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Abstract
Random and directed motility of bacterial populations were assayed by monitoring the
flux of bacteria through a microchannel plate (a porous glass plate comprising a fused
array of capillary tubes) separating two identical stirred chambers. Cells, washed
free of growth medium by a new filtration method, were added to one chamber at a low
density. Their number in the other chamber was determined from the amount of light
scattered from a beam of a laser diode and recorded on a strip chart. Diffusion coefficients
were computed from fluxes observed in the absence of chemical gradients, and chemotaxis
drift velocities were computed from fluxes observed in their presence. Cells migrated
through tubes of diam 10 microns more rapidly than through tubes of diam 50 microns,
suggesting that the straight segments of their tracks were aligned with the axes of
the smaller tubes. Mutants that are motile but nonchemotactic could be selected because
they move through the microchannel plate in the face of an adverse gradient. Weak
chemotactic responses were assessed from ratios of fluxes observed in paired experiments
in which the sign of the gradient of attractant was reversed. Studies were made of
wild-type Escherichia coli and mutants that are nonmotile, tumblely, smooth-swimming,
aspartate-blind, or defective in methylation and demethylation. Chemotaxis drift velocities
for the latter mutants (cheRcheB) were quite small.