Rippling is a predatory behavior in Myxococcus xanthus

Abstract

Cells of Myxococcus xanthus will, at times, organize their movement such that macroscopic traveling waves, termed ripples, are formed as groups of cells glide together on a solid surface. The reason for this behavior has long been a mystery, but we demonstrate here that rippling is a feeding behavior which occurs when M. xanthus cells make direct contact with either prey or large macromolecules. Rippling has been observed during two fundamentally distinct environmental conditions: (i) starvation-induced fruiting body development and (ii) predation of other organisms. Our results indicate that case (i) does not occur in all wild-type strains and is dependent on the intrinsic level of autolysis. Analysis of predatory rippling indicates that rippling behavior is inducible during predation on proteobacteria, gram-positive bacteria, yeast (such as Saccharomyces cerevisiae), and phage. Predatory efficiency decreases under genetic and physiological conditions in which rippling is inhibited. Rippling will also occur in the presence of purified macromolecules such as peptidoglycan, protein, and nucleic acid but does not occur in the presence of the respective monomeric components and also does not occur when the macromolecules are physically separated from M. xanthus cells. We conclude that rippling behavior is a mechanism utilized to efficiently consume nondiffusing growth substrates and that developmental rippling is a result of scavenging lysed cell debris.

FIG. 1.

Predatory behavior of M. xanthus. M. xanthus strain DZ2 cells mixed with India Ink (left) and E. coli cells (right) were pipetted as colonies 1 mm apart on CF medium. The M. xanthus swarm expands from the initial spot in a tangled motility pattern. Lysis of E. coli prey cells occurs as M. xanthus cells make direct contact with the E. coli colony. Expansion of the swarm through the E. coli colony induces rippling motility behavior. Beyond the E. coli colony, the swarm resumes the tangled motility behavior. Pictures were taken at the following time points: 16 h (A and B), 40 h (C and D), and 64 h (E and F). The panels on the left were captured at a magnification of ×20. The panels on the right were captured at a magnification of ×200 and correspond to the regions marked with rectangles on the left.

FIG. 2.

Rippling behavior is essential for efficient predation. Replicate 10-μl aliquots containing ∼10⁹ E. coli prey cells were pipetted onto dried 10 μl spots of ∼10⁷ M. xanthus cells on either CF or CYE agar. At the times shown, E. coli cells were harvested and the number of E. coli survivors was measured as CFU. The lines represent E. coli only on CF (▪) and CYE (□), E. coli in the presence of DZ2 on CF (•) and CYE (○), and E. coli in the presence of DZ2 ΔpilA mutant on CF (▴) and CYE (▵).

FIG. 3.

Rippling differences observed in wild-type strains DK1622 and DZ2. M. xanthus cells from strains DK1622 and DZ2 were pipetted in the presence and absence of E. coli prey on CF agar and incubated at 32°C for 72 h. (A) DK1622 alone; (B) DZ2 alone; (C) DK1622 with E. coli; (D) DZ2 with E. coli. Although rippling is observed in both M. xanthus strains in the presence of prey E. coli cells, rippling was only observed in the pure cultures of strain DK1622.

FIG. 4.

Lytic differences observed in wild-type strains DK1622 and DZ2. Log-phase cultures of strains DK1622 (⧫) and DZ2 (•) were harvested and washed in MMC buffer. One-milliliter cell suspensions containing ∼10⁸ cells/ml were added to 12-well plates and incubated at 32°C. Cells were counted directly in a hemacytometer at the times indicated. Due to different levels of cell lysis, ∼10⁸ cells were added to wells, and the viability was measured through direct cell counts at the times indicated.

FIG. 5.

Effect of dead cells on developmental rippling. Ten-microliter aliquots of M. xanthus cells were pipetted onto CF media as mixtures of live and dead (heat-killed) cells and photographed after 24 h of incubation at 32°C. (A) 10% dead cells; (B) 80% dead cells. Ten-microliter aliquots of live cells were also pipetted separate from but adjacent to the same quantity of dead cell material. (C) 10% dead cells; (D) 80% dead cells. The volume of the dead cell material in each of these aliquots was raised to 10 μl with water.