Bacterial autoinducer 2 (AI-2) is proposed to be an interspecies mediator of cell-cell communication that enables cells to operate at the multicellular level. Many environmental stimuli have been shown to affect the extracellular AI-2 levels, carbon sources being among the most important. In this report, we show that both AI-2 synthesis and uptake in Escherichia coli are subject to catabolite repression through the cyclic AMP (cAMP)-CRP complex, which directly stimulates transcription of the lsr (for "luxS regulated") operon and indirectly represses luxS expression. Specifically, cAMP-CRP is shown to bind to a CRP binding site located in the upstream region of the lsr promoter and works with the LsrR repressor to regulate AI-2 uptake. The functions of the lsr operon and its regulators, LsrR and LsrK, previously reported in Salmonella enterica serovar Typhimurium, are confirmed here for E. coli. The elucidation of cAMP-CRP involvement in E. coli autoinduction impacts many areas, including the growth of E. coli in fermentation processes.Bacteria have evolved complex genetic circuits to modulate their physiological states and behaviors in response to a variety of extracellular signals. In a process termed quorum sensing, or density-dependent gene regulation, bacteria produce, release, and respond to signaling molecules (autoinducers), which accumulate as a function of cell density. Quorum sensing allows bacteria to communicate with each other and coordinate their activities at a multicellular level. The autoinducers of many gram-positive bacteria are secreted peptides (30, 42), while gram-negative bacteria use small chemical molecules (60). Among gram-negative bacteria, the LuxI/LuxR signal synthase-signal receptor system is the most studied at the molecular level, with the signaling species being a family of N-acylhomoserine lactones. However, the cross-species autoinducer, autoinducer 2 (AI-2), has received intense interest recently because the gene for its terminal synthase, luxS, is present in over 55 bacteria and its activity can be readily assayed biologically (61). It is known that quorum sensing regulates diverse cellular processes, including bioluminescence (19, 34), spore formation (33), motility (18, 22), competence (35), conjugation (20), antibiotic synthesis (2, 17), virulence (38,44,50), and biofilm maturation (13, 45).Our laboratory is interested in understanding and controlling microbial behavior in bioreactors in order to enhance recombinant protein synthesis and yield. Since quorum sensing is emerging as a global regulator of many intracellular processes, including those that influence protein synthesis, efforts to understand this "tunable" controller are essential. In our previous work using chemostat cultures (14), many stimuli were found to affect the level of AI-2. Among these, the pulsed addition of glucose, a common carbon source for recombinant Escherichia coli fermentations, resulted in increased AI-2 levels, but with the dynamic response dependent on the steadystate growth rate (e.g., dil...

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Electroaddressing of biological components at specific device addresses is attractive because it enlists the capabilities of electronics to provide spatiotemporally controlled electrical signals. Here, the electrodeposition of calcium alginate hydrogels at specific electrode addresses is reported. The method employs the low pH generated at the anode to locally solubilize calcium ions from insoluble calcium carbonate. The solubilized Ca2+ can then bind alginate to induce this polysaccharide to undergo a localized sol‐gel transition. Calcium alginate gel formation is shown to be spatially controlled in the normal and lateral dimensions. The deposition method is sufficiently benign that it can be used to entrap the bacteria E. coli. The entrapped cells are able to grow and respond to chemical inducers in their environment. Also, the entrapped cells can be liberated from the gel network by adding sodium citrate that can compete with alginate for Ca2+ binding. The capabilities of calcium alginate electrodeposition is illustrated by entrapping reporter cells that can recognize the quorum sensing autoinducer 2 (AI‐2) signaling molecule. These reporter cells were observed to recognize and respond to AI‐2 generated from an external bacterial population. Thus, calcium alginate electrodeposition provides a programmable method for the spatiotemporally controllable assembly of cell populations for cell‐based biosensing and for studying cell‐cell signaling.

