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For a non-motile colony, assume that the only
contribution to colony size is from the bacteria on the periphery and that
the bacteria density is roughly constant.
Growth is parameter independent. |
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The cooperative whirls and jets of motile
bacteria can destabilize the colony boundary, which can result in
branching. Surface wetness regulates the colony expansion. The density is
not necessarily constant. |
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Motile strains like M22(84) can, under ideal
conditions, swim towards a chemical signal, increasing colony size. |
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Non-motile strains, such as 5:7(M), cannot
swim. Increase in colony size is
due to the increase in total mass. |
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Nutrient levels and temperature regulate the
reproduction rate of the bacteria. |
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In soft agar, bacteria and nutrients can
diffuse. |
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While the agar media does lose moisture, this
can be limited by placing the plates in a humidity chamber and minimizing
exposure to air. |
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For colonies grown on dry agar (10 g/l), the
growth is linear and depends on nutrient concentration. |
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For colonies grown on soft agar (6 g/l), the
growth is much faster. In these wet
conditions, the edge of the colony is destabilized by the whirls and jets
of bacteria. |
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Agar plates with Tryptose Blood nutrient were
made with varying amounts of agar and TB.
The conditions, chosen for variety of expected results, were 6
grams/liter agar and 10 g/l agar, with either 0.5xTB or 2.0xTB. |
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After inoculation with either motile or
non-motile strains of bacteria, the plates were stored in a humidity
chamber in an incubator at 24°C. |
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After nine hours, measurements began and
continued until 60.5 hours after inoculation. |
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Golding, Kozlovsky, Cohen, and Ben-Jacobs
propose that the velocity of the bacteria define the colony shape. They use a death term and chemotactic
signaling in their models. [1] |
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Kawasaki claims that the diffusion coefficient
of the bacteria depends on their own density, the density of the nutrients,
and a stochastic term to introduce frontal instability. [4] |
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Kessler and Levine employ a cutoff mechanism in
the growth term, which allows the development of branching patterns. [4] |
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Kitsunezaki has developed a density dependent
model that divides bacteria based on their motility properties: active or
inactive. [4] |
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Mimura, Sakaguchi, and Matsushita account for
the motility rate of the active bacteria, the diffusion rate of nutrients,
the growth rate of the bacteria, and the rate of conversion of active
bacteria into inactive bacteria. By
varying the agar and nutrient concentrations, they obtain four different
colony patterns. [4] |
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Mendelson, Bourque, Wilkening, Anderson,
Watkins, and Salhi have described the dynamics of the bacteria as
interconnected levels of coordination that result in superpatterns which
expend the colony edge. [2,3] |
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Golding, I., Y. Kozlovsky, I. Cohen, and E.
Ben-Jacobs. 1998. Studies of bacterial branching growth
using reaction-diffusion models for colonial development. Physica A. 260: 510-554. |
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2. Mendelson, N. H., A. Bourque, K.
Wilkening, K. R. Anderson, and J. C. Watkins. 1999. Organized cell
swimming motions in Bacillus subtilis colonies: patterns of short-lived whirls and jets. J. Bacteiol. 181: 600-609. |
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Mendelson, N. H., and B. Salhi. 1996.
Patterns of reporter gene expression in the phase diagram of Bacillus
subtilis colony forms. J.
Bacteriol. 178: 1980-1989. |
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Mimura, M., H. Sakaguchi, and M.
Matsushita. 2000. Reaction-diffusion modeling of bacterial
colony patterns. Physica A. 282:
283-303. |
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I would like to express my gratitude towards
Drs. J. Lega and N. Mendelson for their advising on this project, and also
Tim Carrol for assisting in laboratory experiments. |
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This material is based upon work supported by
the National Science Foundation under VIGRE Grant No DMS9977116. Any opinions, findings, and conclusions
or recommendations expressed in this material are those of the author and
do not necessarily reflect the views of the National Science Foundation. |
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