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Social interactions and cheating
in the microbial world
Steve Allison
UC Irvine
Ecology and Evolutionary Biology
Earth System Science
[email protected]
What is a microbe?
• Bacteria, fungi, and protists
• Interact with environment at microscopic
scales
NIH
David Read
Microbes are diverse and abundant
• 1 billion bacteria in a handful
of soil
• 1 million in a mouthful of
seawater
• More biomass than in plants
• 10 bacterial cells for every 1
human cell in your body
• 1-10 million species of
microbes globally
Whitman et al. 1998, Norm Pace
Why microbes are important
• Global carbon cycling
• Nutrient cycling, soil fertility (agriculture)
• Biomolecules; drug discovery
Microbe “social lives”
• Many microbes live in complex communities
• They need energy and nutrients
• Microbial behaviors:
 Cooperation
 Communication
 Cheating
• Behaviors require production of chemicals
that operate outside the cell
Cooperation example: Biofilms
Polysaccharides
Where are biofilms?
•
•
•
•
•
On your teeth
Inside showerheads
On rocks in streams
Between particles of soil
Almost anywhere with liquid water
Communication: Quorum sensing
• A behavior is triggered when cell densities
cross a threshold
• Requires production of an inducer molecule
Inducer
Product
Low density
High density
Reasons for quorum sensing
• Coordination of group behavior
• Evasion of host defenses by pathogen
• Can trigger biofilm formation (or cessation)
Costs and benefits of cooperation
Nutrients
Oxygen
Polysaccharides
Evolution of cheating
• Making biofilm is expensive
• All microbes benefit
• Natural selection favors cheater mutants
Cheaters
Paul Rainey
Do cheaters win?
• What might prevent cheaters from taking
over?
 Producers must get a bigger share of the benefits
 Products benefit close relatives, not cheaters (kin
selection)
 Spatial structure
Spatial structure favors producers
Cheater
Bottleneck
Producer
Chuang et al. 2009
Extracellular enzymes
• Some microbes “forage” with enzymes
• They eat complex chemicals produced by
plants, animals, and other microbes
• Prevents the world from filling with dead
bodies
• Enzymes contain carbon and nitrogen
Enzyme model
Substrate
Enzyme
Product
Processes:
•Metabolism
•Reproduction
•Input
•Enzyme synthesis
•Diffusion
•Enzyme catalysis
•Product uptake
Allison 2005
Producer
Cheater
Coexistence at low diffusion
Population size
Increasing enzyme diffusion
Cheater
Producer
Time
Cheater
Producer
Population size
Time
Time
Cheaper enzymes
Population size
Increasing enzyme diffusion
Cheating reduces enzyme function
Product
concentration
(mg/l)
Low product formation
Cheater experiments
• Pseudomonas fluorescens
• Producer: degrades
protein with protease
• Cheater: protease
knockout
• Well mixed flasks
• Protein medium
• Digested protein medium
Collaborators: Mindy Ta,
Lucy Lu, Jennifer Chen
Cheaters suppress producers on
protein
Protein
1.2
Producers
Optical Density
1.0
0.8
Competition
0.6
Cheaters
0.4
0.2
0.0
0
20
40
Time (h)
60
80
Cheaters suppress producers on
protein
Protein
Digested protein
1.2
1.5
Producers
0.8
Competition
0.6
Cheaters
0.4
Optical Density
Optical Density
1.0
1.0
0.5
0.2
0.0
0.0
0
20
40
Time (h)
60
80
0
20
40
Time (h)
60
80
Cheaters suppress protease production
Protein
Activity (mg/ml/h)
0.8
0.6
Producers
0.4
0.2
Competition
Cheaters
0.0
0
10
20
30
40
Time (h)
50
60
70
Role of enzymes in the ocean
MICRO time series
Newport Beach, CA
Collaborators: Adam
Martiny, Sunny Jiang,
numerous undergrads
WHOI
Marine snow
LAP Activity (micromol/L/h)
Most peptidase activity is bound
<0.2 micron
<2.7 micron
Unfiltered
600
500
400
300
200
100
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
PHOS Activity (micromol/L/h)
Phosphatase is freely dissolved
<0.2 micron
<2.7 micron
Unfiltered
150
100
50
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Ocean carbon sequestration
CO2
CO2, Nutrients
Phytoplankton
Cheater
Surface ocean
Deep ocean
Producer
Take-home messages
• Microbes are diverse and important
• Microbes cooperate, but natural selection can
favor cheating
• Microbial behavior and evolution has
consequences for ecosystems
• Thank you: Lab members, NSF, audience