Seeing Bacterial Bones with Cryo-EM Tomography
Until this decade, the notion that bacteria had cytoskeletons that maintained their cylindrical morphologies was not seriously considered. “People more or less thought the bacterial cell was a swimming pool and the chromosome was this ball of spaghetti,” says Stanford microbiologist Lucy Shapiro. The lack of apparent internal organization to bacteria supported this view. But then, as Ewen Callaway describes in Nature, MreB and FtsZ were identified as distant homologs to eukaryotic actin and tubulin, respectively.
Callaway’s history lesson is interesting in its own right, but what I thought interesting was the reference to new, higher-resolution attempts at discerning intracellular structure in bacteria with Cyro-electron tomography. Grant Jensen’s lab at CalTech, for instance, has been using this technique to image the actin-like protein MamK, found in bacteria endowed with iron-containing structures called magnetosomes. MamK filaments (yellow, at right) organizes a chain of magnetosomes (iron-containing structures) in the magnetic bacterium Magnetospirillum magnetotacticum.
Cryo-EM tomography has also been used to construct a three-dimensional visualization of a complete eurkarotic cell (S. pombe). The power of such high-power resolution to confirm sequence homologies with functional data is really quite neat. As Callaway notes:
A more promising technology — cryo-electron tomography — might be the answer [to lack of resolution of organization within bacteria]. Instead of coating cells with gold or dousing them in harsh fixatives, cryo-EM, as it is often called, takes pictures of flash-frozen samples. “We’re looking at cells in a nearly native state,” says Grant Jensen, a biologist at California Institute of Technology in Pasadena. The gentle treatment keeps the bacterial cytoskeleton intact. “If you thawed them out, most of them would probably swim away.”
Cryo-EM has the added benefit of allowing researchers to combine numerous angles of a cell into a three-dimensional picture, just like a computed tomography scan does. Recently, Jensen’s lab collected images of rings of FtsZ lining the insides of a bacterium called Caulobacter and pinching its membrane — a model predicted by others but never seen before.
Just think of what else Cryo-EM can do.
- Ewen Callaway. Cell biology: Bacteria’s new bones, Nature 451, 124-126 (2008). DOI: 10.1038/451124a
- Komeili A, Li Z, Newman DK, Jensen GJ. Magnetosomes Are Cell Membrane Invaginations Organized by the Actin-Like Protein MamK, Science 311, 242-245 (2006). DOI: 10.1126/science.1123231
About the Author
Dan Rhoads
Dan is a postdoc working at the University of Cyprus in developmental biology. He has a BSc in molecular biology and a PhD pharmacology and biochemistry.
Share