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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