Statistical fluctuations limit the precision with which a microorganism can, in a given time T, determine the concentration of a chemoattractant in the surrounding medium. The best a cell can do is to monitor continually the state of occupation of receptors distributed over its surface. For nearly optimum performance only a small fraction of the surface need be specifically adsorbing. The probability that a molecule that has collided with the cell will find a receptor is Ns/(Ns + pi a), if N receptors, each with a binding site of radius s, are evenly distributed over a cell of radius a. There is ample room for many indenpendent systems of specific receptors. The adsorption rate for molecules of moderate size cannot be significantly enhanced by motion of the cell or by stirring of the medium by the cell. The least fractional error attainable in the determination of a concentration c is approximately (TcaD) - 1/2, where D is diffusion constant of the attractant. The number of specific receptors needed to attain such precision is about a/s. Data on bacteriophage absorption, bacterial chemotaxis, and chemotaxis in a cellular slime mold are evaluated. The chemotactic sensitivity of Escherichia coli approaches that of the cell of optimum design.

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Flagellated bacteria, such as Escherichia coli, swim by rotating thin helical filaments, each driven at its base by a reversible rotary motor, powered by an ion flux. A motor is about 45 nm in diameter and is assembled from about 20 different kinds of parts. It develops maximum torque at stall but can spin several hundred Hz. Its direction of rotation is controlled by a sensory system that enables cells to accumulate in regions deemed more favorable. We know a great deal about motor structure, genetics, assembly, and function, but we do not really understand how it works. We need more crystal structures. All of this is reviewed, but the emphasis is on function.

Cells swimming in confined environments are attracted by surfaces. We measure the steady-state distribution of smooth-swimming bacteria (Escherichia coli) between two glass plates. In agreement with earlier studies, we find a strong increase of the cell concentration at the boundaries. We demonstrate theoretically that hydrodynamic interactions of the swimming cells with solid surfaces lead to their reorientation in the direction parallel to the surfaces, as well as their attraction by the closest wall. A model is derived for the steady-state distribution of swimming cells, which compares favorably with our measurements. We exploit our data to estimate the flagellar propulsive force in swimming E. coli.

Bacteria swim by rotating flagellar filaments that are several micrometers long, but only about 20 nm in diameter. The filaments can exist in different polymorphic forms, having distinct values of curvature and twist. Rotation rates are on the order of 100 Hz. In the past, the motion of individual filaments has been visualized by dark-field or differential-interference-contrast microscopy, methods hampered by intense scattering from the cell body or shallow depth of field, respectively. We have found a simple procedure for fluorescently labeling cells and filaments that allows recording their motion in real time with an inexpensive video camera and an ordinary fluorescence microscope with mercury-arc or strobed laser illumination. We report our initial findings with cells of Escherichia coli. Tumbles (events that enable swimming cells to alter course) are remarkably varied. Not every filament on a cell needs to change its direction of rotation: different filaments can change directions at different times, and a tumble can result from the change in direction of only one. Polymorphic transformations tend to occur in the sequence normal, semicoiled, curly 1, with changes in the direction of movement of the cell body correlated with transformations to the semicoiled form.

Chemoreceptors in Escherichia coli are coupled to the flagella by a labile phosphorylated intermediate, CheYϳP. Its activity can be inferred from the rotational bias of flagellar motors, but motor response is stochastic and limited to a narrow physiological range. Here we use fluorescence resonance energy transfer to monitor interactions of CheYϳP with its phosphatase, CheZ, that reveal changes in the activity of the receptor kinase, CheA, resulting from the addition of attractants or repellents. Analyses of cheR and͞or cheB mutants, defective in receptor methylation͞demethylation, show that response sensitivity depends on the activity of CheB and the level of receptor modification. In cheRcheB mutants, the concentration of attractant that generates a half-maximal response is equal to the dissociation constant of the receptor. In wild-type cells, it is 35 times smaller. This amplification, together with the ultrasensitivity of the flagellar motor, explains previous observations of high chemotactic gain.Escherichia coli ͉ fluorescence ͉ fluorescence resonance energy transfer B acteria move up spatial gradients of chemical attractants in a biased random walk, now running smoothly, now tumbling randomly, extending runs that carry them in a favorable direction (1). The central point of regulation in the chemotaxis signal transduction pathway is the level of phosphorylation of the diffusible signaling protein CheY (2). CheY is phosphorylated by the kinase CheA that is coupled to specific receptors (3). CheYϳP binds to FliM, a component of the switch complex, at the cytoplasmic face of the flagellar motor and modulates the direction of motor rotation (4-6). The phosphatase CheZ binds to CheYϳP and accelerates its dephosphorylation (7,8). The receptor is thought to exist in two states, active (activating CheA) and inactive (inhibiting CheA) (9). Attractant binding lowers the probability of the active state, decreasing the level of CheYϳP. Attractant removal raises the probability of the active state, increasing the level of CheYϳP. Adaptation is by receptor methylation by CheR and demethylation by CheB (10-12). Receptor methylation compensates for attractant binding, with the modified receptor more likely to be in the active state. Thus, CheR promotes adaptation to increasing levels of attractants, and CheB promotes adaptation to decreasing levels of attractants. Two major chemoreceptors exist in Escherichia coli: the aspartate receptor, Tar, and serine receptor, Tsr. Tar has four methylation sites, which are glutamates; however, two of these are expressed as glutamines, deamidated by CheB. The glutamines are thought to be physiologically equivalent to methylated glutamates (13).We used fluorescence resonance energy transfer (FRET) to measure intracellular changes in phosphorylation-dependent interactions of CheY upon chemotactic stimulation. FRET, which relies on the distance-dependent transfer of energy from an excited donor fluorophore to an acceptor fluorophore, is one of the few tools available for monitoring prot...

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