Lecture 22: Microbial cell-to-cell communication
Today:
(wikipedia)
And now, social behaviors in microbes!
How do social behaviors arise?
DNA/genes
cells
genome
community/colony
organisms
ecosystem
What common principles lead to more complex biological organization?
One way to think about this is to look at how complex behaviors arise in general:
“Traditional” Darwinian view of evolution
Cells growing on a nutrient source
A mutant arises in a gene that allows faster growth under these conditions
The mutant has a fitness advantage and now the new copy of the gene becomes much more frequent in the population
How do you reconcile this view with “social” behaviors?
Worker Bees
(wikipedia)
Multicellularity
(wikipedia)
One approach: redefine fitness as “inclusive”!
Inclusive fitness theory: a model for social evolution
Recipient
Actor
social action
offspring
offspring
+ or -
+ or -
r, how related are Actor/Recipient (probability they share the gene(s) for the social action)?
r1, how related are Actor and its offspring?
r2, how related are Actor and Recipient’s offspring?
m, the change in actor’s expected offspring as a result of the action
n, the change in recipient’s expected offspring as a result of the action
Inclusive fitness theory: a model for social evolution
Recipient
Actor
social action
offspring
offspring
+ or -
+ or -
r
r1
r2
m
n
| Effect on Recipient’s expected offspring number (n) | ||
Gain (n > 0) | Loss (n < 0) | ||
Effect on Actor’s expected offspring number (m) | Gain (m > 0) | | |
Loss (m < 0) | | | |
Cooperation
Altruism
Selfishness
Spite
Hamilton’s Rule
A gene G is associated with a social action.
The gene will spread if the total change in frequency brought about by the social action is positive, or if the inclusive fitness is positive!
Hamilton’s Rule
Bourke, Principles of Social Evolution
Hamilton’s Rule asserts that a social phenotype that hurts the fitness of the actor can still arise evolutionarily if it suitably increases the fitness of a recipient that’s closely related!
What “social behaviors” can bacteria engage in??
Quorum Sensing: Bacterial Communication
Vibrio fischeri
Ocean-dwelling V. fischeri only produce luminescence at high cell density.
How?
Luminescence of the Hawaiian Bobtail Squid
V. fischeri live within E. scolopes tissue
sand
shallow water
shadow
Hawaiian Monk Seal
luminescent V. fischeri (1011 cells / mL!!)
The squid goes to bed in the morning
V. fischeri
V. fischeri mostly purged by pump during sleep
Luminescence turns on at night
V. fischeri
V. fischeri mostly purged by pump during sleep
How do V. fischeri turn on luminescence only at high density?
“Autoinducer” molecule
Low cell density > low concentration of autoinducers
High cell density > high concentration of autoinducers
How do V. fischeri turn on luminescence only at high density?
“Autoinducer” molecule
Low cell density > low concentration of autoinducers
High cell density > high concentration of autoinducers
How do V. fischeri turn on luminescence only at high density?
luminescence genes
autoinducer
LuxI
LuxR
Transcription of luminescence genes!
Quorum Sensing
“Autoinducer” or “Quorum Sensing” molecule
Low cell density > low concentration of autoinducers
High cell density > high concentration of autoinducers
Activate multicellular behaviors
Quorum Sensing molecules are species-specific
Two major QS paradigms
Gram negative
Gram positive
QS molecules (AHL) pass through membrane
QS molecules (polypeptides) activate membrane receptors
What kind of behaviors are regulated by quorum sensing?
Behaviors that would not be useful when there is not a large group of bacteria!
Luminescence in V. fischeri
competence
Competence in B. subtilis
What kind of behaviors are regulated by quorum sensing?
Behaviors that would not be useful when there is not a large group of bacteria!
Virulence in S. typhimurium
Mating/DNA transfer in A. tumefaciens
What makes a phenotype “Quorum Sensing”?
Regulated by a species-specific extracellular molecule
Clear density-dependence
Increasing density
Quorum sensing has remarkable synthetic biology applications!
First, a non-quorum sensing example, a synthetic oscillator circuit:
E. coli
A synthetic oscillating E. coli
The normal repressilator is very noisy, bad
Behaviors of different cells under different conditions:
How can we make this system more robust and reliable?
What if we coupled many cells together to cooperate? Would that reduce the noisiness?
A synthetic quorum sensing oscillator
luxI
produces AHL QS molecule
AHL
aiiA
degrades AHL QS molecule
AHL
yemGFP
AHL
reporters on QS activity
All QS-regulated!
LuxR
How does the system work?
Should result in oscillations in the production of YemGFP. Does it?
GFP
Oscillation period should depend on flow rate
If fluid flow is inhibiting quorum sensing by removing AHL from the system, oscillations should become slower as flow rate increases because it will then take longer for the cells to build up the critical AHL concentration
Oscillation period increases with flow rate
fluid flow velocity
QS oscillations should propagate in space
AHL
Because AHL 1) can diffuse through space, and 2) is coupled to its own production, if a group of cells reaches critical AHL concentration and starts a QS pulse, it should propagate through space.
QS oscillations should propagate in space
AHL
Because AHL 1) can diffuse through space, and 2) is coupled to its own production, if a group of cells reaches critical AHL concentration and starts a QS pulse, it should propagate through space.
A model predicts spatial QS waves
Within one cell
Across a spatially extended population
What have we learned?