Mid-infrared microscope used to look inside bacteria

Researchers at The University of Tokyo (UTokyo) have constructed an improved mid-infrared microscope, enabling them to see the structures inside living bacteria at the nanometre scale. Their breakthrough has been described in the journal Nature Photonics, and opens the door for developing even more accurate mid-infrared-based imaging in the future.

Thanks to modern microscopes, we can venture down to see the inner workings of our very own cells. But even these tools have limitations — for example, super-resolution fluorescent microscopes require specimens to be labelled with fluorescence, which can sometimes be toxic to samples. Electron microscopes can also provide very impressive details, but samples must be placed in a vacuum, so live samples cannot be studied.

By comparison, mid-infrared microscopy can provide both chemical and structural information about live cells, without needing to colour or damage them. However, its use has been limited in biological research because of its comparatively low-resolution capability. While super-resolution fluorescent microscopy can narrow down images to tens of nanometres (1 nanometre being one-millionth of a millimetre), mid-infrared microscopy can typically only achieve around 3 microns (1 micron being one-thousandth of a millimetre). However, the Tokyo researchers now claim to have attained a higher resolution of mid-infrared microscopy than ever before.

“We achieved a spatial resolution of 120 nanometres — that is, 0.12 microns,” said UTokyo’s Professor Takuro Ideguchi. “This amazing resolution is roughly 30 times better than that of conventional mid-infrared microscopy.”

The team used a ‘synthetic aperture’ — a technique combining several images taken from different illuminated angles to create a clearer overall picture. Typically, a sample is sandwiched between two lenses. The lenses, however, inadvertently absorb some of the mid-infrared light. They solved this issue by placing a sample of bacteria on a silicon plate which reflected visible light and transmitted infrared light. This allowed the researchers to use a single lens, enabling them to better illuminate the sample with the mid-infrared light and get a more detailed image.

“We were surprised at how clearly we could observe the intracellular structures of bacteria,” Ideguchi said. “The high spatial resolution of our microscope could allow us to study, for example, antimicrobial resistance, which is a worldwide issue.

“We believe we can continue to improve the technique in various directions. If we use a better lens and a shorter wavelength of visible light, the spatial resolution could even be below 100 nanometres. With superior clarity, we would like to study various cell samples to tackle fundamental and applied biomedical problems.”

Image caption: This illustration represents bacteria being illuminated with mid-infrared in the top left, while visible light from a microscope underneath is used to help capture the image. Image ©2024 Ideguchi et al/Nature Photonics