Transcript Read Berg, Chapter 3

Chemotaxis and motility
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~2M years
Homo erectus
3.6 B years
~400,000 years
Homo sapiens
1678
AVL
~400 years
Chemotaxis
1881: oxygen and light
Engelmann
Read Berg, Chapter 2
1836
Ehrenberg
~200 years 2
Anton van Leeuwenhoek
(1632 -1723)
“I now saw very plainly that these were little eels, or worms, lying all huddled up together
and wriggling; just as if you saw, with the naked eye, a whole tubful of very little eels and water,
with the eels a-squirming among one another: and the whole water seemed to be alive with these
multifarious animalcules. This was for me, among all the marvels that I have discovered in nature,
the most marvelous of all; and I must say, for my part, that no more pleasant sight has ever yet come
before my eye than these many thousands of living creatures, seen all alive in a little drop of water,
moving among one another, each several creatures having its own proper motion”.
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(Original image was a hand-drawing)
Pfeffer assay (1884): Congregation of bacteria at mouth of capillary containing meat extract
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80 years later….
1969 Julius Adler describes chemoreceptors in bacteria, a discovery demonstrating that
bacteria can sense and process environmental information. His method involved inserting
a tube of chemicals into a solution of bacteria and then counting the number of bacteria
that swam to the chemical.
"That's one small step for a man, one giant leap for mankind."
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Adler introduced the quantitative capillary assay, ushering in the modern era of
chemotaxis
10 mM L-serine
10 mM phenol
Attraction and repulsion
Read Berg, Chapter 3
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A kid in a candy store
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Summary of Adler’s experiments (1969)
Q: What is E. coli attracted to?
1. Defined chemicals were tested as attractants.
Galactose
Ribose
Aspartate
Serine
Chemotaxis towards glucose but not toward glycerol
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Q: Is attraction related to the ability to metabolize the chemical?
What would be a simple way to test if an attractant was being metabolized?
2. Attractants were metabolized, but not all metabolized chemicals
were attractants
For example, both glucose and glycerol are metabolized,
but only glucose is an attractant
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Q: For attractants identified, was metabolism essential?
Galactose
Ribose
Aspartate
Serine
3. Attractants could be metabolized, but metabolism was not necessary.
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Q: Do non-metabolizable analogs attract?
4. Attractants could be metabolized, but metabolism was not necessary.
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E. coli pays attention to things of low mw, among them oxygen, acids and
bases, sugars, amino acids and dipeptides.
Taste will do. Consumption is not necessary.
Howard Berg ‘E. coli in Motion’
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Q: How are the attractants being recognized?
5. Genetic evidence for external receptors for attractants.
6. Receptors recognize structure of attractants.
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Q: How many receptors?
Fucose/galactose
Aspartate/serine
galactose
serine
Structurally related compounds compete
7. Evidence for structural recognition of attractant by receptors, and for
different classes of receptors.
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Receptors have specificity
Gal BP-Trg
Glu BP–Trg
Rib BP -Trg
Tar
Tsr
8. Five different classes of receptors.
How can you test this conclusion?
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What if there were mutants that did not respond to all five attractants?
Non-chemotactic point mutations
9. First molecular scheme for how chemotaxis might work.
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Plate assay for chemotaxis
Wild-type
Che-
LB (nutrient-rich) soft agar (0.3%)
Read Berg, Chapter 3
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Swimming E. coli
3D Random Walk
Fluorescent Anti-antibody
EM
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Darkfield Microscopy (1976)
Run-Tumble
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Helical transformations in fluorescently-labeled flagella (2000)
Alexa fluor dyes
Rowland Institute at Harvard – Howard Berg
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The chemotaxis signaling pathway in E. coli/Salmonella
CW
CCW
CCW
CCW
CW
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Taken from various reviews
Small size, big problems
The miniscule size of bacteria consigns them to a life that is dominated by viscous drag and Brownian
motion. (An equivalent scenario would be a human trying to swim in a pool of molasses.) Their small
mass confers such little momentum, that if the flagellar motors of an E. coli cell were to stop turning,
water viscosity would bring the cell to a full stop after a coasting distance of less than 1 Å. Yet, despite
these daunting physical constraints, E. coli cells swim at speeds of 10-20 body lengths per second.
Amazing.
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Taken from J. Parkinson