The book Behavior of the Lower Organisms, written by naturalist Herbert Spencer Jennings in 1906, contains a novel idea: bacteria may alter their thoughts.
Stentor, a single cell bristling with beating hairs, was his topic. These trumpet-shaped predators are so enormous that they can be eaten by giant fish and seen by humans, and they are so fearless that they can catch and consume rotifers, which are natural organisms with hundreds of cells and a primitive brain. In the microbial galaxy, stentors are found halfway between the Star Destroyer and the Sarlacc Pit.
Jennings decided to irritate it to see what would happen. Stentor would bend aside when presented with a spray of stinging carmine powder skilfully targeted at their mouths by his steady hand, then reverse the beating of its hairs (called cilia) to expel the powder, then constrict and eventually detach.
He noticed that the order of activities changed slightly depending on the stimulus (he tested different chemicals) and that steps were occasionally skipped. “However, it is true,” he continued, “that under situations that increasingly interfere with the organism’s regular operations, behavior consists in ‘testing’ successively diverse reactions until one provides relief.”
In other words, stentors may respond to a stimulus with one behavior, then switch to a more expensive strategy if the irritation continued. It could “remember” that it had tried one approach and failed, at least for a short time (a timeframe that Jennings described as difficult to quantify experimentally and still unresolved).
However, experts from a different school of animal behavior repeated his experiment in 1967 and could not replicate his results. And with that, Jennings’ discoveries were sent to the dustbin of history.
The experiment and its defenestration were then uncovered by Jeremy Gunawardena, an associate professor of systems biology at Harvard Medical School, roughly ten years ago. He believed it merited further examination. He was surprised to learn that the 1967 team had not utilized the proper Stentor species (Being behaviorists, they could have thought species didn’t matter because variation came from the environment rather than DNA.) Unlike Jennings’ Stentor roeselii, which likes to relax poolside, the one they picked, Stentor coeruleus, loves to swim.
Gunawardena was enthralled by the prospect of recreating the experiment and learning more about what single cells are capable of. After years of futile dangling of the concept at lab meetings, he discovered that undergrad Joseph Dexter and postdoc Sudhakaran were eager to give it a go at night and on weekends—with no financing.
This time, the Harvard team was able to find the suitable species in an English golf course pond and build their own “Device for Irritating Stentors.” (They didn’t have Jennings’ extraordinary pipette abilities because they were quantitative biologists) and discovered something astonishing.
Stentor did not react to carmine powder the manner Jennings stated in their setup. However, when confronted with swarms of 21st-century plastic microbeads, Individual Stentor roeseli acted as described by Jennings, with one notable exception that Jennings did not notice in 1906.
If Stentor can honestly “decide,” it’s not the only way the ciliates—the hairy bacteria that Stentor belongs to—resemble humans. At the scale of a single massive cell, a ciliate behaves like an animal, and the likeness may be astonishing.
Some animals, for example, glue cilia bundles together to form cirri, which they can utilize as legs, jaws, paddles, or teeth—Euplotes skitters nimbly across cirri’s surfaces, like a water flea from Close Encounters. Nervelike neurofibrils connect the cirri. The cirri become limp when the fibrils are severed.
Some ciliates carry tethered darts that they can use to assault prey, ward off predators, or just anchor. Others have food-grabbing tentacles. Ciliates, like sea stars, may regenerate complete bodies in a day or two from tiny parts if those fragments retain both a component of the cell’s cilia-studded armor and a portion of the cell’s genetic heart, the nucleus. Many ciliates split by pinching in half; however, other stalked or sessile ciliates use a specific birth canal to propel tiny spherical larvae into the world.
Diplodinium is a ciliate that lives in the rumen of cows and other hoofed animals, a unique habitat known to support a wide variety of unusual creatures, roughly half of which are ciliates (consider that the next time you see a cow placidly chewing its cud). Neurofibrils, cirri, myonemes (muscle-like striated contractile fibers), a “backbone” comprised of stacked plates, a mouth, and an esophagus that contracts with assistance of a ring connected to its exterior, and an anus are all found in Diplodinium. But keep in your mind that it is a single cell.
In a nutshell, ciliates have pushed the biology of the single cell to its apparent limit on the planet. Once you realize this, having anything like a noggin in there requires less straining of the imagination.
Stentor shifted behaviors in response to repeated puffs or beads, according to new research published in the journal Current Biology in 2019. The order of activities was essentially similar to Jennings’ account. Contracture invariably preceded detachment, and mathematical calculations demonstrated that cilia alternation or bending occurred considerably more frequently before than after contraction.
Another intriguing aspect of their research, which I invite you to investigate for yourself, is that it appears that stentors have personalities. Some were prepared to bear irritation—even to live dangerously—by continually contracting and relaxing, or by bending, contracting, then relaxing. This was the viewpoint of the optimists.
Some people only contract once or twice, never to relax again. Others shrank and withdrew, and that was the end of it. These were the pessimists (or perhaps simply those who had recently completed a successful “door dash”).
Some stentors only ever replied with one or two favorite actions, never with others they were physically capable of. After 13 bead blasts, one calm person reacted with ciliary alternation or contraction, never bending or detaching.
Is Stentor capable of making judgments in the same way that humans do? This study, as well as Jennings’ evidence, strongly implies that this is the case.
Finally, there came a startling discovery. According to this team’s statistical research, the likelihood of a fair coin toss was compatible with the decision of contracting or detaching. In other words, it appeared to be completely random.
The only difficulty is that no known cellular mechanism can achieve this effect. That perplexing phenomenon has yet to be duplicated or explained.
Because they are merely cells, and the cells in our soviet-style bodies are the equal of worker bees, it may be time to let go of our preconceived beliefs about what cells are capable of. Wily, gunslinging, free-living cells may have talents well beyond our feeble primate imaginations.