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Three-dimensional direct current (DC) servo system in which errors in the position of the image of a bacterium sensed at the top of the microscope (where one normally places a camera) are used to control the position of a small chamber holding a cell suspension, so that the image (and hence the bacterium) remains fixed in the laboratory reference frame. To follow the movement of the bacterium, all one has to do is write down the position (the x, y, and z coordinates) of the chamber. It’s rather like following the progress of a worm in a bucket of soil by moving the bucket Tracking Bacteria 33 so that the worm remains fixed in the reference frame of one’s garden.

Those that remain motile leave the spot and move outward, rejoining the band. This raises the concentrations of bacteria at the points where they rejoin, and new spots form there. Thus, one gets radial arrays of spots, as shown in Fig. 7a. The spots are frozen in place, because the cells, having run out of nutrient, soon stop swimming. 7. (a) Cells of a mutant of E. coli chemotactic to aspartate but not to serine that have spread outward in a soft-agar plate to form radial arrays of spots. (b) Cells of the same kind that have formed a hexagonal array of spots.

Greigite). These cause the cells to line up with the earth’s magnetic field, so they behave like swimming compass needles (Blakemore, 1975; Frankel, 1984; Frankel and Blakemore, 1991). E. coli (without the photoreceptor transplant) is damaged by light at high intensities; it does not respond to magnetic fields. It is sensitive in the blue, so when working under a microscope at high intensities, one needs to use a cutoff filter that blocks wavelengths shorter than about 500 nm. , 1979). More Exotic Patterns The formation of chemotactic rings (Figs.