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For decades, scientists have believed the human brain is a sterile fortress, protected from microbial invaders by the robust blood-brain barrier. But a new study, published in Science Advances, challenges this assumption by showing bacteria can’t just make their way to the brain, they can thrive there.

Researchers from the University of New Mexico, led by biologist Irene Salinas, made this startling revelation when studying salmon and trout. Using DNA extraction and microscopic imaging, they identified living bacteria in the fishes’ olfactory bulbs and other brain regions. The results showed the olfactory bulb, which is directly connected to the nasal cavity, harboured bacteria as did deeper-lying brain tissue.

The presence of bacteria in fish brains raised several questions. The foremost was about how they managed to cross the blood-brain barrier. Salinas & co. discovered that many of these microbes possessed unique adaptations that helped them breach the barrier. Some produced molecules called polyamines that can open tight junctions in the barrier fluid; others were able to evade immune responses or outcompete their rivals, ensuring their survival in the brain’s delicate environs.

The group also explored the origins of these brain-dwelling microbes. Some bacteria seemed to have colonised the brain much before the blood-brain barrier had evolved to its present form. Others likely travelled up from the gut or the bloodstream, continuously infiltrating the brain throughout the fishes’ lives. The researchers said the presence of more than one pathway suggests the brain’s microbial community is dynamic, shaped by both early colonisation and ongoing interaction with other bodily systems.

A particularly striking finding was the image of a bacterium caught mid-transit across the barrier, offering direct visual evidence. Some researchers have hypothesised these microbes might be engulfed by immune cells while others have suggested they could play active roles in physiological processes — just like the human gut microbiome does in regulating digestion, immunity, and mood.

Fish are very different from humans yet the study also opens the door to rethinking the brain’s microbiome in vertebrates, including humans. If bacteria can thrive on fish brains, it’s possible they may do so on human brains as well.

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(This article forms a part of the Science for All newsletter that takes the jargon out of science and puts the fun in! Subscribe now!)

When some object stands in the path of light, the object casts a shadow behind it. It’s the place the light can’t reach because of the object. The question is if some light can block some other light, thus casting a shadow of its own. Sometimes light can interfere with other light, but that’s not really the same thing: the dark patches in an interference pattern aren’t shadows so much as darkness created by the elimination of light (waves).

In a new study, a team from universities in the U.S. and Canada has reported creating a shadow as a result of one beam of light blocking another. The team members placed three items in a straight line with some distance between them: a blue laser source, a small cubic crystal, and a camera with a blue laser. The blue light illuminated the cube before hitting the camera lens. Then the team hit the cube with a green laser in a perpendicular direction. As the green light cut across the path of the blue light through the cube, a slender dark shadow appeared in the camera’s view. When the researchers changed the position of the green laser source, the shadow moved as well. It was also visible to the naked eye when the team placed a sheet of paper in front of the camera lens.

This result isn’t as straightforward as a lamppost casting a shadow on the ground. It was possible because the crystal, inside which the two light beams intersected, was made of ruby — a mineral consisting of aluminium oxide and a small population of chromium atoms. When the green light struck the chromium nuclei, they absorbed the energy and jumped to a higher energy level. Then they shed only a part of this energy to slide down to an intermediate level.

At this point the nuclei could absorb photons from the blue light and jump to a new higher energy level. This absorption effectively blocked the blue light and created a shadow in the camera’s image.

The green and blue beams have to have just the right colour or they’d carry too much or too little energy to result in these energy transitions.

Even if the shadow was real and true to its name, it was also peculiar. First, it wasn’t due to the ruby crystal. Instead the green light changed the crystal’s response to blue light. Without the green light also illuminating the cube, the chromium atoms respond differently to the blue. Second, photons — the particles of light — don’t have mass or a physical size and thus can’t cast shadows. Yet there is a shadow thanks to polaritons: packets of energy consisting of photons strongly coupled to the atomic excitations. Polaritons can have mass and thus cast shadows.

Playful though the experiment seems to be, the research team already foresees value and important applications of their findings. As the team’s paper, published in the journal Optica, concludes: “Potential applications can be envisioned in areas such as optical switching, controllable shade or transmission, control of the opaqueness of light with light, and lithography” — the last an image-making technique that uses flat surfaces.

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