motion; walking enabled rapid local exploration.

In part A of this figure, red represents the "walking" orientation, and is a pictorial representation of the motion of individual bacteria - each line represents a single cell's motion over time. Comparing those lines to the blue ones, you can see that the crawling orientation tends to be much longer and much straighter.

Part B is just quantifying (putting into numbers) what is represented in A. They actually analyzed about 70,000 individual bugs, and if you look at the axis labeled "L," it shows the distance that each individual traveled, and you can pretty clearly see that the red guys all cluster in the much shorter distances.

They also mention some observations about the cell division behaviors - the TFP seem to be important for daughter cells to move away from each other after they divide - but this seems like more of an appeal to increase the relevancy of the paper. As I said before, it was known for a while that TFP were required for movement, and none of the data presented demonstrates that this walking movement is necessary. In fact, they say

daughters left the division site by detaching, walking, or crawling

It's just that TFP are required for all of these events.

Understanding he way that biofilms affect the life cycle of bacteria is crucial to understand the role of biofilms in disease, but the data presented here is really just observation. It doesn't provide any mechanistic insight, though it will hopefully lead the way to more detailed understanding of how and why this is important. On the other hand, it clearly impressed the editors of Science enough to get included.

UPDATE: After I originally posted this, one of the authors dropped by and left a comment linking to one of the actual videos. It's pretty awesome - and he promised a full-length paper is in the works and almost ready for submission.