Chemotaxis Control of Transient Cell Aggregation

Abstract

Chemotaxis affords motile cells the ability to rapidly respond to environmental challenges by navigating cells to niches favoring growth. Such a property results from the activities of dedicated signal transduction systems on the motility apparatus, such as flagella, type IV pili, and gliding machineries. Once cells have reached a niche with favorable conditions, they often stop moving and aggregate into complex communities termed biofilms. An intermediate and reversible stage that precedes commitment to permanent adhesion often includes transient cell-cell contacts between motile cells. Chemotaxis signaling has been implicated in modulating the transient aggregation of motile cells. Evidence further indicates that chemotaxis-dependent transient cell aggregation events are behavioral responses to changes in metabolic cues that temporarily prohibit permanent attachment by maintaining motility and chemotaxis. This minireview discusses a few examples illustrating the role of chemotaxis signaling in the initiation of cell-cell contacts in bacteria moving via flagella, pili, or gliding.

FIG 1

Chemotaxis-dependent cell aggregation. Chemotaxis signaling, by modulating motility patterns, can promote transient cell-cell interactions between cells motile by flagella, by type IV pili, or by gliding. The examples shown are discussed in the text and illustrate transient cell aggregation in A. brasilense (left), M. xanthus (middle), and Synechocystis (right). The double arrows indicate reversible events modulated by chemotaxis signal transduction. The unidirectional arrows represent a committed transition to irreversible attached states.

FIG 2

Clumping in A. brasilense. The image sequences represent frames taken at 0.6-s intervals (T, time) from a video recording of free-swimming A. brasilense cells. The cultures were prepared by growing A. brasilense from a single colony, under elevated aeration, as described previously (8). The images were obtained by dark-field microscopy at ×40 magnification. Cells in transient stable clumps are visible. The arrows point to instances when a motile cell leaves a clump (top row) or joins a clump (bottom row). Cells in the suspension and the clumps were motile.

FIG 3

Twitching and gliding and cell aggregation in M. xanthus. Scanning electron (top) and phase-contrast (bottom) micrographs and artist sketches from the prints illustrating the initiation of aggregation during M. xanthus development. The movement of motile cells in spiral patterns depends on transient cell contacts, and motile cells may leave or remain within the aggregates. Scale bar, 10 μm. Adapted from reference with permission.

FIG 4

Phototaxis and aggregation in twitching Synechocystis cells. Left, communities of Synechocystis cells exhibit phototaxis, i.e., they move directionally toward white light (incident from the top). This image was taken 2 days after a drop of cells was placed on a low-concentration (0.4%) agarose plate. The original drop can be seen, as well as the finger-like projections of cells moving toward the light. Right, image of single cells within these finger-like projections moving toward white light (incident from the top), taken at ×20 magnification under the microscope. Time-lapse video microscopy of the cell populations can be used to track the motility behavior of cells (33, 70). Courtesy of Rosanna Chau and Devaki Bhaya, reproduced with permission.