Gliding motility in bacteria: insights from studies of Myxococcus xanthus

Abstract

Gliding motility is observed in a large variety of phylogenetically unrelated bacteria. Gliding provides a means for microbes to travel in environments with a low water content, such as might be found in biofilms, microbial mats, and soil. Gliding is defined as the movement of a cell on a surface in the direction of the long axis of the cell. Because this definition is operational and not mechanistic, the underlying molecular motor(s) may be quite different in diverse microbes. In fact, studies on the gliding bacterium Myxococcus xanthus suggest that two independent gliding machineries, encoded by two multigene systems, operate in this microorganism. One machinery, which allows individual cells to glide on a surface, independent of whether the cells are moving alone or in groups, requires the function of the genes of the A-motility system. More than 37 A-motility genes are known to be required for this form of movement. Depending on an additional phenotype, these genes are divided into two subclasses, the agl and cgl genes. Videomicroscopic studies on gliding movement, as well as ultrastructural observations of two myxobacteria, suggest that the A-system motor may consist of multiple single motor elements that are arrayed along the entire cell body. Each motor element is proposed to be localized to the periplasmic space and to be anchored to the peptidoglycan layer. The force to glide which may be generated here is coupled to adhesion sites that move freely in the outer membrane. These adhesion sites provide a specific contact with the substratum. Based on single-cell observations, similar models have been proposed to operate in the unrelated gliding bacteria Flavobacterium johnsoniae (formerly Cytophaga johnsonae), Cytophaga strain U67, and Flexibacter polymorphus (a filamentous glider). Although this model has not been verified experimentally, M. xanthus seems to be the ideal organism with which to test it, given the genetic tools available. The second gliding motor in M. xanthus controls cell movement in groups (S-motility system). It is dependent on functional type IV pili and is operative only when cells are in close proximity to each other. Type IV pili are known to be involved in another mode of bacterial surface translocation, called twitching motility. S-motility may well represent a variation of twitching motility in M. xanthus. However, twitching differs from gliding since it involves cell movements that are jerky and abrupt and that lack the organization and smoothness observed in gliding. Components of this motor are encoded by genes of the S-system, which appear to be homologs of genes involved in the biosynthesis, assembly, and function of type IV pili in Pseudomonas aeruginosa and Neisseria gonorrhoeae. How type IV pili generate force in S-motility is currently unknown, but it is to be expected that ongoing physiological, genetic, and biochemical studies in M. xanthus, in conjunction with studies on twitching in P. aeruginosa and N. gonorrhoeae, will provide important insights into this microbial motor. The two motility systems of M. xanthus are affected to different degrees by the MglA protein, which shows similarity to a small GTPase. Bacterial chemotaxis-like sensory transduction systems control gliding motility in M. xanthus. The frz genes appear to regulate gliding movement of individual cells and movement by the S-motility system, suggesting that the two motors found in this bacterium can be regulated to result in coordinated multicellular movements. In contrast, the dif genes affect only S-system-dependent swarming.

FIG. 1

Occurrence of gliding bacteria among the eubacteria. The figure depicts examples of bacteria (indicated in italics) that have been reported to move by “gliding.” Note that “gliding” is an operational definition, and not all bacteria may glide by the same mechanism. No gliding archaea have been reported. Modified from reference .

FIG. 2

Two gliding motility systems are operative in M. xanthus. (Left) Colony morphology. (Right) Gliding speed plotted against cell-cell distance of individual cells (the detection limit for active movement was 1 μm/min [117]). (A) Wild-type DK1622 (A⁺S⁺). Gliding-speed data are from reference . (B) JZ315 (cglB, A⁻S⁺). Gliding-speed data are from reference . Note that when the cells were separated by more than 2 μm, no active single-cell movement was observed. When the cells were in contact, the velocities observed were similar to those found when wild-type cells moved in close proximity (A). (C) DK3473 (pilR, A⁺S⁻); cell movement of 50 individual DK3473 cells was examined as described previously (117). A total of 4,531 speed and cell-cell distance values were obtained and plotted. The speed of isolated cells was similar to that of wild-type cells. However, cells did not exhibit high-speed movements, as observed for wild-type and cglB cells (A and B) when in close proximity.

FIG. 3

Models for localization of motility elements on the surface of M. xanthus cells. Multiple motor units are located along the cell body. (A) In this model, a single motor unit can generate displacement in two directions that are opposite to each other (◂•▸). (B) In this model, two types of unidirectional motors exist (•▸ and ◂•), each capable of generating force in only one direction. The motor elements are arranged opposite each other, so that selected activation of one or the other results in movement in one or the other direction.

