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H C Berg

Publications and source records attributed to H C Berg.

At least 91 records · Page 5Linked to original sources
PubMed1983

Adaptation kinetics in bacterial chemotaxis.

Cells of Escherichia coli, tethered to glass by a single flagellum, were subjected to constant flow of a medium containing the attractant alpha-methyl-DL-aspartate. The concentration of this chemical was varied with a programmable mixing apparatus over a range spanning the dissociation constant of the chemoreceptor at rates comparable to those experienced by cells swimming in spatial gradients. When an exponentially increasing ramp was turned on (a ramp that increases the chemoreceptor occupancy linearly), the rotational bias of the cells (the fraction of time spent spinning counterclockwise) changed rapidly to a higher stable level, which persisted for the duration of the ramp. The change in bias increased with ramp rate, i.e., with the time rate of change of chemoreceptor occupancy. This behavior can be accounted for by a model for adaptation involving proportional control, in which the flagellar motors respond to an error signal proportional to the difference between the current occupancy and the occupancy averaged over the recent past. Distributions of clockwise and counterclockwise rotation intervals were found to be exponential. This result cannot be explained by a response regular model in which transitions between rotational states are generated by threshold crossings of a regular subject to statistical fluctuation; this mechanism generates distributions with far too many long events. However, the data can be fit by a model in which transitions between rotational states are governed by first-order rate constants. The error signal acts as a bias regulator, controlling the values of these constants.

Aspartic Acid↗
PubMed1983

Coordination of flagella on filamentous cells of Escherichia coli.

Video techniques were used to study the coordination of different flagella on single filamentous cells of Escherichia coli. Filamentous, nonseptate cells were produced by introducing a cell division mutation into a strain that was polyhook but otherwise wild type for chemotaxis. Markers for its flagellar motors (ordinary polyhook cells that had been fixed with glutaraldehyde) were attached with antihook antibodies. The markers were driven alternately clockwise and counterclockwise, at angular velocities comparable to those observed when wild-type cells are tethered to glass. The directions of rotation of different markers on the same cell were not correlated; reversals of the flagellar motors occurred asynchronously. The bias of the motors (the fraction of time spent spinning counterclockwise) changed with time. Variations in bias were correlated, provided that the motors were within a few micrometers of one another. Thus, although the directions of rotation of flagellar motors are not controlled by a common intracellular signal, their biases are. This signal appears to have a limited range.

Cell Division↗
PubMed1982

Impulse responses in bacterial chemotaxis.

The chemotactic behavior of Escherichia coli has been studied by exposing cells tethered by a single flagellum to pulses of chemicals delivered iontophoretically. Normally, wild-type cells spin alternately clockwise and counterclockwise, changing their direction on the average approximately once per second. When cells were exposed to a very brief diffusive wave of attractant, the probability of spinning counterclockwise quickly peaked, then fell below the prestimulus value, returning to baseline within a few seconds; repellent responses were similar but inverted. The width of the response indicates that cells integrate sensory inputs over a period of seconds, while the biphasic character implies that they also take time derivatives of these inputs. The sensory system is maximally tuned to concentration changes that occur over a span of approximately 2 sec, an interval over which changes normally occur when cells swim in spatial gradients; it is optimized to extract information from signals subject to statistical fluctuation. Impulse responses of cells defective in methylation were similar to those of wild-type cells, but did not fall as far below the baseline, indicating a partial defect in adaptation. Impulse responses of cheZ mutants were aberrant, indicating a serious defect in excitation.

Chemotaxis↗
PubMed1982

Gliding motility of Cytophaga sp. strain U67.

