Sunday, February 22, 2009

Viewing the arrangement of Borrelia burgdorferi flagella by electron cryotomography

ResearchBlogging.orgThe most peculiar feature of spirochetes may be the location of their flagella, the thin motility structures that propel bacteria through liquids. Flagella typically extend out from the surface of bacteria into the surroundings. Spirochetes, being not so typical, keep their flagella hidden in the periplasm between the cytoplasmic and outer membranes (see figure). For example, the Lyme disease spirochete Borrelia burgdorferi has 7-11 flagella attached near each end of the "protoplasmic" or cell cylinder, with each flagellum extending through the periplasm towards the center of the spirochete. The flagella impose a flat-wave shape (not a spiral shape!) on B. burgdorferi by wrapping around its protoplasmic cylinder.


How do flagella that are located in the periplasm drive the spirochete through the medium? B. burgdorferi motility is thought to require the rotation of its flagella against the cell cylinder, causing the cell body to gyrate.

B. burgdorferi flagella often appear as a bundle when observed by standard transmission electron microscopy. Here is one such image from a 2000 study revealing at least 10 flagella in a cross section of B. burgdorferi. With the flagella arranged in this manner, it is difficult to imagine how the flagella that are not in direct contact with the cell cylinder could contribute to its gyration.

A study by Charon and colleagues in the January 2009 issue of Journal of Bacteriology suggests that the flagellar bundle is an artifact of the standard techniques used to prepare the samples for electron microscopy. They employed the emerging technique of electron cryotomography to avoid the fixation and staining procedures that often introduce artifacts into samples. Electron cryotomography consists of the following steps:
  1. To preserve structure, the specimen is plunge frozen at -165°C or less. Fixing or staining is not necessary.
  2. While maintaining the sample at the ultralow temperature, 2D projections of the sample are obtained at different angles by transmission electron microscopy.
  3. Computer software assembles the 3D structure of the specimen from the 2D projections.
The software also permits slices of the specimen to be observed without having to actually perform thin sectioning.

Here's a cross-section of B. burgdorferi as viewed by electron cryotomography. Note that the flagella are arranged in a single layer within the periplasm, not in a bundle.

Figure 1 of Charon et al. Bar, 50 nm.
PFs, periplasmic flagella; PS, periplasmic space; PM, plasma (or cytoplasmic) membrane; OM, outer membrane.

A longitudinal slice through the periplasm of B. burgdorferi reveals nine flagella neatly arranged in a parallel fashion along the surface of the protoplasmic cylinder. The authors refer to this array as a "flat ribbon." Each flagellum in the ribbon is separated by ~3 nm, allowing each to rotate in the same direction without interference from neighboring flagella.

Figure 5 of Charon et al. Bar, 200 nm.

3D reconstruction of a section of the spirochete illustrates the flat ribbon of flagella (in red) wrapping around the cell cylinder (in blue). Only a section of the cell cylinder is shown, and the outer membrane has been removed from the image.

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These new images support a model for for B. burgdorferi motility that was first described back in the 1990s. In this model, the rotation of the flagella against the cell cylinder generates gyrating waves that progress backwards along the cell body. As explained in the discussion of the Charon et al. paper, it is conceivable that all 7-11 flagella must lie against the cell cylinder as a flat ribbon to exert the force necessary to generate the waves; a flagella bundle may not exert enough force. The torque generated by the rotating flagella causes a counter rotation of the cell cylinder (panel a below). The backward-propagating, gyrating waves push the spirochete through the medium. Flagella arranged in a bundle would not generate enough torque because of potential interference between rotating flagella (panel b).

Figure 8 of Charon et al. a. Flagella arranged in a flat ribbon. b. Flagella arranged in a bundle.

This model also explains why B. burgdorferi moves so well through viscous gel-like material such as the extracellular matrix; the gel provides traction for the backward-progressing waves to drive the spirochete through the medium.

Here's a movie animating B. burgdorferi motility, first presented at a meeting in 2001 .

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SOURCE

You can also see real B. burgdorferi gyrating and generating backward-moving waves in a movie embedded in Dr. Nyles Charon's website.

Reference

N. W. Charon, S. F. Goldstein, M. Marko, C. Hsieh, L. L. Gebhardt, M. A. Motaleb, C. W. Wolgemuth, R. J. Limberger, N. Rowe (2009). The Flat-Ribbon Configuration of the Periplasmic Flagella of Borrelia burgdorferi and Its Relationship to Motility and Morphology Journal of Bacteriology, 191 (2), 600-607 DOI: 10.1128/JB.01288-08

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