This post will bring together some of the previous posts, namely flagella and optical tweezers. An interesting property of flagella is that they can have a definite shape to them depending on the kind of bacteria you get them from. In our lab, we have several different strains of Salmonella that have differently shaped flagella. Some are straight and some are curly. In fact, they can take on a whole range of curliness, if you will. Some are much more curly than others depending on the strain of bacteria. Here is a representative range of the different shapes of flagella we can get in the lab.
This in itself is an interesting biology question that a lot of biologists worked on for a long time and only recently have understood. However, why the flagella take on different shapes is not what I am interested in with my research, it is the implications of the different shapes they take that I study.
For instance, some of those curly flagella look an awful lot like a spring. We realized that and decided that we wanted to find out if they could stretch just like a spring. For those of you not familiar with Hooke’s Law, a spring stretches linearly with force. This means that if I stretch a spring a distance x and it takes a force F, then to stretch it twice as far, 2x, then it would take twice as much force, 2F.
So, we wanted to stretch a flagella and see if the force it took to stretch it was linear as well. If you are thinking that we used the optical tweezers to pull on the ends of a curly flagella, you would be right! But it is a bit more complicated than that. We can move beads around with the optical tweezers, not flagella. That means we need some way of attaching the beads to the flagella. Fortunately, biologists have figured out a way to attach things to flagella using a pair of proteins call biotin and streptavidin. How they work is not really important, but you can think of it like velcro, when you get them close to each other, they will stick.
So, the procedure is essentially:
- Make a sample containing beads coated in streptavidin and flagella coated in biotin.
- Grab two free floating beads with the optical tweezers
- Attach a bead to either end of a freely floating flagella
- Slowly move the beads farther and farther apart, pulling on the flagella
As we pull the flagella apart, the beads attached at either end are pulled slightly out of the center of the optical trap. It is this shift that we measure. From it we can calculate the force on the bead. In addition to knowing the force on the beads, we can measure the distance between them very accurately. This means that we can measure the force on the flagella as we stretch it out and see what happens.
Did you see what happened there? Look at the four pictures on the left. They are pictures of the flagella as we stretch it out. So, going from picture a to b, we see that the flagella just extends like we would expect a spring to stretch out. Indeed, if we look at the graph on the right, between the points a and b, the force just increases as a relatively straight line, just like a normal spring. However, between pictures b and c, there is a change in the flagella. It might be hard to notice in the picture, but in picture c, the top half of the flagella has changed shape and is much less curly than the bottom half. This is because the flagella is changing from one state to another, from a more curly state to a less curly state. This change has some interesting consequences. For instance, look at the graph on the right between points b and c. As we stretch the flagella through this transition, the amount of force it takes to stretch the flagella is less. This is equivalent to pulling a spring and the farther you pulled it the less it pulled back. That is really strange and that is why it is really interesting to study. Finally, after the transition, picture d shows the new, not as curly state of the flagella.
This type of experiment is called a force extension experiment because you measure the force as you extend an object. These experiments are primarily what I will be doing for this summer. I will be measuring the force extension curves of different strains of flagella and see if they undergo a transition and if so, what it does to the force extension curve. I hope that these posts have helped you understand what I have been up to! I plan on doing one or two more about this system. The next installment will be about what happens when we take a bunch of flagella and try to make a liquid crystal out of them.