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Biomedical subjects

I R Lapidus

Publications and source records attributed to I R Lapidus.

12 recordsLinked to original sources
PubMed1990

Pausing, switching and speed fluctuation of the bacterial flagellar motor and their relation to motility and chemotaxis.

Wild-type Escherichia coli and Salmonella typhimurium cells, tethered to glass by their flagella, rotate with brief intermittent pauses, the prevalence of which is decreased by attractants and increased by repellents. By attaching latex beads to filaments of a S. typhimurium mutant having straight rather than helical flagella, it was established that the flagella on free cells also pause intermittently. Pausing is therefore an intrinsic feature of the motor and not an artifact associated with tethering. In tethered cells of wild-type strains and non-chemotactic mutants defective in transducers, chemotaxis proteins, or the flagellar switch, both the classical response to chemotactic stimuli (change in direction of rotation from counterclockwise to clockwise or vice versa), and the pausing response to such stimuli, were linked together. No separate signal for pausing was found. In comparing different strains under different stimulation conditions, it was found that cells that never reversed seldom if ever paused, while cells that reversed frequently paused frequently. It is suggested that pausing is the result of futile switching events. A modified description of tumbling and chemotaxis is provided in which pausing, as well as reversal, has a role. Suppression of reversals and pauses by attractant stimuli commonly resulted in an increase in the speed of counterclockwise rotation; this may be because of suppression of pauses or reversals that are too brief to be detected. The clockwise rotation rate of unstimulated cells, which commonly was faster than their counterclockwise rate, was not further increased by repellent stimuli. The rotation rate of any given cell under any given condition was found to fluctuate on all time-scales measured. The study also revealed that some of the common repellents of E. coli and S. typhimurium slow down or stop the motor; these effects are not mediated by the chemotaxis machinery or intracellular pH.

Cell-Free System↗
PubMed1988

Pausing of flagellar rotation is a component of bacterial motility and chemotaxis.

When bacterial cells are tethered to glass by their flagella, many of them spin. On the basis of experiments with tethered cells it has generally been thought that the motor which drives the flagellum is a two-state device, existing in either a counterclockwise or a clockwise state. Here we show that a third state of the motor is that of pausing, the duration and frequency of which are affected by chemotactic stimuli. We have recorded on video tape the rotation of tethered Escherichia coli and Salmonella typhimurium cells and analyzed the recordings frame by frame and in slow motion. Most wild-type cells paused intermittently. The addition of repellents caused an increase in the frequency and duration of the pauses. The addition of attractants sharply reduced the number of pauses. A chemotaxis mutant which lacks a large part of the chemotaxis machinery owing to a deletion of the genes from cheA to cheZ did not pause at all and did not respond to repellents by pausing. A tumbly mutant of S. typhimurium responded to repellents by smooth swimming and to attractants by tumbling. When tethered, these cells exhibited a normal rotational response but an inverse pausing response to chemotactic stimuli: the frequency of pauses decreased in response to repellents and increased in response to attractants. It is suggested that (i) pausing is an integral part of bacterial motility and chemotaxis, (ii) pausing is independent of the direction of flagellar rotation, and (iii) pausing may be one of the causes of tumbling.

Cell Movement↗
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↗
PubMed1978

A model for traveling bands of chemotactic bacteria.

A theoretical model is used to study band formation by chemotactic populations of Escherichia coli. The model includes the bacterial response to attractant gradients, the chemotactic sensitivity of the bacteria to the concentration of the attractant, and population growth. For certain values of the parameters in the model, traveling bands of bacteria form and propagate with or without growth. Under specific growth conditions the band profile is maintained and the band propagates at constant speed. These predictions are in general agreement with the experiment results of J. Adler and earlier theoretical work by L. Segel and his collaborators. However, our theory differs in several important respects from the latter efforts. Suggestions are made for further experiments to test the proposed model and to clarify the nature of the processes which lead to band formation.

Chemotaxis↗
PubMed1976

Model for the chemotactic response of a bacterial population.

We present a mathematical model for the motion of a bacterial population in prescribed attractant or repellent gradients. The model is suggested by the observations of Mesibov et al. (1973, J. Gen. Physiol. 62:203) and Brown and Berg (1974, Proc. Natl. Acad. Sci. U.S.A. 71:1388) who found that the sensitivity of the chemotactic response depends on the concentration of attractant. Predictions of the theory are in general agreement with the experiments of Dahlquist et al. (1972, Nat. New Biol. 236:120) and of Mesibov et al. on populations of motile bacteria in fixed attractant gradients. Additional tests of the model are proposed.

Bacterial Physiological Phenomena↗
PubMed1974

A mathematical model for bacterial chemotaxis.

A differential equation describing the chemotactic migration of a bacterial population in a fixed exponential gradient of attractant has been integrated using the appropriate boundary conditions. The solution predicts an initial bacterial accumulation at the concentration "knee" with the final distribution of bacteria approaching a time-independent state. Specific additional experiments to obtain further data for a rigorous test of the theory are suggested.

Bacteria↗
PubMed1971

Analysis of a model for multiseptation in bacteria.

The predictions of a model for multiseptation in bacteria recently proposed by Paulton are derived mathematically. It is shown that independent of the growth rate, the time between septum formation and cell division is given by t(s) = S . t, where S is the average number of successive division sites and t is the generation time. This result is in good agreement with the experimental data.

Bacteria↗