Did you know that many bacteria — some of the oldest lifeforms on Earth — can talk? Really.
And not only between the same kind of single-cell bacteria, but back and forth with members of other species, too.
Okay, they don’t talk in words or with sounds at all. But they definitely communicate in a meaningful and essential way, especially in the microbial mats and biofilms (microbes attached to surfaces surrounded by mucus) that constitute their microbial “cities.”
Their “words” are conveyed via chemical signaling molecules — a chemical language — going from one organism to another, and are a means to control when genes in the bacterial DNA are turned “on” or “off.” The messages can then be translated into behaviors to protect or enhance the larger (as in often much, much larger) group.
Called “quorum sensing,” this microbial communication was first identified several decades ago. While the field remains more characterized by questions than definitive answers, is it clearly growing and has attracted attention in medicine, in microbiology and in more abstract computational and robotics work.
Most recently, it has been put forward as chemically-induced behavior that can help scientists understand how bacteria living in extreme environments on Earth — and potential on Mars — survive and even prosper. And the key finding is that bacteria are most successful when they form communities of microbial mats and biofilms, often with different species of bacteria specializing in particular survival capabilities.
Speaking at the recent Astrobiology Science Conference in Seattle, Rebecca Prescott, a National Science Foundation Postdoctoral Research Fellow in Biology said this community activity may make populations of bacteria much more hardy than otherwise might be predicted.
“To help us understand where microbial life may occur on Mars or other planets, past or present, we must understand how microbial communities evolve and function in extreme environments as a group, rather than single species,” said Prescott,
“Quorum sensing gives us a peek into the interactive world of bacteria and how cooperation may be key to survival in harsh environments,” she said.
And because “quorum sensing” has not been investigated in the world of astrobiology, “this study will be the first to illuminate how microbial interactions might influence survival on Mars and early Earth conditions.”
This makes quorum sensing of interest to NASA, Prescott told me, because it potentially broadens the universe of environments where bacteria might survive.
“Microbes don’t function as single species in nature, like we have them in most of our experiments.,” Prescott told me. “It’s therefore important for us to try and understand them as interactive communities – the socialites that they are.”
Prescott’s research has taken her to extreme environments such as hypersalty ponds with strong ultraviolet light in the Bahamas, the hot springs of Iceland and the lava caves beneath the Hawaiian Islands, to name a few
In some of these locales, such as the Bahamas hypersaline mats, it is not unusual for lifeforms to desiccate — a profound drying that few organisms can survive. Yet certain microbes — when enclosed in their protective, slimy biofilms formed with the assistance of quorum sensing — are able go dry for years and then regain activity when water returns.
Prescott’s colleague and supervisor in the research, University of South Carolina Environmental Health Sciences Professor and Associate Dean for Research Alan Decho, said of these sites: “These are incredibility harsh environments, where very little life other than bacteria can exist.”
The bacterial samples are now going into a Mars simulator chamber in Scotland. That simulator, in the University of Edinburgh lab of astrobiologist Charles Cockell, will be where the examples of extremophile bacteria are tested for compatibility with an early and then a later Mars atmosphere and to determine how and if their chemical “talking” changes.
The presence of quorum sensing might also lead some day to the discovery of biosignatures on Mars. This is because the bacteria signaling molecules — acyl homoserine lactones (AHLs) — are neutral lipids, and lipids are often preserved in the rock record.
In this tale of “talking bacteria” and their biofilms, it seems only proper that the species most associated with the discovery of quorum sensing by bacteria is the unusual bobtail squid of Hawai`i. The squid develops a striking bioluminescence at night, and it turns out that bacteria in its body are a source of the light.
The bacteria in the squid (Vibrio fischeri) start the night dark and only become bioluminescent as the density of bacteria grows. That density leads, thanks to the quorum sensing phenomenon, to a changed expression of genes and release of proteins that lead to the bioluminescence. Most of the bacteria are later expelled when daytime comes.
The tiny squid bacteria and the squid have their own symbiotic relationship: the bacteria collect a sugar and amino acid solution produced by the squid and the bacteria-induced light hides the squid’s silhouette when viewed from below.
For bacteria to use quorum sensing, they must possess three characteristics: the ability to secrete a signaling molecule, the ability to detect a change in concentration of signaling molecules, and an ability to regulate gene expression as a response to that change.
This process is highly dependent on how the signaling molecules spread. Quorum sensing signaling molecules generally released by individual bacteria in tiny amounts that can slip away undetected if the cell density in the area is low. At high cell densities, the concentration of signaling molecules may exceed its threshold level and trigger changes in gene expressions.
As a result, a main focus of quorum sensing research is on microbial mats and biofilms, the kind of slime-covered collections found most visibly in ponds and other waterways but most everywhere else too — on shower curtains, n the International Space Station orbiting the Earth, the plaque on your teeth, your cellphone and in fact in a number of places throughout our bodies. (Prescott makes a point of saying most bacteria are harmless, and even are essential for life.)
Producing the protective biofilm mucus to make microbial “cities” is done as part of the quorum sensing process — an activity that helps create an environment that is more stable, with different cells or species doing different tasks. A bit like ants, perhaps, but on a microscopic level.
The biofilms are also organized in part through quorum sensing in ways that result in bacteria that are more resistant to radiation being on the surface of the film while those that are harmed by oxygen would be found deeper in the mat.
“Biofilm genes are controlled by quorum sensing,” Prescott told me. “Basically there has to be a lot of you for a mucus layer to make a difference, so microbes start making mucus once they sense other neighbors around.“
Radiation protection provides a good model for how members of a mixed species biofilm will have different roles to play.
”The species that are more tolerate of radiation—or individual cells of same species—will exist at surface, and sometimes produce chemicals that are UV protectants. That also provides protection for others below that are less tolerate to UV. In addition, the biofilm mucus (exopolysaccahride) is a UV protectant itself.”
“So certain members may be producing more mucus, while others are breaking down nutrients. Many biofilm researchers say biofilms are more like multi-cellular organisms than single cell, and it is certainly a step towards multicellularity.”
And these organized activities are often coordinated through some sort of quorum sensing; i.e, chemical “talking.”
Armed with a protective covering and other community-based strengths, biofilms are adaptable. Consider, for instance, the inside of the International Space Station, some 250 miles above the Earth. Biofilms can be found there all the time, and not because they were purposefully brought up.
One batch of mixed bacterial biofilms, however, was intentionally delivered to the ISS for a European Space Agency-led study of bacterial microbes and larger species including fungi and lichen. The samples were exposed to the pressures, temperatures, radiation and more of space over a two-year period.
While not all of the biofilm material survived and prospered, much of it did — more than most other samples.
Prescott’s astrobiology work in Cockell’s Edinburgh lab will expose her collected biofilms to different but also harsh conditions — simulated Mars environments that can be changed to explore the effects of different conditions including extreme temperature, pressure, dryness, and radiation.
The simulator is part of a cutting-edge effort to test microbes for potential future uses on Mars including manufacturing, “bio-mining,” and transforming elements available on Mars into a form that plants can use. Prescott will use the chamber to look for changes in the biofilm’s gene expression and quorum sensing under Mars conditions and will look at the AHL signaling molecules to see which species can maintain them.
“We have no idea what will happen in the Mars environments; maybe they’ll die and maybe they’ll live,” she said. “And who knows? There may be quorum sensing systems on Mars different from anything we know.”