1. Source-Sink Population Dynamics
Facilitate Plasmid Host Range Evolution
Wesley Loftie-Eaton
Genevieve Metzger
Jacob Bayless da Costa
John Mittler
Eva Top

2. Plasmid Reproduction- Vertical

Mother to daughter cell
– When mother splits, some
plasmids end up in
daughter
– Occasionally, none make
it to daughter
• Known as
segregational loss

3. Plasmid Reproduction- Horizontal

Spread horizontally
(conjugation)
– Encodes pilus which
extends & attaches to
recipient cell
– Plasmid moves through
tube
– Host plasmid remains

4. Plasmid Fitness

Plasmids need their host

Accessory Genes
− Some plasmids are
symbiotic

Cells get protection

Plasmids get a home to
reproduce

5. Cryptic plasmids

do not confer fitness
− Use resources
− Offer nothing in return

Cells not burdened by plasmid
out compete plasmid bearers

6. Cryptic plasmid persistence

Cells not burdened by plasmid cost out
compete plasmid bearers

Requires adaptation to host to
compensate for lower host fitness:
− Lowered cost
− Increased conjugation
− Decreased segregation loss

This takes time and many adaptations
are host specific

7. How do cryptic plasmids adapt to naïve hosts?

8. Source Sink theory

Movement from good to poor patches
of habitat

Good habitat is known as source
− Positive net growth rate
− Often specialized for habitat

Poor habitat is known as sink
− Negative net growth rate
− Requires migration to maintain
population
− Animal has not had time to evolve
specific adaptations to selective
pressures

9. Source Sink theory- Example

10. Source-Sink Theory and Genetic Diversity

11. Genetic variation in the genome of
a plasmid from a source host
can facilitate persistence in a sink host
HYPOTHESIS

12. To determine experimentally if plasmid host range
expansion follows predictions of the source-sink theory of
ecology.
To use our experimental and computational approaches to
generate additional hypotheses about the spread of plasmid
borne accessory genes to naïve hosts.
1.
2.
OBJECTIVES

13. METHODOLOGY

14. Plasmid
pBP136.Gm
IncP-1β
{Natively cryptic}
Source host
E. coli AT1306 (dap4, ΔmutS)
Auxotrophic mutant, hypermutator
Sink host
• Shewanella oneidensis MR-1
• Does not maintain pBP136
• Evolution of pBP136 can increase
persistence of pBP136 in this
host
MODEL SYSTEM

15. METHODOLOGY

16. First Result- What does it tell us?

No Alteration of washout curve
– Can assume that no major
evolutionary events took place

Washout curve
− In 72 hours plasmid is reduced to
~20%
• Plasmid is not stable in the sink
host.

17. dN/dt = rN*N*C/(C+Km) + sp2*rp2*P2*C/(C+Km) - h1P1N - h2P2N - D*N
change in N = {growth} + {segregation of P2} - {conjugation from P1 and P2} - {chemostat drainage}
dP2/dt = rp2*P2*(1-sp2)*C/(C+Km) + h1P1N + h2P2N - D*P2
change in P2 = {growth} - {segregation of P2} + {conjugation from P1 and P2} - {chemostat drainage}
dC/dt = D*C0 - D*C - y*rN*N*C/(C+Km) - y*rp2*P2*C/(C+Km)
change in C = {flow into chemostat} - {flow out of chemostat} - {consumption by N} - {consumption by P2}
Assumptions
• Cells grow with Monod type kinetics in which growth rate slows down once resource approaches
the saturation constant, Ks.
• Conjugation is growth independent. (under investigation)
• Conjugation rate increases proportionally with density
Mathematical Model

18. change in N = {growth} + {segregation of P2} - {conjugation from P1 and P2} - {chemostat drainage}
change in P2 = {growth} - {segregation of P2} + {conjugation from P1 and P2} - {chemostat drainage}
change in C = {flow into chemostat} - {flow out of chemostat} - {consumption by N} - {consumption by P2}
N -> Plasmid-free cells | P2 -> Plasmid-containing cells | C -> Resource concentration
Mathematical Model
Assumptions
• Cells grow with Monod type kinetics in which growth rate slows down once resource approaches
the saturation constant, Ks.
• Conjugation is growth independent. (under investigation)
• Conjugation rate increases proportionally with density

19. EXPECTATION – Mutations Increasing Stability

20. First Result- Interpreted in Model

21. First Result- Interpreted in Model

22. 
Measuring values of biological constants
− Required to test assumptions of the mathematical model
Where we are NOW

23. 
If cryptic plasmids do follow source sink, we can predict their
spread
– Extrapolate to plasmids accessory genes which act
cryptic in new habitats
• Ab resistance genes.
Why?

24. Funding
BEACON NSF Cooperative Agreement No. DBI-0939454
NIGMS NIH Award Number P30 GM103324
Acknowledgements
People
Kiara Garcia (HOIST)
Brandon Cornwell | Thibault Stalder