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| Top left: Thirty percent of people have harmless Staphylococcus aureus
(staph) on their skin and in their nose, but occasionally it causes
infections, such as gastroenteritis (sometimes
called stomach flu) and can be a cause of food poisoning. USDA.
Top right: Helicobacter bilis. Some strains are common in people and on occasion they can cause peptic ulcers,
chronic gastritis, and stomach cancer. CDC. Bottom left: Bacteria.
Brandon
Antonio Segura Torres and Priscilla Vieto Bonilla, CC BY-SA 4.0.
Bottom right: Escherichia coli
(E. coli) is a common bacteria often found in our lower intestines.
Some strains are harmful, causing such ailments as gastroenteritis, urinary
tract infections, and inflammatory bowel disease. USDA. Three
of these photographs are colorized
Every living organism has to be able to sense their environment in order to survive. This includes the ability to process the information it receives, so it can respond, and to store and retrieve information for it can use in a similar situation in the future. That sounds like a lot, but even bacteria—the simplest known living creature—can do it. (I’m setting aside viruses for now, since we’re not sure they’re actually alive, although viruses can taste.) All life needs to sense its surroundings in some way. Touch is one of the oldest senses. Even single cell organisms will recoil when you poke them. They feel their way around their environment, determining whether to move towards or away from whatever they encounter. By using this sense they can even all move in the same direction as a group. They can tell when they come in contact with a surface and can release glue to adhere to it. Interestingly, touch in other animals works pretty much the same way as it does in us, and fish sense with their pectoral fins like we do our hands[1], but some animals do have unusual capabilities. When a duck dabbles in water, it is actually feeling around for food. The tip of its bill, and that of similar wading birds, is packed with sensors and is thought to be just as sensitive as our fingertips. When probing in mud and sand, they create a pressure wave, and by feeling distortions in the wave they can detect hidden food without actually touching it.[2] Mallards have a few taste buds in clusters on their jaws, but none on their tongues. Other ducks may be the same. The narwhal’s long tusk is packed with sensors. This unusual toothed whale has two large teeth, but the left one grows out to an enormous size, apparently becoming a sense organ. The other remains in its mouth and only becomes about a foot long. Narwhals grow up to fifteen feet and their spiral-shaped tusk can grow to nine. It’s mostly males that have them, but some females do too, and in some males both of the teeth become tusks. Scientists think these tusks can detect changes in pressure, temperature, and the saltiness of the water, indicating when the water is about to freeze. They also float with the tusk pointing up in the air like an antenna, probably sensing air pressure and temperature to predict the weather.[3] Snakes and some lizards have raised scales on their heads that they use for feeling things. Sea snakes also use them to sense ripples in the water and track down prey. Likewise, crocodiles, alligators, and caimans are armored with scales, yet they can feel light touches through small bumps on their scales that are sensitive to touch, temperature, and chemicals. In fact, their tough scales are ten times more sensitive to touch than our fingers. Alligators have them on their heads, while crocodiles have them all over their bodies. So if you softly pet a crocodile, he or she will feel it. And they’re most sensitive around their teeth, which may be especially important since they carry their babies around in their jaws.[4]
“How doth the little crocodile Improve its shining tail, And pour the waters of the Nile On every golden scale! “How cheerfully it seems to grin! How neatly spreads its claws! And welcomes little fishes in With gently-smiling jaws!” —Lewis Carroll, Alice’s Adventures Under Ground
Getting back to bacteria, they can sense tiny changes in chemical gradients, which enable them to move towards food and they can tell whether they’re getting closer. They also use this sense to avoid competitors and flee from toxins. Many types of bacteria seek out the walls of your intestines as the ideal place to live—both pathogenic and helpful bacteria. Once they’ve arrived, they sense it and alter their gene expression for the new environment. Bacteria have four or five types of receptors in their membrane wall that send signals to their flagellum—a spinning whip-like hair that serves as their propeller.[5] And they can smell gases.[6] It turns out that much of the distinctive smell of a forest after it rains comes from bacteria that are communicating with each other. Bacteria give off a number of scents that influence and coordinate their behavior, some of which we can smell. Their single-celled predators can smell it too, but for them it signals the location of food, and not just that, it tells them the type of bacteria, so they can head for the kind they like best. Normally the scents travel about four inches (10 cm), but when there’s lots of bacteria communicating at once, they can fill a forest with that wonderful damp forest smell. Terpenes are the most common component of bacterial scents, but bacteria aren’t the only ones that produce it. Plant terpenes are what gives lavender, tangerines, and pine trees their fragrances. Plants use them to warn their neighbors that predators are in the area. Animals also use them and they’re often included in perfumes. Terpenes and other chemical communicators are called pheromones. So, when the soil bacteria Serratia picks up the scent from a deadly fungi Fusarium, they give off their own scent, warning their relatives it’s time to hightail it out of Dodge. And that’s just one example of interspecies communication among some of the most basic creatures you can find.[7] |
Bacteria can also use scents to scare away predators, such as the round worm Caenorhabditis elegans. When bacteria become poisonous, they release a scent—geosmin—to warn their predators to stay away, similar to how the bright colors of the poison dart frogs warn predators to steer clear of them. This scent is what we smell when working in the garden with damp soil or right after it rains. If bacteria produce it in water, it can cause the water to taste like dirt.[8]
While most bacteria use taste, smell, and touch to sense their environment, so far they don’t appear to be able to hear or sense vibrations. Still, simple single-celled bacteria have four of the five primary senses. They do respond to light, meaning they have a very basic form of sight.