Video techniques were used to analyze the motion of the gliding bacterium Cytophaga sp. strain U67. Cells moved singly on glass along the long axis at a speed of about 2 micrometers/s, advancing, retreating, stopping, pivoting about a pole, or flipping over. They did not flex or roll. Cells of different lengths moved at about the same speed. Cells sometimes spun continuously about a pole at a frequency of about 2 HZ, the body moving in a plane parallel to that of the glass or on the surface of a cone having either a large or a small solid angle. Polystyrene latex spheres moved to and fro on the surfaces of cells, also at a speed of about 2 micrometers/s. They moved in the same fashion whether a cell was in suspension, gliding, or at rest on the glass. Two spheres on the same cell often moved in opposite directions, passing by one another in close proximity. Small and large spheres and aggregates of spheres all moved at about the same speed. An aggregate moved down the side of a cell with a fixed orientation, even when only one sphere was in contact with the cell. Spheres occasionally left one cell and were picked up by another. Cell pretreated with small spheres did not adhere to glass. When the cells were deprived of oxygen, they stopped gliding, and the spheres stopped moving on their surfaces. The spheres became completely immobilized; they no longer moved from cell to cell or exhibited Brownian movement. Cytophaga spp. are known to have a typical gram-negative cell envelope: an inner (cytoplasmic) membrane, a thin peptidoglycan layer, and an outer (lipopolysaccharide) membrane. Our data are consistent with a model for gliding in which sites to which glass and polystyrene strongly adsorb move within the fluid outer membrane along tracks fixed to the rigid peptidoglycan framework.

Cell Membrane↗
PubMed1977

Physics of chemoreception.

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.

Bacterial Physiological Phenomena↗
PubMed1977

A protonmotive force drives bacterial flagella.

Streptococcus strain V4051 is motile in the presence of glucose. The cells move steadily along smooth paths (run), jump about briefly with little net displacement (twiddle), and then run in new directions. They stop swimming when deprived of glucose. These cells become motile when an electrical potential or a pH gradient is imposed across the membrane. Starved cells suspended in a potassium-free medium respond to the addition of valinomycin by a brief period of vigorous twiddling. They also twiddle, although less vigorously, when the external pH is lowered. Valinomycin-induced twiddling occurs in the absence of external alkali or alkaline earth cations and without significant net synthesis of ATP. When a chemoattractant is added to cells swimming in the presence of glucose, twiddles are transiently suppressed, and the cells run for a time. Similarly, when starved cells are suspended in a potassium-free medium containing both valinomycin and an attractant, many cells initially run rather than twiddle. We conclude that the flagella are driven by a protonmotive force.

Adenosine Triphosphate↗
PubMed1975

Bacterial behaviour.

Bacteria swim by rotating their flagella. They back up or choose new directions at random by changing the direction of the rotation. The probability of such changes is biased by sensory reception. The bias depends on the way in which the intensity ofthe stimulus changes with time, so that the bacteria tend to swim up a gradient of attractant and down a gradient of repellent chemicals.

Bacteria↗
PubMed1975

Transient response to chemotactic stimuli in Escherichia coli.

We have followed by eye and with the tracking microscope the rotational behavior of E. coli tethered to coverslips by their flagella. The cells change their directions of rotation at random, on the average about once a second. When an attractant is added or a repellent is subtracted, they spin clockwise (as viewed through the coverslip, i.e., along the flagellum toward the body) for many seconds, then counter-clockwise for many seconds, and then gradually resume their normal mode of behavior. The time interval between the onset of the stimulus and the clockwise to counter-clockwise transitiion is a linear function of the change in receptor occupancy. The cells adapt slowly at a constant rate to the addition of an attractant or the subtraction of a repellent. They adapt rapidly to the subtraction of an attractant or the addition of a repellent. Responses to mixed stimuli can be analyzed in terms of one equivalent stimulus.

Aminoisobutyric Acids↗
PubMed1975

Chemotaxis in bacteria.

Bacteria swim by rotating their flagella. They alter course by abruptly changing the direction of this rotation. The probability of the occurrence of this event is biased by chemoreception. The bias depends on the way in which the concentration of the attractant or repellent changes with time. Sugars are detected as they bind to specific proteins which also play a role in transport. The way in which the receptors are coupled to the flagella is not known. The coupling may involve changes in membrane potential.

Amino Acids↗