Bacterial cells reveal skeletal structures

Bacteria are different from you and me. Always the minimalists, they lack features that plant and animal cells usually can’t do without: a nucleus, special organelles, and an internal skeleton made of protein, to name a few. But research reported in the March 23 Cell knocks out one plank of this standard profile–bacteria, too, have a protein skeleton, or cytoskeleton.

“This is akin to finding the platypus, a mammal that lays eggs,” says Laura J.F. Jones, who revealed the skeleton in Bacillus subtilis with her colleagues Rut Carballido-López and Jeffery Errington, all of Oxford University in England.

The researchers say their finding helps illuminate the origins of our own cell structure and eliminates a fundamental difference between two of the most basic groups of organisms, prokaryotes (bacteria and blue-green algae) and eukaryotes (plants, animals, and protozoans).

“Spectacular” is the how cell-mechanics researcher Piet De Boer of Case Western Reserve University in Cleveland rates the Oxford team’s unmasking of a bacterial cytoskeleton. “Bacteria have really been thought of as bags of enzymes without much of an internal structure at all,” says De Boer.

Bacteria were believed to have only a tough cell wall for support. Even powerful electron microscopes have failed to turn up any distinct internal structure. In contrast, eukaryotic cells, which evolved after bacteria, have a network of filaments for support and movement. A protein known as actin forms much of this cytoskeleton, which can look like a bushy spray of fibers.

In the past decade, bacteriologists have searched for complex structures in bacteria by using techniques for tagging proteins with fluorescent markers. These studies, which can illuminate otherwise hidden structures, have yielded evidence of a higher level of organization than previously believed, says De Boer.

Using fluorescent tags made with antibodies that can bind to specific proteins, the Oxford investigators looked for a bacterial cytoskeleton in the rod-shaped B. subtilis. “It seemed likely to me that something as important as the cytoskeleton must have evolved quite early, so I almost expected to find actin in bacteria even though the textbooks say it is absent,” says Errington.

He and his colleagues focused on two bacterial proteins, MreB and Mbl, because of evidence that the genes coding for them have roles in determining cellular shape. Disabling the gene for MreB resulted in rounded cells, while disabling the gene for Mbl yielded elongated, twisted bacteria. Using a different fluorescent antibody to light up the intact protein in each altered cell, the researchers revealed complex internal structures made of either MreB or Mbl.

“We were ecstatic when we saw the first MreB and Mbl images, because they immediately told us that the proteins probably made filaments like actin,” says Errington. Coiling within the cell as they did, the filaments clearly could determine cell shape in normal bacteria, he says.

Errington likens the filamentous structure to a scaffold: It doesn’t have great strength itself, but instead provides the internal framework for a sturdier exterior shell, in this case the bacterium’s tough cell wall.

The finding suggests that the cytoskeleton evolved before bacteria and our own cellular ancestors split into two groups, says Errington. Having a cytoskeleton isn’t a defining feature of eukaryotic cells after all, he asserts.