Chemotaxis is the ability of a bacterial cell to detect chemical stimuli and move toward food or away from toxins.

Chemotaxis is essential for bacteria to find food by swimming towards food or to escape from toxins. The two types of chemotaxis are positive and negative chemotaxis. Positive chemotaxis occurs if the movement is toward a higher concentration of food. Negative chemotaxis arises to avoid places with a higher level of toxins.

Substances that induce the effect of chemotaxis in motile cells are chemoattractants and chemorepellents. Chemoattractants attract motile cells from the substance through chemotaxis receptors. Conversely, chemorepellent repels motile cells by signaling them to leave or stay away.

Chemoattractants and chemorepellents: In response to chemoattractants, cells move toward the stimulant. In response to chemorepellents, cells move away from them.

Some bacteria, such as E. coli, have several flagella per cell (4–10 typically) and can rotate in two ways:

1. Counter-clockwise rotation – aligns the flagella into a single rotating bundle, causing the bacterium to swim in a straight line.

2. Clockwise rotation – breaks the flagella bundle apart such that each flagellum points in a different direction, causing the bacterium to tumble in place.

The directions of rotation are given for an observer outside the cell looking down the flagella toward the cell.

Bacterial Chemotaxis: Correlation of swimming behavior and flagellar rotation in E. coli.

Key Points

• Chemotaxis is the ability of bacteria to find food by swimming towards it or to escape from toxins.

• The two types of chemotaxis are positive and negative chemotaxis. Positive chemotaxis the movement is towards food and negative chemotaxis, avoiding toxins.

• Chemoattractants and chemorepellents are substances that induce the effect of chemotaxis in motile cells. Chemoattractants attract motile cells, and chemorepellent moves them away.

• Some bacteria, such as E.coli, which has flagella, can rotate either counter-clockwise (CCW) or clockwise (CW); CCW causes the bacterium to swim in a straight line; CW causes the bacterium to tumble in place.

Key Terms

chemotaxis: the movement of cells toward or away from a chemical within the environment

positive chemotaxis: occurs if the movement of a bacteria is toward a higher concentration of food.

negative chemotaxis: occurs when a bacteria moves to avoid places with a higher concentration of toxins

motile cells: cells which can often move with the help of flagellum

chemoattractants: a chemical that attracts motile cells

chemorepellent: substances that repel motile cells

e.coli: a gram-negative bacterium commonly found in the lower intestine of organisms

flagellum: a hairlike structure that acts primarily as an organelle of locomotion