Bacterial Chemotaxis

Submitted by:

Swati Kumari

M.Sc. Bioinformatics

4th semester

Roll no: 22

Session: 2014-16

Submitted to:

Dr. Durg Vijay Singh

Bioinformatics Programmme

Centre for Biological Science

Contents

• Introduction

• Biological example

• References

Introduction

• Chemotaxis - Composed of two words “chemo + taxis”.

• Chemo means a “chemical substance” is a form of matter that has constant chemical

composition and characteristic properties.

• Taxis means "arrangement" is the movement of an organism in response to a stimulus such

as light or the presence of food.

• Chemotaxis is the movement of an organism/bacteria in response to a chemical stimulus

i.e., move away or towards substances that are present in the environment through a

non-random process.

• This movement is often directed either

• Positive chemotaxis - movement towards attractants (nutrients) or

• Negative chemotaxis - movement away from the repellents (toxin).

• Chemoattractants - Chemicals that attract bacteria.

• e.g., for E.coli amino acids (serine and aspartic acid), sugars (maltose, ribose, galactose,

glucose), dipeptides, pyrimidines and electron acceptors (oxygen, nitrate, fumarate).

• Chemorepellents - Chemicals that drive bacteria away.

• e.g., for E.coli amino acid (leucine), metal ion (Ni), pH, potentially noxious chemicals

(alcohols and fatty acids).

• E. coli uses temporal gradients to guide its motion.

• Uses a biased-random-walk strategy to sample space and convert spatial gradients to

temporal ones.

• In liquid environments, E. coli swims in a pattern than resembles a random walk.

• E. coli moves up a gradient of attractant -

• Detects a net positive change in attractant concentration,

• Reduces the probability of a tumble and tends to continue going up the gradient.

• Swim/Runs - the cell keeps a rather constant direction which produce Counter clockwise

rotation.

• The runs last about 1 sec on average.

Counter clockwise swimming behavior in E. coli

Chemotaxis Behavior

• E. coli detects a concentration of repellent increases with time -

• The cell increases tumbling frequency,

• Tends to change direction and avoid swimming toward repellents.

• Tumbles - the bacterium stops and randomly changes direction which produce Clockwise

rotation.

• The tumbles about 0.1 sec on average.

Clockwise flagellar rotation in E.coli

Fig. Bacterial runs and tumbles are related to the rotation direction of the flagella

motors. When all motors spin counter-clockwise (CCW), the flagella turn in a bundle and cell is propelled forward. When one or more motors turn clockwise (CW), the cell tumbles and randomizes its orientation. The switching dynamics of a single motor from CCW to CW and back can be seen by tethering a cell to a surface by means of its flagellum, so that the motor spins the entire cell body (at frequencies of a few hertz due to the large viscous drag of the body).

• Addition of an attractant in space, increasing the attractant concentration from 0 to 1, but no

spatial gradients are formed.

• The cells sense an increase in attractant levels and suppress the tumbles: the tumbling

frequency of the cells plummets within about 0.1 sec.

• After a while, the tumbling frequency of the cells begins to increase, even though attractant is

still present. This process is called adaptation.

• Bacterial chemotaxis shows exact

adaptation:

“The steady-state tumbling frequency is

independent of attractant levels.”

Response & Exact Adaptation

Fig. The chemotaxis signal transduction network

Chemoreceptors form complexes with the CheA (kinase proein) and

CheW (adaptor protein)

CheA phosphorylates itself & transfers phosphoryl (P) groups to CheY

(diffusible messenger protein)

phosphorylated CheY interacts with the flagellar motors to induce

tumbles.

Rate of CheY dephosphorylation is greatly enhanced by

CheZ (phosphate activator)

Binding of attractants to the receptors decreases the rate of CheY

phosphorylation and tumbling is reduced

Note: Adaptation is provided by changes in the level of methylation of the

chemoreceptors: methylation increases the rate of CheY phosphorylation.

Pair of enzymes, CheR (methyl transferase) and CheB (methyl esterase),

add and remove methyl (m) groups.

Chemotaxis system of E. coli

• Michael Eisenbach (2011) Bacterial Chemotaxis. Book Title eLS

• Alon U, Surette MG, Barkai N and Leibler S (1999) Robustness in bacterial chemotaxis.

Nature 397:168-171

• An overview of E. coli chemotaxis, Parkinson Lab, Department of Biology - University of

Utah.

References