The collective intelligence of biofilms

Fascinating stuff !

The slime mold Physarum polycephalum sometimes barely qualifies as a microorganism at all: When it oozes across the leaf litter of a forest floor during the active, amoeboid stage of its life cycle, it can look like a puddle of yellowish goo between an inch and a meter across. Yet despite its size, Physarum is a huge single cell, with tens of thousands of nuclei floating in an uninterrupted mass of cytoplasm. In this form, Physarum is a superbly efficient hunter. When sensors on its cell membrane detect good sources of nutrients, contractile networks of proteins (closely related to the ones found in human muscle) start pumping streams of cytoplasm in that direction, advancing the slime mold toward what it needs.

But Physarum is not just reflexively surging toward food. As it moves in one direction, signals transmitted throughout the cell discourage it from pushing counterproductively along less promising routes. Moreover, slime molds have evolved a system for essentially mapping their terrain and memorizing where not to go: As they move, they leave a translucent chemical trail behind that tells them which areas are not worth revisiting.


Within the biofilm, the bacteria divide the labor of maintaining the colony and differentiate into forms specialized for their function. In this biofilm of the common soil bacterium Bacillus subtilis, for example, some cells secrete extracellular matrix and anchor in place, while some stay motile; cells at the edges of the biofilm may divide for growth, while others in the middle release spores for surviving tough conditions and colonizing new locations.

Biofilm behaviors testify to the capacity and openness of bacterial to form collectives — but that openness has limits, as shown in this culture with several cohabiting biofilms. Here, adjacent biofilms that consist of the same bacteria or closely related strains comfortably merge. But the adjacent biofilms made up of more divergent bacteria keep themselves distinct and may even try to eliminate or control each other.

“The blue pigment seen in this video is actinorhodin, which is technically an antibiotic,” Chimileski said, but added that the term is misleading in this context. “Killing or growth inhibition usually occurs only at very high concentrations relative to what is out in nature.” For that reason, he said, there is “an emerging view that killing is probably not the ecological function of many or most antibiotics. Rather, these bioactive molecules act as signals or developmental cues” to other cells.

https://www.quantamagazine.org/the-beautiful-intelligence-of-bacteria-and-other-microbes-20171113/