Memory doesn’t always refer to the mind’s ability to encode, keep, and recall information. Sometimes, a mind, or even any neurons whatsoever, aren't required.
A recent study, spearheaded by the University of California, Los Angeles, reminds us of this fact by revealing that a certain species of bacteria appears to possess a form of sensory-based “multigenerational memory”. The paper’s revelation has been described by one of the team’s senior authors as a “huge surprise to us and to the field.”
The bacterium in question is Pseudomonas aeruginosa, a rod-shaped, pearlescent species that infects both plants and animals. In humans, it’s known to cause a range of conditions, with the more serious ones occurring in those that are hospitalized or have weaker immune systems.
As the team of the Proceedings of the National Academy of Sciences paper note, collections of P. aeruginosa also form biofilms, collections of bacteria that stick to each other and another surface. These agglomerations of singular cells work cooperatively with their comrades, which boosts their overall resilience to any environmental extremes.
Biofilms have been around since time immemorial, with their fossilized remnants seen in stromatolite formations 3.5 billion years old. Today, they’re still ubiquitous in other forms – your dental plaque, for example. P. aeruginosa is just another example, but in humans, its persistent biofilms can prove deadly. As ever, the more we understand, the better.
When bacteria first join a biofilm, they have to establish a proper connection with other bacteria surrounding them. If they’re attached to the alien surface, they have to work out how to bind to that too.
Like plenty of their evolutionary cousins, these bacteria can tweak the physical or biochemical characteristics of their biofilm – such as the growth rate – using electrochemical signals to “orchestrate community-scale behavior” between their spatial neighbors.
The team wanted to know if these bacteria orchestrate similar communication between ancestor and descendants, otherwise known as “temporal neighbors”. In other words, do the later generations actually “remember” the adhesive instructions first calculated by their forebears?
In order to find out, the team developed a novel, multigenerational tracking method to track at every step of the way how each cell in a biofilm is behaving. They even threw in an electronic processing technique, normally used to assess changes in pitches in sound, to follow the bioelectrochemical signaling used by the bacteria.
Turns out that there’s a rhythmic pattern that ripples through the cells, one involving the activity level of their movement-based appendages (pili) and the expression of cyclic AMP (cAMP), a type of signaling molecule that tells cells what to do. Both take place just hours apart.
It seems that surface sentient cells retain a memory of the surface “imprinted” using cAMP. This allows ancestral cells to transmit or convey this rhythm to their descendants, which ultimately suppresses their movement and encourages surface attachment, leading to the development of a sturdy, populous biofilm.
In a way, these bacteria form a multigenerational conga line, with the surface sentient signals “propagated as a type of memory across multiple bacterial generations.”
“This behavior has nothing to do with genetics and mutations, which is what people usually think about for bacteria,” study lead author Calvin Lee, a graduate student at UCLA, told IFLScience. “The bacteria in this community are genetically identical, but behave differently due to their sensory memory.”
The groundbreaking nature of this discovery, one that has major implications for how we understand dangerous infections, cannot be overstated. It’s analogous to finding out how cities are planned out and carefully built by architects spanning centuries – just on a microbial scale.