Genetic and biochemical requirements for chemotaxis to L-proline in Escherichia coli

Chemotaxis by Pseudomonas putida (ATCC 17453) towards camphor involves cytochrome P450cam (CYP101A1)

The camphor-degrading microorganism, Pseudomonas putida strain ATCC 17453, is an aerobic, gram-negative soil bacterium that uses camphor as its sole carbon and energy source. The genes responsible for the catabolic degradation of camphor are encoded on the extra-chromosomal CAM plasmid. A monooxygenase, cytochrome P450_cam, mediates hydroxylation of camphor to 5-exo-hydroxycamphor as the first and committed step in the camphor degradation pathway, requiring a dioxygen molecule (O₂) from air. Under low O₂ levels, P450_cam catalyzes the production of borneol via an unusual reduction reaction. We have previously shown that borneol downregulates the expression of P450_cam. To understand the function of P450_cam and the consequences of down-regulation by borneol under low O₂ conditions, we have studied chemotaxis of camphor induced and non-induced P. putida strain ATCC 17453. We have tested camphor, borneol, oxidized camphor metabolites and known bacterial attractants (d)-glucose, (d) - and (l)-glutamic acid for their elicitation chemotactic behavior. In addition, we have used 1-phenylimidazole, a P450_cam inhibitor, to investigate if P450_cam plays a role in the chemotactic ability of P. putida in the presence of camphor. We found that camphor, a chemoattractant, became toxic and chemorepellent when P450_cam was inhibited. We have also evaluated the effect of borneol on chemotaxis and found that the bacteria chemotaxed away from camphor in the presence of borneol. This is the first report of the chemotactic behaviour of P. putida ATCC 17453 and the essential role of P450_cam in this process.

Decoding the Chemical Language of Motile Bacteria by Using High-Throughput Microfluidic Assays

Motile bacteria navigate chemical environments by using chemoreceptors. The output of these protein sensors is linked to motility machinery and enables bacteria to follow chemical gradients. Understanding the chemical specificity of different families of chemoreceptors is essential for predicting and controlling bacterial behavior in ecological niches, including symbiotic and pathogenic interactions with plants and mammals. The identification of chemical(s) recognized by specific families of receptors is limited by the low throughput and complexity of chemotaxis assays. To address this challenge, we developed a microfluidic-based chemotaxis assay that is quantitative, simple, and enables high-throughput measurements of bacterial response to different chemicals. Using the model bacterium Escherichia coli, we demonstrated a strategy for identifying molecules that activate chemoreceptors from a diverse compound library and for determining how global behavioral strategies are tuned to chemical environments.

Ecological role of energy taxis in microorganisms

Motile microorganisms rapidly respond to changes in various physico-chemical gradients by directing their motility to more favorable surroundings. Energy generation is one of the most important parameters for the survival of microorganisms in their environment. Therefore it is not surprising that microorganisms are able to monitor changes in the cellular energy generating processes. The signal for this behavioral response, which is called energy taxis, originates within the electron transport system. By coupling energy metabolism and behavior, energy taxis is fine-tuned to the environment a cell finds itself in and allows efficient adaptation to changing conditions that affect cellular energy levels. Thus, energy taxis provides cells with a versatile sensory system that enables them to navigate to niches where energy generation is optimized. This behavior is likely to govern vertical species stratification and the active migration of motile cells in response to shifting gradients of electron donors and/or acceptors which are observed within microbial mats, sediments and soil pores. Energy taxis has been characterized in several species and might be widespread in the microbial world. Genome sequencing revealed that many microorganisms from aquatic and soil environments possess large numbers of chemoreceptors and are likely to be capable of energy taxis. In contrast, species that have a fewer number of chemoreceptors are often found in specific, confined environments, where relatively constant environmental conditions are expected. Future studies focusing on characterizing behavioral responses in species that are adapted to diverse environmental conditions should unravel the molecular mechanisms underlying sensory behavior in general and energy taxis in particular. Such knowledge is critical to a better understanding of the ecological role of energy taxis.

