This week, I visited the lab of Dr. Lars Dietrich at Columbia University. His team is currently studying an almost forgotten area of microbiology, the cooperation and complex organization of bacteria. Unicellular organisms are usually discussed as single-celled and incapable of forming complex structures. However, as I was shown by Dr. Dietrich and his staff, these assumptions are incorrect for at least one extraordinary species.
"Considering the abundance of bacteria, there is relatively little known about them," Dr. Dietrich told me. And, after a little research of my own, I wholeheartedly agree. The progress of our understanding of Bacteria, the most diverse and numerous (by many orders of magnitude) domain of life, has been largely limited to practical concerns, and those bacteria most relevant to human health, even indirectly, comprise the smallest fraction of the total. In terms of how little we know compared to what we could be examining, the only area of scientific inquiry that comes close is astronomy. But despite Carl Sagan's millions and billions of stars, the stars we can currently observe number in septillions (10^
|Meet the colonists, Pseudomonas aeruginosa, a bacterium that can infect animals and is also ubiquitous in soil and water throughout the world. It is a tenacious survivor, out-competing most other bacteria and adapting to a variety of environments, including the weightless labs on the International Space Station. It has a very helpful property for lab experiments, in that it changes color when exposed to oxygen, employing various redox-active phenazines (including pyocyanine-blue and pyoverdine-green) for metabolic redox reactions. This also lends to P aeruginosa's competitive edge, as the pyocyanine it generates kills competing organisms by virtue of the molecule's redox properties. As remarkable as these adaptations are, to the lay observer they would seem fairly small compared to what they can do together, because P auruginosa also cooperates, and Dr. Dietrich's team has discovered how. But first, let's discuss a little fundamental biology.|
When an organism requires oxygen, its adaptations are all about surface area. Breathing organs almost universally evolve folds and wrinkles to increase the amount of oxygen they can absorb, including gills, lung sacs, the skin of amphibians like the Titicaca water frog or the Hellbender Salamander, the spongy mesophyll tissues in leaves, and the aerated roots of the Mangrove tree. Surface area increases the rate of absorption/diffusion, as summarized by Fick's law:
This is an elegant statement of a long known biological fact, the rate at which substances are exchanged with an organism's environment is limited by its surface area and the concentration of the substance required. Complex organisms therefore seem to have an advantage over single celled organisms in general because they can form complex structures with lots of surface area, increasing their diffusion potential to form really large structures like redwoods or whales or anything else that requires massive metabolic processes carried out over trillions of cells. This also means that incidental fluctuations in oxygen concentration are tolerable, assuming the oxygen deprivation doesn't go on too long. But that is the counter-advantage of single-celled organisms; because their metabolic needs are modest, they don't require as much oxygen. However, if a colony of bacteria hopes to survive prolonged oxygen deprivation, it too has to adapt, and that means forming more surface area, which is exactly what P aeruginosa does.
What you are seeing here is a colony of P auruginosa grown in an oxygen-depleted environment (15%). When one of Dr. Dietrich's graduate students, Chinweike Okegbe, showed me a set of these colonies from the refrigerator, each was about the size of a fingerprint. Even to the naked eye, it was clear that the bacteria had organized themselves into a complex structure, and, when examined with an extremely fine electrode at different depths, were found to have formed an outer layer of oxygen absorbing bacteria and an internal or structural layer that receives the products of the oxygen-absorbing layer for metabolism. In other words, the bacteria stacked under and inside the folds and squiggles require no oxygen because they receive electron rich molecules from the outer colonists, and the same molecules that turn these bacteria blue and green in oxygen is responsible. It was therefore Dr. Dietrich's hypothesis that not only do phenazines serve to kill competing organisms, but they also assist other P auruginosa bacteria to survive. This is a profound example of cooperation, which is a strategy thought generally limited to complex organisms. While quorum sensing had been discovered between bacteria before, this is perhaps the first time a metabolic pathway has been demonstrated to interoperate across many single-celled organisms. It is the fundamental reason P auruginosa can organize into complex colonies as it does, and proves that bacteria possess the means to adapt to their environments by forming complex structures like those seen in eukaryotes. What is more extraordinary is that they change into these structures in a matter of days, while complex organisms require massive time periods to reproduce and evolve such structures. In fact, it is likely that complex organisms would not have evolved at all were it not for the advantages of efficiency and stress-mitigation that these complex structures provide, and adequate prolonged selective stress such that a colony like this one never returns to living as a stress-free flattened blob of non-specialized organisms.
Thanks to the work of Dr. Dietrich's team, we now know that bacteria cooperate on a level much more complex than we thought possible. The P auruginosa colonies grown in this lab are the key to an astounding discovery about the most diverse and numerous domain of life on our planet.
-f. f. white
Dr. Dietrich with Chinweike Okegbe and their specimens
- Dietrich LE, Okegbe C, Price-Whelan A, Sakhtah H, Hunter RC, Newman DK. (2013). "Bacterial community morphogenesis is intimately linked to the intracellular redox state.". J. Bacteriology 195 (7): 1371–80. doi:10.1128/JB.02273-12. PMID 23292774.
- "Dietrich Lab." Doi:http://www.dietrichlab.com/index.
html; Retrieved 21DEC2013.
- Kim W et al. (29 April 2013). "Spaceflight Promotes Biofilm Formation by Pseudomonas aeruginosa". PLOS ONE 8 (4): e6237. doi:10.1371/journal.pone.0062437. Retrieved 21DEC2013.
- " Working together for one species" Bolivian Amphibian Initiative 01DEC2011. doi: http://bolivianamphibianinitiative.
blogspot.com/2011/12/working-together- for-one-species.html. Retrieved 21DEC2013.
- Garcia, David M. "Great Adaptations," Natural History. doi: http://www.naturalhistorymag.com/
perspectives/212397/great-adaptations. Retrieved 21DEC2013.