Myxobacterial tools for social interactions

Abstract

Myxobacteria exhibit complex social traits during which large populations of cells coordinate their behaviors. An iconic example is their response to starvation: thousands of cells move by gliding motility to build a fruiting body in which vegetative cells differentiate into spores. Here we review mechanisms that the model species Myxococcus xanthus uses for cell-cell interactions, with a focus on developmental signaling and social gliding motility. We also discuss a newly discovered cell-cell interaction whereby myxobacteria exchange their outer membrane (OM) proteins and lipids. The mechanism of OM transfer requires physical contact between aligned cells on a hard surface and is apparently mediated by OM fusion. The TraA and TraB proteins are required in both donor and recipient cells for transfer, suggesting bidirectional exchange, and TraA is thought to serve as a cell surface adhesin. OM exchange results in phenotypic changes that can alter gliding motility and development and is proposed to represent a novel microbial interacting platform to coordinate multicellular activities.

Fig. 1

Cellecell interactions within biofilms. A) SEMs that reconstruct temporal steps in M. xanthus fruiting body formation. Left-most panel shows a biofilm that has formed on a plastic surface when the bacteria were grown in a rich medium. Upon medium replacement with starvation buffer, development ensued. These developmental events occurred over 3 days. Fig. was adapted with permission from Kuner and Kaiser (1982). B) SEM of a M. xanthus biofilm showing the fibrils that connect individual cells. Fig. was adapted with permission from Kearns and Shimkets (2001). Scale bar, 2 µm. C) Graph depicts an idealized sliding scale of cell–cell interactions between bacteria in biofilms. At one extreme, cells only aggregate into a biofilm; there is no cell–cell signaling. At the other extreme are complex cell–cell interactions exemplified by myxobacteria and discussed in the text.

Fig. 2

Signaling pathways during M. xanthus development. Cells secrete an A-signal into the extracellular milieu that is taken up by responder cells and by themselves early (~2 h) during development. The C-signal requires cellecell contact and triggers developmental gene expression after 6 h of starvation. Cells produce and respond to developmental signals in a similar manner (i.e., bidirectionally); however, for simplicity the signal transduction pathways are depicted unidirectionally.

Fig. 3

Type IV pili and fibril EPS interactions. A pilus binds EPS on a neighboring cell, which triggers its retraction, pulling the cell forward. The pilus is also proposed to function as a sensor to activate the Dif signal transduction pathway, which results in EPS production. Other proteins known to influence EPS production are also shown and described in the text. Arrows with dashed lines indicate positive regulation, and bars indicate negative regulation. Circles with P represent a phosphate group; gray shaded ovals represent the pilin subunits and Dif proteins are labeled with letters. For details, see text and Black et al. (2006).

Fig. 4

Model for Tgl transfer and stimulation of S-motility. Juxtaposed cell poles of a Tgl⁺ donor (left) and a Δtgl recipient are shown. A tgl mutant expresses PilQ monomer (PilQ_M), but these monomers fail to assemble into a stable multimeric OM channel. Following physical contact, the Tgl protein is transferred from donor to recipient, where Tgl facilitates assembly of the PilQ secretin (Nudleman et al., 2005). Once PilQ is assembled, type IV pili are made at the cell pole to power S-motility.

Fig. 5

Cell motility aligns cells for OM exchange. A) Merged fluorescent micrograph on a hard agar surface of non-motile fluorescently-labeled donor (red, SS_OM-mCherry) and recipient (green, GFP) cells mixed with a third-party strain that is motile and non-fluorescent (1:1:1 ratio). B) Same cells as in A, except cells were harvested and placed on a glass slide for clear analysis of protein transfer; green recipients become yellow/orange in merged micrograph. C) Same non-motile labeled donor/recipients cells as in A, except without the motile strain. Cells are not aligned. D) Same cells as in C, except harvested cells are placed on a glass slide, showing that the non-motile cells do not transfer, as red and green cells remain distinct in color in the merged micrograph. For details, see text and Wei et al. (2011).

Fig. 6

Fluorescent markers used to monitor cell envelope exchange. The ability of markers to label specific cell envelope compartments and to be transferred is shown (Pathak et al., 2012; Wei et al., 2011). DiD is a lipid fluorescent dye, which based on our experience, becomes irreversible imbedded in the M. xanthus OM.

Fig. 7

A working model of OM exchange. The blue rectangles represent TraA, and the red and gray lollipops represent OM lipoproteins. See text for details.

Fig. 8

OM exchange regulates swarming and development. A) Motile and non-motile strains mixed at a 1:1 ratio and incubated for two days. Swarming is inhibited by a Tra-dependent mechanism. B) Non-motile cells are proposed to produce a signal(s) that is transferred to motile cells via TraAB that blocks swarming and development.