Chemotaxis of a Ralstonia sp. SJ98 toward co-metabolizable nitroaromatic compounds

We have earlier reported chemotaxis of a Gram-negative, motile Ralstonia sp. SJ98 towards p-nitrophenol (PNP), 4-nitrocatechol (NC), o-nitrobenzoate (ONB), p-nitrobenzoate (PNB), and 3-methyl-4-nitrophenol (MNP) that also served as sole source of carbon and energy to the strain [S.K. Samanta, B. Bhushan, A. Chauhan, R.K. Jain, Biochem. Biophy. Res. Commun. 269 (2000) 117; B. Bhushan, S.K. Samanta, A. Chauhan, A.K. Chakraborti, R.K. Jain, Biochem. Biophy. Res. Commun. 275 (2000) 129]. In this paper, we report chemotaxis of a Ralstonia sp. SJ98 toward seven different nitroaromatic compounds (NACs) by drop assay, swarm plate assay, and capillary assay. These NACs do not serve as sole carbon and energy source to strain SJ98 but are partially transformed in the presence of an alternate carbon source such as succinate. This is the first report showing chemotaxis of a bacterial strain toward co-metabolizable NACs.

Energy taxis is the dominant behavior in Azospirillum brasilense

Energy taxis encompasses aerotaxis, phototaxis, redox taxis, taxis to alternative electron acceptors, and chemotaxis to oxidizable substrates. The signal for this type of behavior is originated within the electron transport system. Energy taxis was demonstrated, as a part of an overall behavior, in several microbial species, but it did not appear as the dominant determinant in any of them. In this study, we show that most behavioral responses proceed through this mechanism in the alpha-proteobacterium Azospirillum brasilense. First, chemotaxis to most chemoeffectors typical of the azospirilla habitat was found to be metabolism dependent and required a functional electron transport system. Second, other energy-related responses, such as aerotaxis, redox taxis, and taxis to alternative electron acceptors, were found in A. brasilense. Finally, a mutant lacking a cytochrome c oxidase of the cbb(3) type was affected in chemotaxis, redox taxis, and aerotaxis. Altogether, the results indicate that behavioral responses to most stimuli in A. brasilense are triggered by changes in the electron transport system.

Aerotaxis and other energy-sensing behavior in bacteria

Energy taxis is widespread in motile bacteria and in some species is the only known behavioral response. The bacteria monitor their cellular energy levels and respond to a decrease in energy by swimming to a microenvironment that reenergizes the cells. This is in contrast to classical Escherichia coli chemotaxis in which sensing of stimuli is independent of cellular metabolism. Energy taxis encompasses aerotaxis (taxis to oxygen), phototaxis, redox taxis, taxis to alternative electron acceptors, and chemotaxis to a carbon source. All of these responses share a common signal transduction pathway. An environmental stimulus, such as oxygen concentration or light intensity, modulates the flow of reducing equivalents through the electron transport system. A transducer senses the change in electron transport, or possibly a related parameter such as proton motive force, and initiates a signal that alters the direction of swimming. The Aer and Tsr proteins in E. coli are newly recognized transducers for energy taxis. Aer is homologous to E. coli chemoreceptors but unique in having a PAS domain and a flavin-adenine dinucleotide cofactor that is postulated to interact with a component of the electron transport system. PAS domains are energy-sensing modules that are found in proteins from archaea to humans. Tsr, the serine chemoreceptor, is an independent transducer for energy taxis, but its sensory mechanism is unknown. Energy taxis has a significant ecological role in vertical stratification of microorganisms in microbial mats and water columns. It plays a central role in the behavior of magnetotactic bacteria and also appears to be important in plant-microbe interactions.

The Aer protein and the serine chemoreceptor Tsr independently sense intracellular energy levels and transduce oxygen, redox, and energy signals for Escherichia coli behavior

We identified a protein, Aer, as a signal transducer that senses intracellular energy levels rather than the external environment and that transduces signals for aerotaxis (taxis to oxygen) and other energy-dependent behavioral responses in Escherichia coli. Domains in Aer are similar to the signaling domain in chemotaxis receptors and the putative oxygen-sensing domain of some transcriptional activators. A putative FAD-binding site in the N-terminal domain of Aer shares a consensus sequence with the NifL, Bat, and Wc-1 signal-transducing proteins that regulate gene expression in response to redox changes, oxygen, and blue light, respectively. A double mutant deficient in aer and tsr, which codes for the serine chemoreceptor, was negative for aerotaxis, redox taxis, and glycerol taxis, each of which requires the proton motive force and/or electron transport system for signaling. We propose that Aer and Tsr sense the proton motive force or cellular redox state and thereby integrate diverse signals that guide E. coli to environments where maximal energy is available for growth.

