Biology Reference
In-Depth Information
culture well, all three subpopulations grew to establish a stable consortium. By varying the
distance between the culture wells, and thus the subpopulations, the stability of the
consortium was altered. When the culture wells were seeded with a mixture of all three
populations (i.e. a separation distance of 0
s
viability was observed. Similarly, when the distance of the culture wells was increased too
much (
μ
m), a marked decrease in the consortium
'
μ
m), the consortium also declined in viability. Only when the culture wells
were separated at an intermediate distance was the consortium viable. From modeling, the
authors determined that spatially separating the subpopulations allowed for the balance
between consumption of nutrients (competition) and the performance of each population
B
1800
s
'
vital task (cooperation) within the consortium.
Recently, it has been demonstrated that coculturing populations within close proximity to
each other can also generate a functional consortium. Park et al. created a microfluidic
device to encapsulate distinct populations of bacteria in aqueous droplets.
41
Using this
system, they coencapsulated three auxotrophic populations, where the distribution of each
population within a droplet could be predicted using a Poisson distribution. When tyrosine
(Y-) and tryptophan (W-) auxotrophs were encapsulated in separate droplets, neither could
grow in a minimal medium. However, when coencapsulated within the same droplet, both
auxotrophic populations grew in minimal medium by exchanging essential metabolites.
Interestingly, amino acids secreted and exchanged within the same droplet could not
diffuse across the droplet membrane. As such, while a population of droplets consisted
of a distribution of Y- only, W- only, and Y- and W- strains, only those droplets
containing Y- and W- were observed to grow. The authors then extended this concept and
coencapsulated three auxotrophic populations, the aforementioned auxotrophs and a serine
auxotroph (S-), which could not exchange metabolites with either Y- or W-. Only when the
droplets contained a combination of S-, Y-, and W-, was S- able to grow. By varying the
initial ratios of the populations, the authors were able to predict and experimentally verify
the distribution of the coencapsulated populations.
254
The spatial organization of distinct bacterial populations can also affect their ability to
cooperate (
Fig. 13.4b
). In 2011, Kim et al. created a synthetic consortium consisting of
two bacteria strains,
Sphingobium chlorophenolicum
, which can degrade the environmental
pollutant pentachlorophenol (PCP), and
Ralstonia metallidurans
, which can convert Hg(II)
into Hg(0).
42
The authors sought to develop a consortium that could metabolize both
environmental pollutants simultaneously. When both populations were well-mixed,
the consortium could not metabolize both pollutants, as the PCP concentration remained
nearly unchanged. The authors determined that Hg(II) was inherently toxic to
S.
chlorophenolicum
, and thus prevented it from degrading PCP. As such, they hypothesized that
a spatially structured community, consisting of a core-shell spatial structure, would improve
the ability of the consortium to metabolize both pollutants. They hypothesized that they
could shelter
S. chlorophenolicum
from Hg(II) by placing it on the inside of the structure,
while placing
R. metallidurans
on the outside of the structure, thus forming a protective shell
around
S. chlorophenolicum
. Using a mathematical model, they rationally determined the
spatial scale of the structure. They predicted that an outer layer of
R. metallidurans
that was
too thin would be insufficient to convert Hg(II) to Hg(0) before Hg(II) diffused into the
S.
chlorophenolicum
layer killing the population. Alternatively, if the
R. metallidurans
outer layer
was too thick, the diffusion rate of PCP would be insufficient to allow sufficiently fast
PCP degradation by
S. chlorophenolicum.
After predicting the appropriate scale of the outer
layer, they used a microfluidic laminar flow system to create core-shell fibers consisting of
the two bacterial populations. In fibers consisting of an
R. metallidurans
shell and an
S. chlorophenolicum
core, both PCP and Hg(II) were metabolized. Alternatively, when the
orientations of the populations were switched, a significant reduction in PCP degradation
was observed. As such, the authors developed a novel method by which a consortium can
be created and arranged to perform novel tasks.