Synthetic biology and metabolic engineering have expanded the possibilities for engineered cell-based systems. The addition of non-native biosynthetic and regulatory components can, however, overburden the reprogrammed cells. In order to avoid metabolic overload, an emerging area of focus is on engineering consortia, wherein cell subpopulations work together to carry out a desired function. This strategy requires regulation of the cell populations. Here, we design a synthetic co-culture controller consisting of cell-based signal translator and growth-controller modules that, when implemented, provide for autonomous regulation of the consortia composition. The system co-opts the orthogonal autoinducer AI-1 and AI-2 cell-cell signaling mechanisms of bacterial quorum sensing (QS) to enable cross-talk between strains and a QS signal-controlled growth rate controller to modulate relative population densities. We further develop a simple mathematical model that enables cell and system design for autonomous closed-loop control of population trajectories.

Bacterial quorum sensing (QS) is a cell-cell communication process, mediated by signaling molecules, that alters various phenotypes including pathogenicity. Methods to interrupt these communication networks are being pursued as next generation antimicrobials. We present a technique for interrupting communication among bacteria that exploits their native and highly specific machinery for processing the signaling molecules themselves. Specifically, our approach is to bring native intracellular signal processing mechanisms to the extracellular surroundings and "quench" crosstalk among a variety of strains. In this study, the QS system based on the interspecies signaling molecule autoinducer-2 (AI-2) is targeted because of its prevalence among prokaryotes (it functions in over 80 bacterial species). We demonstrate that the Escherichia coli AI-2 kinase, LsrK, can phosphorylate AI-2 in vitro, and when LsrK-treated AI-2 is added ex vivo to E. coli populations, the native QS response is significantly reduced. Further, LsrK-mediated degradation of AI-2 attenuates the QS response among Salmonella typhimurium and Vibrio harveyi even though the AI-2 signal transduction mechanisms and the phenotypic responses are species-specific. Analogous results are obtained from a synthetic ecosystem where three species of bacteria (enteric and marine) are co-cultured. Finally, the addition of LsrK and ATP to growing co-cultures of E. coli and S. typhimurium exhibits significantly reduced native "cross-talk" that ordinarily exists among and between species in an ecosystem. We believe this nature-inspired enzymatic approach for quenching QS systems will spawn new methods for controlling cell phenotype and potentially open new avenues for controlling bacterial pathogenicity.

Many reports have elucidated the mechanisms and consequences of bacterial quorum sensing (QS), a molecular communication system by which bacterial cells enumerate their cell density and organize collective behavior. In few cases, however, the numbers of bacteria exhibiting this collective behavior have been reported, either as a number concentration or a fraction of the whole. Not all cells in the population, for example, take on the collective phenotype. Thus, the specific attribution of the postulated benefit can remain obscure. This is partly due to our inability to independently assemble a defined quorum, for natural and most artificial systems the quorum itself is a consequence of the biological context (niche and signaling mechanisms). Here, we describe the intentional assembly of quantized quorums. These are made possible by independently engineering the autoinducer signal transduction cascade of Escherichia coli (E. coli) and the sensitivity of detector cells so that upon encountering a particular autoinducer level, a discretized sub-population of cells emerges with the desired phenotype. In our case, the emergent cells all express an equivalent amount of marker protein, DsRed, as an indicator of a specific QS-mediated activity. The process is robust, as detector cells are engineered to target both large and small quorums. The process takes about 6 h, irrespective of quorum level. We demonstrate sensitive detection of autoinducer-2 (AI-2) as an application stemming from quantized quorums. We then demonstrate sub-population partitioning in that AI-2-secreting cells can 'call' groups neighboring cells that 'travel' and establish a QS-mediated phenotype upon reaching the new locale.

Signaling MoleculeFmoc-Phe -pH-Responsive Proteins Reporter E. coli Scheme 1 . Biofabrication of multifunctional soft matter. a) Schematic illustrating the coupling of triggered self-assembly (i.e., electrodeposition) and enzymatic-assembly. b) Schematic illustrating that conjugated enzymes (Pfs and LuxS) allow for in situ generation of a bacterial signaling molecule (AI-2) that is detected by entrapped reporter cells.

Escherichia coli were engineered to enable programmed motility, sensing and phenotypic response to the density of epidermal growth factor receptor expressed on the surface of cancer cells.