A nonlinear stimulus-response relation in bacterial chemotaxis

Biomimetic control based on a model of chemotaxis in Escherichia coli

In the field of molecular biology, extending now to the more comprehensive area of systems biology, the development of computer models for synthetic cell simulation has accelerated extensively and has begun to be used for various purposes, such as biochemical analysis. These models, describing the highly efficient environmental searching mechanisms and adaptability of living organisms, can be used as machine-control algorithms in the field of systems engineering. To realize this biomimetic intelligent control, we require a stripped-down model that expresses a series of information-processing tasks from stimulation input to movement. Here we selected the bacterium Escherichia coli as a target organism because it has a relatively simple molecular and organizational structure, which can be characterized using biochemical and genetic analyses. We particularly focused on a motility response known as chemotaxis and developed a computer model that includes not only intracellular information processing but also motor control. After confirming the effectiveness and validity of the proposed model by a series of computer simulations, we applied it to a mobile robot control problem. This is probably the first study showing that a bacterial model can be used as an autonomous control algorithm. Our results suggest that many excellent models proposed thus far for biochemical purposes can be applied to problems in other fields.

Synthetic multivalent ligands as probes of signal transduction

Cell-surface receptors acquire information from the extracellular environment and coordinate intracellular responses. Many receptors do not operate as individual entities, but rather as part of dimeric or oligomeric complexes. Coupling the functions of multiple receptors may endow signaling pathways with the sensitivity and malleability required to govern cellular responses. Moreover, multireceptor signaling complexes may provide a means of spatially segregating otherwise degenerate signaling cascades. Understanding the mechanisms, extent, and consequences of receptor co-localization and interreceptor communication is critical; chemical synthesis can provide compounds to address the role of receptor assembly in signal transduction. Multivalent ligands can be generated that possess a variety of sizes, shapes, valencies, orientations, and densities of binding elements. This Review focuses on the use of synthetic multivalent ligands to characterize receptor function.

Emergent properties from organisms to ecosystems: towards a realistic approach

More realistic approaches are needed to understand the complexity of ecological systems. Emergent properties of real systems can be used as a basis for a new, neither reductionist nor holistic, approach. Three systems, termed here BUBBLEs, WAVEs and CRYSTALs, have been identified as exhibiting emergent properties. They are non-hierarchical assemblages of individual components, with amplification and connectedness being two main principles that govern their build-up, maintenance and mutual relationships. Examples from various fields of biological and ecological science are referred to, ranging from individual organisms to landscapes.

Large increases in attractant concentration disrupt the polar localization of bacterial chemoreceptors

In bacterial chemotaxis, the chemoreceptors [methyl-accepting chemotaxis proteins (MCPs)] transduce chemotactic signals through the two-component histidine kinase CheA. At low but not high attractant concentrations, chemotactic signals must be amplified. The MCPs are organized into a polar lattice, and this organization has been proposed to be critical for signal amplification. Although evidence in support of this model has emerged, an understanding of how signals are amplified and modulated is lacking. We probed the role of MCP localization under conditions wherein signal amplification must be inhibited. We tested whether a large increase in attractant concentration (a change that should alter receptor occupancy from c. 0% to > 95%) would elicit changes in the chemoreceptor localization. We treated Escherichia coli or Bacillus subtilis with a high level of attractant, exposed cells to the cross-linking agent paraformaldehyde and visualized chemoreceptor location with an anti-MCP antibody. A marked increase in the percentage of cells displaying a diffuse staining pattern was obtained. In contrast, no increase in diffuse MCP staining is observed when cells are treated with a repellent or a low concentration of attractant. For B. subtilis mutants that do not undergo chemotaxis, the addition of a high concentration of attractant has no effect on MCP localization. Our data suggest that interactions between chemoreceptors are decreased when signal amplification is unnecessary.

Conserved amplification of chemotactic responses through chemoreceptor interactions

Many bacteria concentrate their chemoreceptors at the cell poles. Chemoreceptor location is important in Escherichia coli, since chemosensory responses are sensitive to receptor proximity. It is not known, however, whether chemotaxis in other bacteria is similarly regulated. To investigate the importance of receptor-receptor interactions in other bacterial species, we synthesized saccharide-bearing multivalent ligands that are designed to cluster relevant chemoreceptors. As has been shown with E. coli, we demonstrate that the behaviors of Bacillus subtilis, Spirochaete aurantia, and Vibrio furnissii are sensitive to the valence of the chemoattractant. Moreover, in B. subtilis, chemotactic responses to serine were increased by pretreatment with saccharide-bearing multivalent ligands. This result indicates that, as in E. coli, signaling information is transferred among chemoreceptors in B. subtilis. These results suggest that interreceptor communication may be a general mechanism for modulating chemotactic responses in bacteria.