A strange cave bacterium forms a multicellular “body”, like plants and animals | Science


A newly described bacterium is shaking up what it means to be a complex organism. The bizarre microbe, discovered by Japanese ecologists researching biodegradable plastics, starts with a single cell. But instead of remaining a single cell like most microbes, it then develops an organized body made up of hundreds of cells. When the time is right, the cellular conglomerate produces a new generation of individual cells to begin this multicellular life cycle again, acting much like a plant growing and producing seeds or a human giving birth to babies.

The work “opens up a new perspective for exploring a new form of multicellularity,” says William Ratcliff, an evolutionary biologist at the Georgia Institute of Technology who was not involved in the work.

The appearance of multicellular organisms was a key evolutionary advance for life on Earth. This allowed the creatures to develop cells with specialized roles, grow larger, and further exploit their environment. Plants and animals are the classic examples of multicellular life, but some microbes also exhibit some simple aspects of group behavior. For example, microbes called cyanobacteria form complex chains of cells. And under stress, unicellular Myxococcus the bacteria come together to form a mobile, stalked fruiting body, allowing them to move around in search of better conditions.

Kouhei Mizuno was not looking for other examples of such microbial complexity when he collected a bacterium designated “HS-3” 17 years ago from a cave in Kyushu, an island south of Japan. The microbial ecologist from Japan’s National Institute of Technology and his colleagues had collected microbes from extreme environments, such as hot springs and caves. They hoped to find new enzymes that could be used to make biodegradable plastic. When he grew HS-3 on a lab dish, its unusual iridescent glow suggested it was no ordinary microbe.

The HS-3 strands form a long folding chain.Kouhei Mizuno

Mizuno and his colleagues found that once individual HS-3 cells landed on a surface, they began dividing into long filaments. These strands form a long chain that bends (see video above) and forms a colorful 2D network, similar to the liquid crystals found in cell phones and other electronic displays. No other microbe is known to do this. After about 5 days, the flat sheet began to thicken as an accumulation of opaque rod-shaped cells. When submerged in water, these cells sprout from the microbe’s amassed “body” (see video below), ready to start a new colony, the team reported this month in eLife. The rod-shaped cells “made me feel like I encountered an alien worm in a sci-fi movie,” Mizuno recalls.

Rod-shaped cells sprout from the microbial “body” when immersed in water.Kouhei Mizuno

Unlike other known forms of multicellular bacteria, HS-3 not only had different cell types; each type had a distinct structure and formed at different times in its life cycle, as if responding to specific environmental cues – in this case submersion in water. For a microbe, “these details are quite specific, explains Marco La Fortezza, an evolutionary biologist at ETH Zürich who did not take part in the work.

Yet its way of life follows a certain logic, Mizuno points out: HS-3 was collected from a wall above an underground stream. On this wall, it can form its body and accumulate its rod-shaped cells. Then, when the rain causes the stream to rise and flood its body, these cells are released and disperse into the environment.

“It’s kind of an intriguing organism,” says Paul Rainey, an evolutionary geneticist at the Max Planck Institute for Evolutionary Biology. “I would like to know a lot more about it.” But he and others aren’t convinced that HS-3’s multicellularity is comparable to that of a human, a pine tree, or even a jellyfish.

“The term ‘multicellularity’ is not well defined,” says Katrin Hammerschmidt, an evolutionary biologist at Christian Albrecht University in Kiel, so it’s hard to say whether HS-3 is eligible. In his mind, the cells of a true multicellular organism can only survive and reproduce as part of a larger organism, which is not the case with HS-3. And she and others point out that large, multicellular organisms must be susceptible to natural selection. Thus, for her, “the cave bacterium is another example of a prokaryote with a transient multicellular stage”.

Ratcliff admits that HS-3’s status is up in the air. “There are multicellular,” he says, “and [then] there is multicellularity, where the group of cells becomes the evolutionary unit. It is clear that this bacterium is multicellular, but further studies are needed to demonstrate its multicellularity, according to Rainey and Mizuno.

Either way, the researchers all agree that the discovery of HS-3 provides strong evidence for the role of the environment – ​​in this case, flooding – in encouraging the evolution of complex organisms. “The more different examples we discover and analyze,” says Hammerschmidt, “the more we learn about the origin and maintenance of multicellularity.”


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