Bacterial motility and chemotaxis: the molecular biology of a behavioral system.
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Adenosine Triphosphatases↗
Biomedical subjects
R M Macnab
Publications and source records attributed to R M Macnab.
Normal-to-curly flagellar transitions and their role in bacterial tumbling. Stabilization of an alternative quaternary structure by mechanical force.
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Flagella↗
Bacterial flagella rotating in bundles: a study in helical geometry.
Bacterial flagella are semi-rigid helices that undergo true rotation. In peritrichously flagellated bacteria (e.g., Escherichia and Salmonella) there are many flagella on each cell; during translational cell movement these operate as a coordinated bundle that actively disperses upon reversal of the rotation sense. The dynamic behavior of a set of helices originating on separate rotational axes is explored by a working model, geometrical analysis, and hydrodynamic calculations. A critical relationship exists between the interaxial separation and phase difference of parallel helices with overlapping domains; in the subcritical case the filaments are not intertwisted, whereas in the supercritical case they are intertwisted in the same sense (left-handed) as the helices, with one twist per helical turn. During counter-clockwise rotation (the sense operative in forward swimming) any preexisting twists of this kind are automatically cancelled and the helices brought progressively into phase. Hydrodynamic calculations suggest that some wrapping then occurs in a right-handed sense, opposite to that of the helices; this necessitates a distortion from true helical geometry which is minimized by maintaining a coaxial in-phase relationship. A highly coordinated helical bundle results that is capable of operating smoothly for an indefinite period, in agreement with the observed behavior of swimming bacteria. During reverse rotation, the supercritical case develops to cause jamming of the bundle, as has been observed with bacteria in high-viscosity medium. The explosive dispersal of the bundle during reversal in low-viscosity medium is a consequence of a complicating phenomenon, namely, a drastic change in flagellar quaternary structure. The overall conclusion is that bundle formation and function are perfectly compatible with a rotational mechanism for the individual flagella.
Bacterial Physiological Phenomena↗
Examination of bacterial flagellation by dark-field microscopy.
A method is described for visualizing unstained bacterial flagella by dark-field light microscopy. Since individual filaments can be seen, a genus such as Salmonella, which is peritrichously flagellated, can readily be distinguished from a polarly flagellated genus such as Pseudomonas. Polarly flagellated bacteria generally swim much faster than peritrichously flagellated bacteria, and turn by abrupt reversals. The differences in flagellation and motility provide diagnostic criteria that may be useful in clinical microbiology.
Bacteria↗
The gradient-sensing mechanism in bacterial chemotaxis.
A "temporal gradient apparatus" has been developed that allows the motility of bacteria to be studied after they have been subjected to a sudden change from one uniform concentration of attractant to another. A sudden decrease elicits the tumbling response observed with spatial gradients; it was found, however, that a sudden increase also elicits a response, namely supercoordinated swimming. This demonstrates that chemotaxis is achieved by modulation of the incidence of tumbling both above and below its steady-state value. The initial responses gradually revert to the steady-state motility pattern characteristic of a uniform distribution of attractant. The apparent detection of a spatial gradient by the bacteria therefore involves an actual detection of a temporal gradient experienced as a result of movement through space. Potential models for the chemotactic response based on some "memory" mechanism are discussed.
Cell Movement↗