Chemotaxis: Videos & Practice Problems

Chemotaxis is the movement of motile cells towards chemoattractants and away from chemorepellents, utilizing flagella through a run and tumble mechanism. Positive chemotaxis occurs when cells move towards higher concentrations of attractants, while negative chemotaxis is the movement away from repellents. Cells adjust the length of their runs based on concentration changes, enhancing their ability to navigate towards favorable environments. This process is crucial for understanding microbial behavior and interactions in various ecosystems.

Chemotaxis

In this video, we're going to begin our lesson on chemotaxis. Recall from our previous lesson video that energy from a proton motive force or PMF for short, is used to move a cell towards a more favorable environment using its flagella. The term chemotaxis can be defined as the movement of a cell towards a chemoattractant and away from chemorepellents. A chemoattractant is a chemical that attracts a motile cell. Cells are going to be moving towards a chemoattractant. Scientists refer to this as positive chemotaxis when a cell is moving towards a chemoattractant. Now, a chemorepellant, on the other hand, is going to be a chemical that repels motile cells. Motile cells, cells that are moving, are going to be moving away from a chemorepellant. Scientists refer to this as negative chemotaxis when the cells move away from a chemorepellant. Now, phototaxis is a specific type of chemotaxis where the cell's movement is going to be towards or away from light. You can see the photo root here is associated with light. When the cell is moving towards light, we call this positive phototaxis. When the cell is moving away from light, we call this negative chemotaxis.

If we take a look at our image down below, we can get a better understanding of this. Notice on the left-hand side over here, we're showing you how cells can swim towards a chemoattractant. Notice that the chemoattractant over here is representing positive chemotaxis. Notice that the cell is moving towards the chemoattractant in this direction from right to left. On the right over here, what we're showing you is how cells can also swim away from a chemorepellant. Over here, we're showing you a chemorepellant, which is going to promote negative chemotaxis. Notice that the cell is moving away from the chemorepellant. And so this will allow the cell to move towards more favorable environments.

It turns out that the actual path that the motile cell takes towards or away from something is not going to be a continuous straight line. Because these cells move in a series of runs and tumbles, those runs and tumbles are not going to allow the cell to move in a continuous straight line for a very long period of time. We'll be able to talk more about this idea right here in our next lesson video as we move forward. But for now, this here concludes our brief introduction to chemotaxis, and I'll see you all in our next video.

Structures external to the bacterial cell wall that is used for motility (movement) by chemotaxis:

• A. Endospores
• B. Capsules
• C. Flagella
• D. Pili
• E. Ribosomes

Answer: (C)

A bacterial cell moving away from the light would be an example of _______.

• A. Positive phototaxis
• B. Negative phototaxis
• C. Positive chemotaxis
• D. Negative chemotaxis

Answer: (B)

Cell Motility During Chemotaxis

In this video, we're going to talk more about cell motility during chemotaxis. Recall from our previous lesson videos that motile cells use their flagella in a run and tumble mechanism, changing their directions during each tumble event. The run events allow the cell to move smoothly in a single direction. Now, when an attractant such as a chemoattractant is present, a cell begins to move in the direction of the higher concentration, the highest concentration of that chemoattractant. The cell has this amazing ability to sense concentration changes. The ability to sense concentration changes comes through very complex cellular mechanisms and processes. Ultimately, the cell can sense the concentration changes and is able to respond to the concentration changes by controlling the length of each run. If the concentration gets really low, the runs become longer until the concentration starts to get higher. When the concentration gets higher, the runs become shorter, and the cell knows that it's in an area where the concentration of the chemoattractant is highest.

If we take a look at this image down below, what we're showing you is the swimming motility of a peritrichous cell, in the absence and presence of a chemical attractant. Recall that peritrichous is just the flagellar distribution and how the flagella are distributed along the entire surface of the bacteria. On the left here, we're showing you the swimming motility of a peritrichous cell when there is no attractant. Notice that the cell's movement occurs in a series of runs and tumbles. Every straight line represents a run, and every corner represents a tumble, a change in direction. When there's no attractant, notice that the end result and the starting position are very close to each other. There is no net progress in terms of its overall direction, its overall movement. The cell has moved in a direction here, but it's now ending in a position that's very similar to its starting position, meaning there is no net movement.

