Biology Reference
In-Depth Information
cells were not detected in the CFP cell biomass. Interestingly, when the authors removed
effluent from the biofilm before and after the formation of this structured biofilm, a
difference in the ability to colonize a downstream location was observed. Specifically,
cells removed from the biofilm prior to the formation of the structured biofilm formed a
downstream biofilm that accumulated less biomass as compared to when the CFP strain
was grown alone and did not form a structured biofilm. Conversely, when effluent from the
biofilm was taken after the structured biofilm was formed, the cells quickly initiated
structurally layered biofilms that produced more biomass than their predecessor biofilm.
The authors demonstrated that an essential requirement for the formation of these robust
downstream biofilms was the YFP/CFP population aggregates. If these aggregates were
disrupted during the transfer process, the populations still formed biofilms, but did not
generate a layered structure. While the authors did not pinpoint the exact benefit that these
aggregates have on the formation of downstream biofilms, this interesting study nonetheless
demonstrates that one can design a synthetic consortium that can readily self-organize into
a spatially structured biofilm.
The controllability of synthetic biofilms was significantly enhanced in a 2012 publication
by Hong et al.
37
In this study, a synthetic consortium was engineered to form a biofilm and
disperse in response to externally applied signals. The authors engineered two
E. coli
cell
types: a
cell type. The disperser cells harbor two gene
circuits. One circuit consists of a constitutively expressed GFP (for strain identification) and
LasI, which synthesizes the QS signal 3OC12HSL. The other consists of an IPTG-inducible
(P
lac
) promoter driving the expression of Hha13D6, a protein that causes dispersal of
biofilms. The colonizer cells also contain two circuits. One circuit constitutively expresses
RFP and LasR, the receptor protein for 3OC12HSL. The other contains a 3OC12HSL-
inducible promoter (P
lasI
) driving the expression of BdcAE50Q, another protein that causes
biofilm dispersal. The circuit logic is as follows: when the disperser cells reach a critical cell
density, sufficient 3OC12HSL is produced, causing the colonizer cells to express BdcAE50Q
and disperse. The disperser cells themselves are dispersed through the use of exogenously
supplemented IPTG.
'
disperser
'
cell type and a
'
colonizer
'
252
The authors first verified that the circuit functioned when the two strains were grown
separately as biofilms: the addition of IPTG dispersed a biofilm consisting solely of
disperser cells; whereas the addition of 3OC12HSL dispersed a biofilm consisting solely of
colonizer cells. The authors then sought to determine if the two strains could function
together when seeded into a microfluidic device. To this end, the authors used colonizer
cells to form a biofilm upon which they added disperser cells. Indeed, as the disperser cells
grew, the colonizer cells began to disperse from the biofilm. After
40 hours of coculture,
nearly 80% of the colonizer biofilm had been dispersed. Upon addition of IPTG to the
microfluidic device, 92% of the disperser biofilm was successfully dispersed. Furthermore,
by replacing the 3OC12HSL responsive promoter with an arabinose responsive promoter,
the authors demonstrated that they could control the amount of colonizer cells dispersed
from colonizer/disperser biofilms.
B
While biofilms can self-organize, they can also be rationally organized. Stubblefield et al.
developed a flow system to modulate the formation of biofilms.
38
The flow system uses a
peristaltic pump that passes a bacterial culture over a flow cell, the contents of which can be
monitored using a microscope.
39
The authors first created a biofilm consisting of two
E. coli
strains, one of which expresses GFP. Independent passage of either population over the flow
cell resulted in a near linear relationship between the initial inoculum density and the
amount of cells deposited on the flow cell. However, when the two bacterial populations
were passed over the flow cell as a well-mixed culture, the linearity observed during the single
population analysis was abolished. Nonetheless, the authors predicted that sequential
passage of each population could reintroduce the previously observed linear relationship,
