Source-Sink Population Dynamics Facilitate Plasmid Host Range Evolution

Wesley Loftie-EatonGenevieve Metzger

Jacob Bayless da CostaJohn Mittler

Eva Top

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

Plasmid Reproduction- Horizontal

Spread horizontally (conjugation)

– Encodes pilus which extends & attaches to recipient cell

– Plasmid moves through tube

– Host plasmid remains

Plasmid Fitness

Plasmids need their host Accessory Genes

Some plasmids are symbiotic

Cells get protection Plasmids get a home to

reproduce

Cryptic plasmids

do not confer fitness

Use resources Offer nothing in return

Cells not burdened by plasmid out compete plasmid bearers

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

How do cryptic plasmids adapt to naïve hosts?

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

Source Sink theory- Example

Source-Sink Theory and Genetic Diversity

Genetic variation in the genome of a plasmid from a source host

can facilitate persistence in a sink host

HYPOTHESIS

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

METHODOLOGY

PlasmidpBP136.GmIncP-1β{Natively cryptic}

Source hostE. 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

METHODOLOGY

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.

dN/dt = rN*N*C/(C+Km) + sp2*rp2*P2*C/(C+Km) - h1P1N - h2P2N - D*Nchange 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*P2change 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

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

EXPECTATION – Mutations Increasing Stability

First Result- Interpreted in Model

First Result- Interpreted in Model

Measuring values of biological constants

Required to test assumptions of the mathematical model

Where we are NOW

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?

Funding

BEACON NSF Cooperative Agreement No. DBI-0939454

NIGMS NIH Award Number P30 GM103324

Acknowledgements

People

Kiara Garcia (HOIST)Brandon Cornwell | Thibault Stalder