近距离观测细菌运动

Bacteria are the most abundant form of life on Earth, and they are capable of living in diverse habitats ranging from the surface of rocks to the insides of our intestines¹. Over millennia², these adaptable³ little organisms have evolved a variety of specialized⁴ mechanisms⁶ to move themselves through their particular environments. In two recent Caltech studies, researchers used a state-of-the-art imaging technique to capture, for the first time, three-dimensional views of this tiny complicated machinery⁷ in bacteria. "Bacteria are widely considered to be 'simple' cells; however, this assumption is a reflection of our limitations, not theirs," says Grant Jensen, a professor of biophysics and biology at Caltech and an investigator⁸ with the Howard Hughes Medical Institute (HHMI). "In the past, we simply didn't have technology that could reveal the full glory of the nanomachines--huge complexes comprising many copies of a dozen or more unique proteins--that carry out sophisticated functions."

 

Jensen and his colleagues used a technique called electron cryotomography to study the complexity⁹ of these cell motility nanomachines. The technique allows them to capture 3-D images of intact cells at macromolecular resolution--specifically, with a resolution that ranges from 2 to 5 nanometers (for comparison, a whole cell can be several thousand nanometers in diameter). First, the cells are instantaneously frozen so that water molecules¹⁰ do not have time to rearrange to form ice crystals; this locks the cells in place without damaging their structure. Then, using a transmission electron microscope, the researchers image the cells from different angles, producing a series of 2-D images that--like a computed¹¹ tomography, or CT, scan--can be digitally reconstructed into a 3-D picture of the cell's structures. Jensen's laboratory is one of only a few in the entire world that can do this type of imaging.

 

In a paper published in the March 11 issue of the journal Science, the Caltech team used this technique to analyze¹² the cell motility machinery that involves a structure called the type IVa pilus machine (T4PM). This mechanism⁵ allows a bacterium¹³ to move through its environment in much the same way that Spider-Man travels between skyscrapers¹⁴; the T4PM assembles a long fiber¹⁵ (the pilus) that attaches to a surface like a grappling hook and subsequently retracts¹⁶, thus pulling the cell forward.

 

Although this method of movement is used by many types of bacteria, including several human pathogens, Jensen and his team used electron cryotomography to visualize¹⁷ this cell motility mechanism in intact Myxococcus xanthus--a type of soil bacterium. The researchers found that the structure is made up of several parts, including a pore on the outer membrane¹⁸ of the cell, four interconnected ring structures, and a stemlike structure. By systematically¹⁹ imaging mutants, each of which lacked one of the 10 T4PM core components²⁰, and comparing these mutants with normal M. xanthus cells, they mapped the locations of all 10 T4PM core components, providing insights into pilus assembly, structure, and function.

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