(283c) Design and Mathematical Modeling of a Quorum Sensing Based Synthetic Ecosystem with Applications to Competitive Mixed Cultures

Cooperation in macroscopic social organisms is essential for food acquisition, protection, reproduction, and dispersal to new localities. Although implemented at much smaller length and time scales, a variety of cooperative systems has been observed in microbial populations. Examples include bacterial production of extracellular polymer to form biofilms with heterogeneous three dimensional structures, the release of virulence factors by Salmonella when the cell population density is high enough to tolerate the human immune response, and reduced reproduction in Rhizobium to allow other bacteria to benefit from carbon obtained from a host during nitrogen fixation. The use of microbial ecosystems to explore macroscopic ecosystems remains controversial. Nevertheless, artificial microbial ecosystems have had impacts on traditional ecology by allowing exploration of many hypotheses that have proven difficult to test in field systems. Despite the wealth of experimental evidence demonstrating bacterial cooperation, the genetic and molecular bases of the associated signal transduction processes are not well understood.

Quorum sensing is an intercellular bacterial communication mechanism found in many gram-negative bacteria. The most studied quorum sensing system is the LuxI-LuxR system identified from Vibrio fischeri, which involves the signaling molecule 3-oxo-hexanoyl-homoserine lactone (3-oxo-C6-HSL). This acyl hormone serine lactone (AHL) is synthesized constitutively by a LuxI synthetase. When the AHL concentration reaches a threshold, AHL interacts with a LuxR transcription activator to activate gene expression regulated by a PluxI promoter. Because the AHL concentration is proportional to cell density under certain conditions, this mechanism allows coordination of cell population behavior such as bioluminescence, biofilm formation, and virulence according to prevailing cell density.

In this paper, we present the conceptual design of a two-species artificial bacterial cooperative ecosystem (symbiosis) in which the survival of one species is dependent on the cell density of the other species. A simple mathematical model is developed to analyze the behavior of this synthetic ecosystem in continuous culture with a single growth limited substrate. The model consists of nonlinear differential equations for the substrate concentration and the cell mass concentrations of species A and B. To capture intercellular signaling via two interacting quorum sensing systems, the specific growth rate of each species is assumed to be nonlinearly dependent on the cell mass concentration of the other species such that cell growth is severely retarded when the cell density of the other population falls below a threshold value. Bifurcation analysis shows the existence of multiple steady-state solutions, including two non-trivial solutions for the coexistence of the two species. One coexistence solution is shown to be stable over a wide range of dilution rates and to produce large variations in the fractions of the two species. Consequently, the proposed design has potential applications in competitive mixed culture bioreactors by allowing stable production of two microbial populations with different growth rates. The presentation is concluded with a discussion of our experimental progress in realizing this synthetic ecosystem design and an overview of our more fundamental modeling work at the gene/protein level.