Cyanobacteria—also known as blue-green algae, although it’s not really an algae—have been around for 2.5 billion years making them one of the oldest creatures on the planet and they were the first organisms to produce oxygen. They can be found just about everywhere all over the Earth. They’re the green slime growing in your aquarium, when you forget to clean it. These are one type of single-celled microorganism that can detect light and respond to it. Using the same principle as our eyes, light hits one side of the cell and is focused onto the other side where they sense it. They then move away from that side in order to head towards the light, or they towards that side to move away from the light.[9]
 |
| A single-celled warnowiid. The dark spot at the base of its eye is its retina. Mona Hoppenrath, Tsvetan R. Bachvaroff, Sara M. Handy, Charles F. Delwiche, and Brian S. Leander, CC BY 2.0 (modified). |
Several might even have a primitive eye built into them, which is pretty astonishing for single-celled organisms. One is a warnowiid, a rare free-swimming predatory plankton—it’s also a protozoa, a protist, and a dinoflagellate—that’s smaller than this period “.”, yet it contains a clear sphere with parts that look like a retina, lens, cornea, and an iris—all arranged like an eye. We’re not sure yet what this possible eye does because these creatures are difficult to find, hard to keep alive, and so far, impossible to raise, so researchers haven’t been able to run many tests on them. Pretty much all it could sense would be light and dark, or perhaps the presence of circular polarized light reflected from their prey. This would enable warnowiids to swim towards their victims where they could attack it with their harpoon-like stingers.[10]
Our own cells have senses too. We were once a single cell, when two of our parents’ cells fused. Now our cells are much more specialized, compared to single-celled organisms. Our cells travel to where they’re supposed to be, seeking out other cells in their specialty, and then line up in rows, even when there’s a plate of glass between them, possibly by sensing infrared light.[11]
Some cells are so sensitive that they can locate a molecule within a nanometer, which is about a third the diameter of a carbon atom. Some seem to be able to determine distances.[12] They have finger-like projections they can extend, retract, and bend, that they use to feel their environment. When they sense food, they curve around it so they can engulf it.[13]
Many of your cells are constantly awash in chemical information, yet are able to determine which signals to act on, in addition to how and when to act.[14] They use chemicals to communicate and to know where to congregate. Others that are part of your immune system swim to threats and destroy them. A study by scientists at the Marine Biological Laboratory in Massachusetts suggests that cells can sense their shape, even when it’s constantly changing.[15] They can also sense the curvature and the density of the cells near them.[16]
And they do all of this without any nervous system or a brain.
Sneezing Sponges
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| A yellow tube sponge, a gray rope sponge, a red encrusting sponge, and a purple vase sponge together in the Caribbean Sea. Twilight Zone Expedition Team 2007, NOAA-OE, CC BY 2.0. |
Sponges sneeze. This is pretty amazing since they're among the simplest animals in the world and don’t have muscles or even a nervous system. They are filter feeders, which is a pretty passive way to survive, relying on water currents to bring them food, but sometimes they sneeze to cast out sediment, waste, and mucus (snot). Biologists can also use certain chemicals to get them to sneeze. It’s not quick, though. A sneeze can take up to forty-five minutes.
The question is, how does a simple sponge know when it needs to sneeze? It must sense it somehow, so what kind of senses does a sponge have? We’re not sure. What we do know is that they sense with cilia—tiny hair-like structures—which have been repurposed in us and other animals for use in hearing and smell.
And I’ll bet you didn’t know that roundworms (Caenorhabditis elegans—the laboratory favorite commonly shortened to C. elegans) can spit. Normally they float around in water hoovering up whatever bacteria they find, but when they taste something they don’t like, they immediately spit it out and run away…that is, as fast as they can crawl.
