In humans, repeated encounters with a wide variety of stimuli can reactivate specific neurons to produce long-term memories. In new research, biologists at the University of Texas at Austin discovered a multigenerational memory in Escherichia coli’s swarming motility. They found that Escherichia coli use iron levels as a way to store information about different behaviors that can then be activated in response to certain stimuli.
Bhattacharyya et al. show that a prior experience of swarming is remembered when Escherichia coli encounters a new surface, improving its future swarming efficiency. Image credit: James Archer, CDC.
Constantly changing environments present a challenge to the survival of all organisms.
Vertebrates use their nervous system for faster decision-making by storing information about their prior experiences.
The process of storage and retrieval of information is called memory, which can be stored for a few seconds to several years.
A stronger memory can be brought to bear on the system by ‘conditioning’ — a process where repeated encounters with a stimulus can be stably linked to a specific response, drastically reducing decision-making time.
Biologists had previously observed that bacteria that had a prior experience of swarming (moving on a surface as a collective using flagella) improve subsequent swarming performance.
University of Texas at Austin researcher Souvik Bhattacharyya and his colleagues set out to learn why.
“Bacteria don’t have brains, but they can gather information from their environment, and if they have encountered that environment frequently, they can store that information and quickly access it later for their benefit,” Dr. Bhattacharyya said.
“It all comes back to iron, one of the most abundant elements on Earth. Singular and free-floating bacteria have varying levels of iron.”
The authors observed that Escherichia coli cells with lower levels of iron were better swarmers.
In contrast, bacteria that formed biofilms, dense, sticky mats of bacteria on solid surfaces, had high levels of iron in their cells.
Bacteria with antibiotic tolerance also had balanced levels of iron.
These iron memories persist for at least four generations and disappear by the seventh generation.
“Before there was oxygen in the Earth’s atmosphere, early cellular life was utilizing iron for a lot of cellular processes,” Dr. Bhattacharyya said.
“Iron is not only critical in the origin of life on Earth, but also in the evolution of life.”
“It makes sense that cells would utilize it in this way.”
The researchers theorize that when iron levels are low, bacterial memories are triggered to form a fast-moving migratory swarm to seek out iron in the environment.
When iron levels are high, memories indicate this environment is a good place to stick around and form a biofilm.
“Iron levels are definitely a target for therapeutics because iron is an important factor in virulence,” Dr. Bhattacharyya said.
“Ultimately, the more we know about bacterial behavior, the easier it is combat them.”
The team’s paper was published in the Proceedings of the National Academy of Sciences.
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Souvik Bhattacharyya et al. 2023. A heritable iron memory enables decision-making in Escherichia coli. PNAS 120 (48): e2309082120; doi: 10.1073/pnas.2309082120