Glycerol elicits energy taxis of Escherichia coli and Salmonella typhimurium

Escherichia coli and Salmonella typhimurium show positive chemotaxis to glycerol, a chemical previously reported to be a repellent for E. coli. The threshold of the attractant response in both species was 10(-6) M glycerol. Glycerol chemotaxis was energy dependent and coincident with an increase in membrane potential. Metabolism of glycerol was required for chemotaxis, and when lactate was present to maintain energy production in the absence of glycerol, the increases in membrane potential and chemotactic response upon addition of glycerol were abolished. Methylation of a chemotaxis receptor was not required for positive glycerol chemotaxis in E. coli or S. typhimurium but is involved in the negative chemotaxis of E. coli to high concentrations of glycerol. We propose that positive chemotaxis to glycerol in E. coli and S. typhimurium is an example of energy taxis mediated via a signal transduction pathway that responds to changes in the cellular energy level.

Selection of motile nonchemotactic mutants of Escherichia coli by field-flow fractionation

We have developed a chromatographic method for isolating bacterial cells that are motile but nonchemotactic. Separation of strains of different phenotype occurs along a thin horizontal channel between two stirred chambers, the lower one containing a chemical attractant. The channel is bounded above and below by rigid filters, permeable to the attractant but not to the bacteria. The lower part of the channel is occupied by a porous plate comprising a vertical array of capillary tubes. An aliquot of cells is injected at one end of the channel and eluted by continuous flow of cell-free medium. Fluid leaving the other end of the channel is collected in a fraction collector. Cells that respond to the gradient swim to the bottom of the channel where they are retarded by the capillary array. Nonmotile cells sink to the bottom and are trapped in a similar manner. Motile cells that fail to respond to the gradient diffuse across the full height of the channel and, thus, travel through the apparatus at the average velocity of the eluent. When mixed with wild-type cells at a ratio of 1:1000 and subjected to an aspartate gradient, aspartate-blind cells were recovered quantitatively. The enrichment was approximately 200 to 1. The wild-type cells that survived the selection had a poorly motile phenotype.

Proline porters effect the utilization of proline as nutrient or osmoprotectant for bacteria

Proline is utilized by all organisms as a protein constituent. It may also serve as a source of carbon, energy and nitrogen for growth or as an osmoprotectant. The molecular characteristics of the proline transport systems which mediate the multiple functions of proline in the Gram negative enteric bacteria, Escherichia coli and Salmonella typhimurium, are now becoming apparent. Recent research on those organisms has provided both protocols for the genetic and biochemical characterization of the enzymes mediating proline transport and molecular probes with which the degree of homology among the proline transport systems of archaebacteria, eubacteria and eukaryotes can be assessed. This review has provided a detailed summary of recent research on proline transport in E. coli and S. typhimurium; the properties of other organisms are cited primarily to illustrate the generality of those observations and to show where homologous proline transport systems might be expected to occur. The characteristics of proline transport in eukaryotic microorganisms have recently been reviewed (Horak, 1986).

Involvement of transport in Rhodobacter sphaeroides chemotaxis

The chemotactic response to a range of chemicals was investigated in the photosynthetic bacterium Rhodobacter sphaeroides, an organism known to lack conventional methyl-accepting sensory transduction proteins. Strong attractants included monocarboxylic acids and monovalent cations. Results suggest that the chemotactic response required the uptake of the chemoeffector, but not its metabolism. If a chemoeffector could block the uptake of another attractant, it also inhibited chemotaxis to that attractant. Sodium benzoate was not an attractant but was a competitive inhibitor of the propionate uptake system. Binding in an active uptake system was therefore insufficient to cause a chemotactic response. At different concentrations, benzoate either blocked propionate chemotaxis or reduced the sensitivity of propionate chemotaxis, an effect consistent with its role as a competitive inhibitor of uptake. Bacteria only showed chemotaxis to ammonium when grown under ammonia-limited conditions, which derepressed the ammonium transport system. Both chemotaxis and uptake were sensitive to the proton ionophore carbonyl cyanide m-chlorophenylhydrazone, suggesting an involvement of the proton motive force in chemotaxis, at least at the level of transport. There was no evidence for internal pH as a sensory signal. These results suggest a requirement for the uptake of attractants in chemotactic sensing in R. sphaeroides.

Chemotactic response of Escherichia coli to chemically synthesized amino acids

In Escherichia coli, seven of the commonly occurring amino acids are strong attractants: L-aspartate, L-serine, L-glutamate, L-alanine, L-asparagine, glycine, and L-cysteine, in order of decreasing effectiveness. The chemotactic response to each amino acid attractant is mediated by either methyl-accepting chemotaxis protein I or II, but not by both. Seven of the commonly occurring amino acids are repellents. This work was carried out with chemically synthesized amino acids.