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[2] Tim Birkhead, “Instant Expert 34: Bird Senses”, New Scientist, no. 2928, August 3, 2013, pp. i-viii, and as “Bird senses: Touch and hearing”, July 31, 2013, https://www.newscientist.com/article/mg21929282-700-bird-senses-touch-and-hearing/, citing Proceedings of the Royal Society B, vol. 265, p 1377.
[3] John M. Henshaw, A Tour of the Senses, Baltimore: The Johns Hopkins University Press, 2012.
[4] BioMed Central, “Croc supersense: Multi-sensory organs in crocodylian skin sensitive to touch, heat, cold, environment”, ScienceDaily, July 2, 2013, http://www.sciencedaily.com/releases/2013/07/130702101506.htm, citing Nicolas Di-Poï, Michel C Milinkovitch, “Crocodylians evolved scattered multi-sensory micro-organs”, EvoDevo, 2013; 4 (1): 19 https://doi.org/10.1186/2041-9139-4-19.
And Ed Yong, “Crocodile Faces Are More Sensitive Than Human Fingertips”, National Geographic, November 8, 2012, https://www.nationalgeographic.com/science/article/crocodile-faces-are-more-sensitive-than-human-fingertips, citing Duncan B. Leitch and Kenneth C. Catania, “Structure, innervation and response properties of integumentary sensory organs in crocodilians”, Journal of Experimental Biology, 2012, http://dx.doi.org/10.1242/jeb.076836.
[5] Institute of Industrial Science, The University of Tokyo, “E. Coli calculus: Bacteria find the derivative optimally”, ScienceDaily, March 24, 2021, https://www.sciencedaily.com/releases/2021/03/210324094729.htm.
And Hebrew University of Jerusalem. “ ‘Smart’ bacteria remodel their genes to infect our intestines”, ScienceDaily, February 22, 2017. https://www.sciencedaily.com/releases/2017/02/170222082914.htm, citing Naama Katsowich, Netanel Elbaz, Ritesh Ranjan Pal, Erez Mills, Simi Kobi, Tamar Kahan, and Ilan Rosenshine, “Host cell attachment elicits posttranscriptional regulation in infecting enteropathogenic bacteria”, Science, 2017; 355 (6326): 735, https://doi.org/10.1126/science.aah4886.
And David H Freedman, “In the Realm of the Chemical”, Discover Magazine, June 1993, pp. 68-76.
[6] Newcastle University, “Bacteria can have a ‘sense of smell’ ”, ScienceDaily, August 17, 2010, http://www.sciencedaily.com/releases/2010/08/100816095719.htm, citing Reindert Nijland and Grant Burgess, “Bacterial Olfaction”, Biotechnology Journal, 2010, https://doi.org/10.1002/biot.201000174.
[7] Netherlands Institute of Ecology (NIOO-KNAW), “How miniature predators get their favorite soil bacteria: Sniffing out your dinner in the dark”, ScienceDaily, December 8, 2016, https://www.sciencedaily.com/releases/2016/12/161208125841.htm, citing Kristin Schulz-Bohm, Stefan Geisen, E R Jasper Wubs, Chunxu Song, Wietse de Boer, and Paolina Garbeva, “The prey’s scent: Volatile organic compound mediated interactions between soil bacteria and their protist predators”, The ISME Journal, 2016, https://doi.org/10.1038/ismej.2016.144.
And Netherlands Institute of Ecology (NIOO-KNAW), “World’s most spoken language is ‘Terpene’: Micro-organisms communicate with each other, and the rest of the world, through smells”, ScienceDaily, April 13, 2017, https://www.sciencedaily.com/releases/2017/04/170413190718.htm, citing Ruth Schmidt, Victor de Jager, Daniela Zühlke, Christian Wolff, Jörg Bernhardt, Katarina Cankar, Jules Beekwilder, Wilfred van Ijcken, Frank Sleutels, Wietse de Boer, Katharina Riedel, and Paolina Garbeva, “Fungal volatile compounds induce production of the secondary metabolite Sodorifen in Serratia plymuthica PRI-2C”, Scientific Reports, vol. 7, no. 1, 2017, https://doi.org/10.1038/s41598-017-00893-3.
[8] Concordia University, “The pleasant smell of wet soil indicates danger to bacteria-eating worms, researchers find: The chemical compound geosmin’s powerful taste warns predators to keep away from certain microbes”, ScienceDaily, April 5, 2022, https://www.sciencedaily.com/releases/2022/04/220405143530.htm, citing Liana Zaroubi, Imge Ozugergin, Karina Mastronardi, Anic Imfeld, Chris Law, Yves Gélinas, Alisa Piekny, and Brandon L. Findlay, “The Ubiquitous Soil Terpene Geosmin Acts as a Warning Chemical”, Applied and Environmental Microbiology, 2022, https://doi.org/10.1128/aem.00093-22.