However, notice that in the presence of a chemoattractant, the starting position is very different from the ending position. Ultimately, the cell is able to move towards the higher concentration. The higher concentration of the chemoattractant is in the dark shaded green area, and a lower concentration of the attractant is on the left side of the box. The runs are much longer when the concentrations are low, and as the concentrations start to get higher, the runs get shorter. The red lines represent the shorter runs when the concentrations are getting higher. Ultimately, you can see that the net movement is towards the attractant. You can see the starting position and the ending position, indicating that the cell is moving towards the attractant.

If we were using a chemorepellent instead of a chemoattractant, the opposite effect would occur. The movement of these bacteria is not in a straight smooth fashion. Instead, it occurs in a series of runs and tumbles until the cell is making its way towards the area that has the highest levels of chemoattractant. This concludes our brief lesson on cell motility during chemotaxis, and we'll be able to get some practice applying these concepts as we move forward. So, I'll see you in our next video.

A common attractant for bacteria is glucose sugar. Bacteria placed in a cup of water with dissolved glucose are going to change their movements depending on the concentration of the glucose. If the concentration of glucose increases the bacteria will…

• A. Have longer runs
• B. Have an increased number of tumbles
• C. Have shorter runs

Answer: (A)

If an E. coli bacterium finds itself moving away from an attractant how will its movements change?

• A. The runs will become longer and the tumbles less frequent.
• B. The runs will become shorter and the tumbles less frequent.
• C. The runs will become longer and the tumbles more frequent.
• D. The runs will become shorter and the tumbles more frequent.

Answer: (D)

Here’s what students ask on this topic:

What is chemotaxis and how does it work in bacteria?

Chemotaxis is the movement of motile cells, such as bacteria, towards chemical attractants (chemoattractants) and away from chemical repellents (chemorepellents). This movement is facilitated by the cell's flagella through a run and tumble mechanism. During a 'run,' the cell moves in a straight line, and during a 'tumble,' it changes direction. When a chemoattractant is present, the cell senses the concentration gradient and adjusts the length of its runs to move towards higher concentrations. Conversely, in the presence of a chemorepellent, the cell moves away by shortening its runs as it senses increasing concentrations of the repellent. This process allows bacteria to navigate towards favorable environments.

What is the difference between positive and negative chemotaxis?

Positive chemotaxis refers to the movement of motile cells towards higher concentrations of a chemoattractant. In this process, cells sense the increasing concentration of the attractant and adjust their movement to move towards it. Negative chemotaxis, on the other hand, is the movement of cells away from a chemorepellent. Here, cells detect the repellent and alter their movement to avoid it. Both types of chemotaxis involve the run and tumble mechanism, where cells change the length of their runs based on the concentration of the chemical they are moving towards or away from.

How do bacteria sense and respond to concentration changes during chemotaxis?

Bacteria sense and respond to concentration changes during chemotaxis through complex cellular mechanisms. They have receptors on their surface that detect the concentration of chemoattractants or chemorepellents. When a bacterium senses a higher concentration of an attractant, it prolongs its runs, moving smoothly towards the source. Conversely, if the concentration of a repellent increases, the bacterium shortens its runs and changes direction more frequently to move away. This ability to sense and respond to concentration gradients allows bacteria to navigate effectively towards favorable environments or away from harmful substances.

What role do flagella play in bacterial chemotaxis?

Flagella are crucial for bacterial chemotaxis as they enable the motility required for movement towards or away from chemical stimuli. Bacteria use their flagella in a run and tumble mechanism to navigate their environment. During a 'run,' the flagella rotate in a coordinated manner, propelling the cell in a straight line. During a 'tumble,' the flagella rotate differently, causing the cell to change direction. By adjusting the length of runs and the frequency of tumbles, bacteria can move towards higher concentrations of chemoattractants or away from chemorepellents, thus optimizing their movement towards favorable conditions.

What is the run and tumble mechanism in bacterial chemotaxis?

The run and tumble mechanism is a key aspect of bacterial chemotaxis, allowing bacteria to navigate their environment. During a 'run,' the bacterium's flagella rotate in a coordinated manner, propelling the cell in a straight line. During a 'tumble,' the flagella rotate differently, causing the cell to change direction. This alternating pattern of runs and tumbles enables the bacterium to explore its surroundings. When a chemoattractant is present, the bacterium adjusts the length of its runs to move towards higher concentrations. Conversely, in the presence of a chemorepellent, the bacterium shortens its runs to move away from the repellent.