FIG. 4

Flexing of an M. xanthus cell. Cells of DK1680 (dsp) were grown in CTT liquid medium (117) to a density of about Klett 100 (∼5 × 10⁸ cells/ml), and 10-μl drops were placed on nitrocellulose-coated coverslips. Cell movement was monitored under an inverted microscope, and images were recorded during the observation period. In the absence of net movement, cells were observed to flex. This can be easily shown when superimposing two images that were recorded at different times. Dark areas indicate overlap of the cell during both recordings, and lighter areas show displacement of cell sections. The time difference between the two superimposed images was 1 min 48 s for the picture at top left (size bar included), 1 min 10 s for the picture at bottom left, 7 s for the picture at top right, and 27 s for the picture at bottom right. In the last picture, the lower cell was gliding in direction of the long axis of the cell while the upper cell flexed. Bar, 0.5 μm.

FIG. 5

Chain-like structures proposed to be involved in gliding motility. The images are from Myxococcus fulvus, and similar structures were observed in M. xanthus. (A) Isolated aggregates of chain-like strands showing different orientations of periodic structural elements. Arrowheads indicate rings. Bar, 50 nm. (B) Regular spacing and positioning of rings normal to the longitudinal axis of the strands (large arrowheads). Three strands form a morphological unit, the ribbon (solid triangles). Long arrows indicate proposed contracted units resulting in a herringbone pattern. Bar, 50 nm. (C) Three-dimensional model of the architecture of strands localized in the periplasmic space and anchored to the peptidoglycan layer and outer membrane. Picture courtesy of Freese et al. Three strands are presented. Conformation of ring elements which are located normal to the outer membrane is shown. The model is not drawn to scale. RE, ring element, framed; EE elongated element; CM, cytoplasmic membrane; OM, outer membrane; cema, central mass; pm peripheral mass. For a description, see the text. Panels A and B reprinted from reference with permission. Panel C reprinted from reference with permission.

FIG. 6

Polar pili on an M. xanthus cell. Picture kindly provided by Dale Kaiser. Bar, 0.5 μm. Reprinted from reference with permission.

FIG. 7

Localization of motility genes on the M. xanthus genome. AseI (outer circle) and SpeI (inner circle) restriction maps of the DK1622 chromosome are shown. Modified from references and with permission.

FIG. 8

Stimulation of motility of the agl and cgl A-motility mutants. Drops (on the left of each image) of a liquid culture of A-motility mutants (A⁻S⁺) (recipient cells) were spotted on CTT agar plates and allowed to dry, and another drop of DK6204 (donor cells), a nonswarming ΔmglBA mutant, was added to intersect with the former spot. (A) cgl recipient strain (A⁻S⁺). After a few hours, single cells of DK1219 are visibly gliding as individual cells. (B) agl recipient strain (A⁻S⁺). No stimulation of single-cell movement is observed. Bar, 20 μm.

FIG. 9

Organization of M. xanthus pil genes. Compiled from references , , and to . For details, see the text.

FIG. 10

Models for generation of cell movement by type IV pili in M. xanthus. To illustrate the model of pilus-dependent S motility, individual cells are not drawn to be in contact with other cells. It should be noted that cell-cell contact is required for S motility. (A) A depolymerization-polymerization (↔) shortens a pilus, thus generating displacement mostly in the direction of the long axis of the cell. (B) Pili attach to neighboring cells at “adhesion sites” (◂○▸), e.g., which are part of the gliding motor of A motility and are involved in A-motility-dependent gliding. Linked to the moving adhesion sites by a pilus, a neighboring cell is “dragged” along, thus performing a translocation in the direction of the long axis of the cell. (C) A combination of pilus retraction-extension and adhesion site-dependent displacement (A plus B).

FIG. 11

Speculative model for single-cell gliding (A motility) in M. xanthus. The picture is a close-up of the interface of the surface of a gliding cell with the substratum and shows one of the numerous motor units in detail. The cell is moving from left to right. In this model, the force generator (molecular motor) is anchored to the cell wall (rigid peptidoglycan layer), which functions as a skeleton of a bacterial cell. A force of action is generated mainly in the direction of the long axis of the cell and is coupled to an adhesion site in the outer membrane, which interacts with the substratum. Because the coupling of the adhesion site to the surface is tight, the force generator moves to the right relative to the adhesion site, and the relative displacement is indicated by the arrows. The chemical energy that the force generator transforms into mechanical work is delivered by an energy transducer and is derived from the electrochemical ion potential across the cytoplasmic membrane.