The wonders of bacteria

Someone on Twitter recently asked me a fascinating question: are bacteria left or right-handed like we are? I jumped into the scientific literature to find out and what I read reminded me why I love biology and microbes so much! To answer that question, I need to tell you about the flagellum, a long helical structure anchored into the surface of many bacteria, like a tail. Not all bacteria have them, but those that do can have one – or a bundle of them.

The best way to think of the flagellum is as the bacterial equivalent of an outboard motor, only one made of 11 stacks of Lego bricks. With me so far? How each of these stacks is arranged gives the flagellum its helical shape. Each of the 11 stacks can have its Lego bricks arranged in one of two positions, let’s call them left or right. That means a stack can either be all left or all right bricks.

Depending on the ratio of left and right stacks, a flagellum can take on one of 12 different positions. If all the stacks are either left or right, the flagellum will be straight.

The remaining 10 positions are what happens when the stacks are in different combinations of left and right. Three of those combinations result in an overall left-handed flagellum, and the other seven make a right-handed flagellum.

To complicate things even more, a bacterium’s flagellum can quickly switch between the 12 different positions in response to changes in its environment.

In other words, flagella have more right-handed positions, but I think we can safely conclude that bacteria are ambidextrous.

What I find even more fascinating is all the ways bacteria use flagella to get around. E coli has several flagella and when all their motors are turning counter-clockwise, the flagella come together in a left-handed bundle and smoothly propel the bacterium forward – the bacterial version of running, say.

If just one of their motors starts turning clockwise instead, the corresponding flagellum transforms into a right-handed helix and leaves the left-handed bundle. This makes E coli stop running and start tumbling.

And viola, the bacterium has now changed direction and can switch back to running. Most studies of how flagella behave have been done on bacteria growing in liquid or on the smooth surface of a Petri dish. But in the real world, bacteria live in complex, structured environments.

Studies of Shewanella putrefaciens, which has just one flagellum, have shown that this bacterium can use that flagellum to free itself if it gets stuck in a tight spot. First, it wraps its flagellum into a spiral around its body. Turning on its motor in that position the bacterium switches from running forward to moving backwards in a screw-like fashion! Isn’t nature amazing!?

The flagellum is as the bacterial equivalent of an outboard motor.