Dr.Hao Wang & Silogini Thanarajah

The role of motility and nutrients in a bacterial colony formation and competition

Outlines

• Definitions• Introduction• Model for simplified case (one bacterial strain

competition)• Mathematical Analysis• Theorems• Competition of two bacterial strains in a petri dish model.• Simulations in 1-D, 2-D space• Conclusion• Model for Liquid case

Definitions

Motile: Moving or having power to move spontaneously.

Immotile: Not moving or lacking the ability to move.

Agar: A dried hydrophilic, colloidal substance extracted from various species of red algae; used in solid culture media for bacteria and other microorganisms.

Introduction

• In most natural environments, bacteria fight with neighbors for space and nutrients.

• Most are harmless, some are beneficial and a few become a threat to our health when they grow and reproduce.

• Many but not all bacteria exhibit motility, i.e. self-propelled motion, under appropriate circumstances.

• Motility is an important part in the colonization of plant roots by bacteria.

• Also, colony formation could help clarify factors influencing biofilm formation and illuminate how groups control the fitness of bacteria.

Naturereviewsamicrobiology

www.nature.com,scienceblogs.com,pasteur.fr

• Bacteria display kinds of colony patterns according to the substrate softness and nutrients concentration.

• Previous studies showed four different colony shapes and recognized a morphological diagram by dividing into four regions like diffusion-limited aggregation-like, eden-like, concentric-ring and fluid spreading.

Pnas.org

• Purpose of this paper is to use bacteria as model organism to study competition and determine which strain will “win” in competition with other strain when the two are mixed in a petri dish.

• We plug these biological characteristics into simulation programs and observe the outcomes.

Agar method vs Liquid method(Bruce Levin’s group experiment)

Observation from experiments results:

• For agar case, motile strain dominates the community.

• For liquid case, immotile strain dominates the community.

Ratio of the 2 strains

immotile/motile

T0 0.9103

T24 0.1714

Ratio of the 2 strains

immotile/motile

T0 0.9057

T24 3.3218

Bacterial competition in a petri dish model

B1-motile strainB2-immotile strain

Bacteria-substrate modelwithout nutrient diffusion

Theorems

we placed motile and immotile bacterial strains in the middle of the petri dish and observed the pattern formation.

Simulations for 1-D space

Motile strain-Agar

Immotile strain-Agar

Motile vs Immotile

Resource-Agar

Motile strain and immotile strain total population over the space

Simulations for 2-D space

We placed motile strain in the middle and the

Immotile strain little far from the middle of the petri dish and observed the pattern formation after 1hr, 5hrs, 8hrs and 15hrs.

Observarion at t=1:• Motile and immotile strains are start to grow on the same position,

we placed.• Some of the nutrients consume by bacterial strains on the same

position.

Observation at t=5:• Motile strain move and grows around the middle of the

petri dish and immotile strain grows on the same position, like narrow.

• Nutrients consume around the middle of the petri dish.

Observation at t=8:• Motile strain move fast and grows to over lab immotile strain and

immotile strain face for the competition with motile strain for nutrients.

• More and more nutrients used by bacterial strains surrounding the middle of the petri dish.

Observation at t=15:• Motile strain grows everywhere even over immotile strain

and immotile strain don’t have enough nutrients to eat and survive.

• Almost all nutrients are used but some are still there.

Depends on the initial conditions we will get different pattern formation.

Conclusion• Bacteria always go extinct due to lack of nutrient after a long time

while some nutrient will always be remaining. If we incorporate a nutrient input as chemostat-type models, then the bacterial community can be sustained (“closed”->”open”).

• From computer stimulations (1-D case): If we put motile and immotile bacterial strains in the middle of the petri dish: initially both grow on the same position after that motile strain move fast and grow on the boundary but the immotile strain grow fast on the middle and finally both will die out. In this case motile strain is dominant. It is consistent to Bruce Levin’s group agar case. For liquid case we have to choose different nutrient equation (Liquid is moving everywhere).

• From 2-D case: If we put motile strain in the middle and the immotile strain little far from the middle of the petri dish: initially both strains grow on the same position as we placed; later, they overlap in some place, then they compete for nutrients such that a some strange patterns occur; after a long time, motile strain passes over immotile one and thus moves fast and grows everywhere and dominate the bacterial community; Finally (not shown in 2-D simulations), all bacteria go extinct due to “closed” system (no nutrient input).

Model for Liquid case

Motile-Liquid

Immotile-Liquid

Resource-Liquid

Motile strain and immotile strain total population over the space

Comparison to agar case

• Motile strain move fast and grow everywhere.

• Immotile strain grow with higher density than motile strain.

• In liquid case immotile strain is dominant.

Conclusion

• In agar case motile strain is dominant while liquid case immotile strain (liquid is moving everywhere but not agar).

Consistent to experiment results of Bruce Levin’s group

Thank you!