Download Silogini Thanarajah

The role of motility and nutrients in a bacterial
colony formation and competition
Dr.Hao Wang & Silogini Thanarajah
Outlines
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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)
Ratio of the 2
strains
immotile/motile
Ratio of the 2
strains
immotile/motile
T0
0.9103
T0
0.9057
T24
0.1714
T24
3.3218
Observation from experiments results:
• For agar case, motile strain dominates the community.
• For liquid case, immotile strain dominates the community.
Bacterial competition in a petri dish model
B1-motile strain
B2-immotile strain
Bacteria-substrate model
without nutrient diffusion
Theorems
Simulations for 1-D space
we placed motile and immotile bacterial strains in the
middle of the petri dish and observed the pattern
formation.
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!