Transcript E3_Virulence_2011Part 3 - MicrobialEvolution.org

The Evolution of Virulence
Lecture Outline
• Introduction to virulence theory
• Transmission mode experiment
• Transmission timing experiment
• Metapopulation experiment
• Summary
A Model Host-Pathogen System
• We used Escherichia coli (host)
and phage T4 (pathogen) to
study the dynamics of a large
host-pathogen metapopulation.
T4
E. coli
• Bacteria and virus are grown in
microtiter plates, which impose
a metapopulation structure.
• Bacteria and phage do not coexist in a well. There are three types of wells:
empty, bacteria-filled, and phage-filled, exhibiting “rock-paper-scissors”
reproduction
infection
dilution
dilution
dilution
• The transitions in the state of any well (due to dilution or immigration) can
be gauged empirically and organized into a transition matrix.
Stochastic Cellular Automata
F
Ecological Dynamics
density (107/mL)
Restricted
Migration Pattern
Spatial Dynamics
phage
bacteria
F
density (107/mL)
Unrestricted
time
phage
bacteria
time
Predictions: Under restricted migration,
(1) metapopulation dynamics are more stable
(2) phage mean density is lower and bacterial mean density is higher
True Cellular Automata
Restricted
Migration Pattern
Ecological Dynamics
Spatial Dynamics
m
Unrestricted
arm
metapopulations
m
sterile
shell
Predictions: Under restricted migration,
(1) metapopulation dynamics are more stable
(2) phage mean density is lower and bacterial
mean density is higher
Evolved Phage Properties
• For each evolved isolate, we measured:
- productivity: the average number of
progeny phage per parent
- competitive ability: how well the
evolved isolate does in head-to-head
competition with a marked mutant
• In both cases, we controlled for the initial
ratio of phage to bacteria (called the
“multiplicity of infection”)
• Restricted phage were significantly more
productive
• Unrestricted phage were significantly
more competitive
• After pooling the data, we found (for 2 of 3
MOI levels) a significant negative
correlation between productivity and
competitive ability.
•
Different migration treatments have
evolutionarily favored different strategies:
-
•
We have a tragedy of the commons:
-
•
“Rapacious” phage in the Unrestricted
treatment
“Prudent” phage in the Restricted treatment.
Rapacious phage outcompete prudent
phage in a mixed population
Pure wells of prudent phage have
higher progeny outputs than pure
wells of rapacious phage.
Why are rapacious phage found in
the Unrestricted treatment?
-
As rapacious mutants are generated,
they take over, lowering productivity
Less productive phage are less
persistent.
The probability of migration to hosts
is lower in the Restricted treatment
This limited host access in the
Restricted treatment makes rapacious
phage extinction-prone
competitive ability
A Microbial ‘Tragedy’
rapacious phage evolve in the
Unrestricted treatment
prudent phage
persist in the
Restricted
treatment
productivity
dilution
dilution
reproduction
dilution
dilutionreproduction
reproduction &
competition
dilution
Averting the Tragedy of the Commons
density (107/mL)
Unrestricted: Tragedy Realized
prudent phage
bacteria
1) Mixing of phage types is more likely
(leading to more tragedies)
rapacious phage
2) Persistence is less important (any well’s
tragedy is less severe)
Restricted Migration with Evolution
Take 3 minutes to talk to your
neighbor about the following:
time
Restricted: Tragedy Averted
density (107/mL)
Rapacious phage fare better in the
Unrestricted treatment for two reasons:
bacteria
prudent phage
rapacious phage
time
So far the description of
rapacious and prudent phage
has been at the population level.
What would you want to know
about the phage itself
in terms of its evolution?
What would you want to know
phenotypically? Genetically?
The Evolution of Phage Life History
• The life cycle of lytic phage:
- Adsorption to host and injection of
phage genome
- Production of progeny particles in
the host
- Lysis of the host and progeny
release
• Phage evolved under Unrestricted
Migration are more infective,
virulent, and tend to be shorterlived outside their host.
Lytic phage life cycle
phage (pathogen)
bacteria (host)
host lysis
adsoption
& injection
progeny production
From Demes to Genes
• Current working model (Tran et al. 2005):
- At a specific time, holins disrupt outer
the inner
membrane
membrane allowing endolysin to pass.
periplasmic space
- Cell wall is degraded and the cell lyses.
inner membrane
- Progeny phage are released.
• Gene t is an attractive candidate locus:
- Non-synonymous mutations in holins
produce different latent periods.
- Null mutants overproduce progeny
without lytic release (‘t’ from Tithonus)
OUTSIDE HOST CELL
T
T
RI T
E
E
T
T
RI
E
INSIDE HOST CELL
E
E
rIA
mutant
E
wild
type
• We found no mutations in gene t.
• Gene rI codes for an antiholin that forms
a complex with the holin; mutations in rI
can hasten lysis (shorten latent period).
• We found two unique deletions in rI:
- More rI mutants were found in the
Unrestricted treatment.
- The rI mutations are sufficient to have
visible effects on the host population.
•••TAAAAAT•••
wild-type
•••TAAAAT•••
rI mutant
rIB
mutant
A Genetic Basis for the Tragedy of the Commons
• Relative to wild type, the
engineered rI mutants have:
- A shorter latent period
- A smaller burst size
(pathogen)
• Relativephage
to wild
type, the adsoption
engineered
rI mutants
are:& injection
bacteria
(host)
- More competitive for hosts
- Less productive when alone
• Mutations at rI are
sufficient to generate a
tradeoff between
competitive ability and
productivity.
progeny production
• Thus, host
we lysis
have rapacious
and prudent alleles at the rI
locus: a genetic basis of the
tragedy of the commons.
Acknowledgements
Roxy
Vouk
Mily
Gualu
Christal
Eshelman
Beth
Halsne
Jodi
Stewart
Josh
Nahum
Stacy
Kelsea
Schneider Laegreid
Jake
Cooper
Yen Nhan
Dang
Sterling Brandon Spencer
Sawaya Rogers
Smith
Kelsey
Hobbs
Sara
Drescher
Chris
Shyue
Shawn
Decew
The Evolution of Virulence
Lecture Outline
• Introduction to virulence theory
• Transmission mode experiment
• Transmission timing experiment
• Metapopulation experiment
• Summary
Summary
• Virulence is the damage to a host caused by an inhabiting pathogen
(increased mortality, decreased reproduction, etc.)
• Virulence varies between and within pathogen species (both naturally
and in the laboratory).
• The conventional wisdom is that virulence should decrease
evolutionarily, but it is sometimes predicted to increase if it trades off
with transmission or within host competition.
• Many factors (host density, superinfection frequency, environmental
reservoirs) will affect the predicted level of virulence and some of these
factors have been experimentally tested:
- Bull et al. found higher virulence in a phage pathogen under horizontal transmission.
- Cooper et al. found higher virulence in an insect pathogen under early transmission.
- The pattern of migration in a metapopulation can affect the evolution of virulence