The Evolution of Virulence 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.

• Bacteria and virus are grown in microtiter plates, which impose a metapopulation structure.

E. coli

• 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”

• The transitions in the state of any well (due to dilution or immigration) can be gauged empirically and organized into a transition matrix.

reproduction infection dilution dilution dilution

T4

Unrestricted

Restricted

Stochastic Cellular Automata

density

(10

7 /m

L)

time

phage

bacteria

density

(10

7 /m

L)

time

phage

bacteriaF

F

Migration Pattern Ecological Dynamics Spatial Dynamics

Predictions: Under restricted migration,(1) metapopulation dynamics are more stable (2) phage mean density is lower and bacterial mean

density is higher

Restricted

Unrestricted

True Cellular AutomataMigration Pattern

m

m

metapopulations

sterileshell

arm

Ecological Dynamics Spatial Dynamics

Predictions: Under restricted migration, (1) metapopulation dynamics are more stable (2) phage mean density is lower and bacterial

mean density is higher

• 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.

Evolved Phage Properties

productivitycompetitive ability

A Microbial ‘Tragedy’• Different migration treatments

have evolutionarily favored different strategies:- “Rapacious” phage in the

Unrestricted treatment- “Prudent” phage in the Restricted

treatment.

• We have a tragedy of the commons:

rapacious

prudent

phage evolve in theUnrestricted treatment

phage persist in theRestricted treatment

dilution dilution dilution

dilutiondilution reproduction

reproduction

- 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

reproduction &competition

Rapacious phage fare better in the Unrestricted treatment for two reasons:

1) Mixing of phage types is more likely (leading to more tragedies)

2) Persistence is less important (any well’s tragedy is less severe)

Averting the Tragedy of the Commons

Restricted Migration with Evolutiondensity

(10

7 /m

L)

prudent phage

bacteria

time

Unrestricted: Tragedy Realized

rapacious phagerapacious phage

prudent phagebacteria

density

(10

7 /m

L)

time

Restricted: Tragedy Averted

rapacious phagerapacious phage

Take 3 minutes to talk to your neighbor about the following:

So far the description of rapacious and prudent phage has been at the population level.What would you want to know about the phage itselfin terms of its evolution?What would you want to knowphenotypically? Genetically?

Lytic phage life cycle

The Evolution of Phage Life History

adsoption& injection

progeny productionhost lysis

phage (pathogen)

bacteria (host)

• 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 shorter-lived outside their host.

From Demes to Genes

T T T T

inner membrane

outer membrane

periplasmic spaceRIRIT

E E E

E E EE E E

•••TAAAAAT•••

wild-type

•••TAAAAT•••

rI mutant

OUTSIDE HOST CELL

INSIDE HOST CELL

rIA

mutantrIB

mutantwildtype

• Current working model (Tran et al. 2005):- At a specific time, holins

disrupt the inner membrane allowing endolysin to pass.

- Cell wall is degraded and the cell lyses.

- 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)

• 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.

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

• Relative to wild type, the engineered rI mutants are:- More competitive for

hosts- Less productive when

alone

• Mutations at rI are sufficient to generate a tradeoff between competitive ability and productivity.

• Thus, we have rapacious and prudent alleles at the rI locus: a genetic basis of the tragedy of the commons.

adsoption& injection

progeny productionhost lysis

phage (pathogen)

bacteria (host)

Acknowledgements

Yen Nhan Dang

RoxyVouk

MilyGualu

JoshNahum

KelseaLaegreid

JodiStewart

Stacy Schneider

Christal Eshelman

Beth Halsne

JakeCooper

BrandonRogers

SpencerSmith

SterlingSawaya

ChrisShyue

KelseyHobbs

SaraDrescher

ShawnDecew

The Evolution of Virulence 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