[9] Albert-Ludwigs-Universität Freiburg, “Slime can see: Tiny cyanobacteria use principle of the lens in the human eye to perceive light direction”, ScienceDaily, February 9, 2016, https://www.sciencedaily.com/releases/2016/02/160209090620.htm, citing Nils Schuergers, Tchern Lenn, Ronald Kampmann, Markus V Meissner, Tiago Esteves, Maja Temerinac-Ott, Jan G Korvink, Alan R Lowe, Conrad W Mullineaux, and Annegret Wilde, “Cyanobacteria use micro-optics to sense light direction”, eLife, 5, 2016, https://doi.org/10.7554/eLife.12620.
[10] Ed Yong, “Single-Celled Creature Has Eye Made of Domesticated Microbes”, National Geographic, July 2, 2015, https://www.nationalgeographic.com/science/article/single-celled-creature-has-eye-made-of-domesticated-microbes, citing Gregory S. Gavelis, Shiho Hayakawa, Richard A. White III, Takashi Gojobori, Curtis A. Suttle, Patrick J. Keeling, and Brian S. Leander, “Eye-like ocelloids are built from different endosymbiotically acquired components”, Nature, vol. 523, pp. 204-207, 2015, http://dx.doi.org/10.1038/nature14593.
[11] Guenter Albrecht-Buehler, “Rudimentary form of cellular ‘vision’ ”, PNAS, vol. 89, no. 17, September 1992, pp. 8288-8292, https://www.pnas.org/content/pnas/89/17/8288.full.pdf, https://doi.org/10.1073/pnas.89.17.8288.
[12] Universidad de Barcelona, “Cells sense their environment to explore it”, ScienceDaily, December 13, 2017, https://www.sciencedaily.com/releases/2017/12/171213125821.htm, citing Roger Oria, Tina Wiegand, Jorge Escribano, Alberto Elosegui-Artola, Juan Jose Uriarte, Cristian Moreno-Pulido, Ilia Platzman, Pietro Delcanale, Lorenzo Albertazzi, Daniel Navajas, Xavier Trepat, José Manuel García-Aznar, Elisabetta Ada Cavalcanti-Adam, and Pere Roca-Cusachs, “Force loading explains spatial sensing of ligands by cells”, Nature, 2017, https://doi.org/10.1038/nature24662.
[13] Ohio State University, “High-resolution lab experiments show how cells ‘eat’: Study solves a 40-year-old problem in cell biology”, ScienceDaily, December 30, 2021, https://www.sciencedaily.com/releases/2021/12/211230130936.htm, citing Nathan M. Willy, Joshua P. Ferguson, Ata Akatay, Scott Huber, Umidahan Djakbarova, Salih Silahli, Cemal Cakez, Farah Hasan, Henry C. Chang, Alex Travesset, Siyu Li, Roya Zandi, Dong Li, Eric Betzig, Emanuele Cocucci, and Comert Kural, “De novo endocytic clathrin coats develop curvature at early stages of their formation”, Developmental Cell, 56 (22): 3146 2021, https://doi.org/10.1016/j.devcel.2021.10.019.
[14] University of California – Irvine, “Researchers eavesdrop on cellular conversations: New computational tool decodes biological language of signaling molecules”, ScienceDaily, February 18, 2021, https://www.sciencedaily.com/releases/2021/02/210218160357.htm.
[15] Diana Kenney, “A Cell Senses Its Curves: New Research from the Whitman Center”, April 28, 2016, https://www.mbl.edu/blog/a-cell-senses-its-curves-new-research-from-the-whitman-center/, citing Andrew A. Bridges, Maximilian S. Jentzsch, Patrick W. Oakes, Patricia Occhipinti, and Amy S. Gladfelter, “Micron-scale plasma membrane curvature is recognized by the septin cytoskeleton”, The Journal of Cell Biology, 213 (1): 23, 2016, https://doi.org/10.1083/jcb.201512029.
[16] Institute of Science and Technology Austria, “How cells feel curvature: Scientists find mechanism that allows cells to sense the curvature of tissue around them”, ScienceDaily, November 18, 2021, https://www.sciencedaily.com/releases/2021/11/211118203618.htm, citing Marine Luciano, Shi-Lei Xue, Winnok H. De Vos, Lorena Redondo-Morata, Mathieu Surin, Frank Lafont, Edouard Hannezo, and Sylvain Gabriele, “Cell monolayers sense curvature by exploiting active mechanics and nuclear mechanoadaptation”, Nature Physics, 2021, https://doi.org/10.1038/s41567-021-01374-1.
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