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New technique sharpens our view of phage assembly inside cyanobacteria

New electron microscopy technique unravels the mysteries of viral infection. Image courtesy Chui Lab.

Combining a device called a Zernike phase plate with electron cryo-tomography, a consortium of scientists that includes those from Baylor College of Medicine can now “see” the process of virus assembly inside cells.

“This unique facility at Baylor College of Medicine allows us to image marine bacteria infected with bacteriophages with stunning clarity,” said Dr. Wah Chiu, professor of biochemistry and molecular biology at BCM and director of the National Center for Macromolecular Imaging. Chiu is corresponding author of a report that appears in the journal Nature on the new biological findings with this technique.

He has long pioneered in electron cryo-microscopy, improving the ability to see smaller and smaller objects in greater detail. The technique begins by freezing a biological object such as a cell in a thin layer of ice so quickly that no crystals form, and avoids the use of harsh chemical fixation. The ice-embedded cell is then tilted at different angles and images are taken. Later, they are combined like a CT-scan to reveal the three-dimensional picture.

In this latest advance, in cooperation with technology developer Dr. Kuniaki Nagayama of the National Institute for Physiological Sciences of National Institutes of Natural Sciences in Japan, Chiu and his colleagues added a Zernike phase plate to the cryo-electron microscope to improve the contrast of the images. Zernike phase plates are made from thin carbon film with a tiny, 1-micron (4/100,000ths of an inch) hole that is drilled by a beam of ions. Images from this phase plate-equipped microscope show contrast three to five times greater than is seen in images taken in a normal cryo-electron microscope. The high contrast is accomplished by shifting the phase of scattered electrons, which enhances low-frequency information, enabling the in-focus, high contrast imaging seen on the attached video.

“Dr. Nagayama built on an old concept,” said Chiu. The concept that phase shifts in light passing through a transparent specimen can be translated into brightness changes in the image began with the work of Dr. Frits Zernike, who received a Nobel Prize in Physics in 1953. Nagayama translated this theory to practical application with his carbon phase plates.

“This marks the first use of this technique to look at the infection process inside the cell and we are able to visualize the assembly process, from baby to mature and infectious entities,” said Chiu.

The marine bacteria under study are photosynthetic organisms responsible for about 25 percent of all carbon fixation (the conversion of inorganic carbon such as carbon dioxide to organic compounds, using sunlight) by living organisms. The viruses (called phages) that infect these bacteria help regulate the marine ecosystem by controlling organization of bacteria community and mediating how genes are transferred from one bacteria to the other in the ocean.

Understanding phage infection and assembly inside the marine bacteria offers the promise of manipulating the bacteria that could be used for bioenergy development, he said. Genes could be inserted to tailor production of specific biofuels, or make it more efficient, or to make the bacteria more resistant to infection.

The technique also has promise in imaging human cells infected with disease-causing viruses, enabling a better understanding of infection.

“Potentially, we could look at neurons and other cells to show what happens inside the cells at different time points after initial viral infection,” said Chiu.

Others who took part in this work include: Wei Dai, Caroline Fu, John Flanagan, Htet A. Khant, Xiangan Liu, Ryan H. Rochat, Steve J. Ludtke, Michael F. Schmid, all of BCM; Desislava Raytcheva, Cameron Haase-Pettingell and Jonathan A. King all from Massachusetts Institute of Technology in Cambridge; and Jacqueline Piret of Northeastern University in Boston, Mass.

Funding for this work came from the Robert Welch Foundation (Grant Q1242) and National Institutes of Health (Grants P41GM123832, AI0175208 and PN2EY016525, GM080139, T15LM007093 through the Gulf Coast Consortia, and T32GM007330, which came from the Medical Scientist Training Program).

Chiu is a Distinguished Service Professor at Baylor College of Medicine and holds the Alvin Romansky Chair in in Biochemistry and Molecular Biology.