As humans, it’s difficult to imagine the world in a scale that is different than our own, especially when that difference is significant, such as the expanse of the universe or the workings of a cell. That’s what makes microorganisms so interesting. There is an entire world among us and in us that is invisible to the naked eye. Now, not only are we aware of these hidden communities, but we use microorganisms in a wide range of industries, and we also know that the bacterial communities living inside and on us help keep us healthy. With all of this unseen activity among us, learning about microorganisms opens up our eyes and minds to an once-invisible world.
What is a microorganism anyway?
The simplest definition of a microorganism, or microbe, is an organism that is too small to be seen by human vision, i.e. can only be seen with a microscope. Of course, microbes are much more complex and diverse than this definition implies. First off, microbes can be single-celled organisms or multicellular, and there are several categories of microbes: bacteria, archaea, fungi (includes yeasts, molds, and mushrooms), protista (algae and protozoa), and viruses.
Viruses are especially interesting in that it has long been debated whether viruses are “living” organisms are not, since they lack one of the seven characteristics of life: the ability to reproduce on their own . Viruses need cells from other organisms in order to replicate, or reproduce. That is also why they are considered such a threat to our health—they can move from cell to cell, replicating and usually killing the cells they inhabited along the way. As I’ll soon discuss though, viruses can also be used for good.
Microbes are actually everywhere
Microbes can be found in any type of environment, including the human body. Since they can be found virtually everywhere, I’m only going to describe the more extreme habitats. Microbes living in these conditions are known as extremophiles. To start with, microbes can be found in the deepest parts of the ocean. When divers first discovered hydrothermal vents at the bottom of the ocean, they were surprised to find thriving communities of life there . It turns out that microbes, especially archaea, are adept at surviving in extreme conditions; in this case those conditions are extreme pressure, and temperatures up to 350°C (662°F)! Microbes can also be found in the freezing temperatures of the arctic. On February 6, 2013 scientists first reported bacteria found a half-mile deep under the ice of Antarctica. In fact, since the arctic isn’t hospitable to other forms of life, bacterial communities dominate the biodiversity .
Radiation is scary for humans even at very low doses, but there are microorganisms that can withstand extremely high levels. These microbes exhibit “radio-resistance”: resistant to ionizing radiation . A lethal dose of radiation for humans is approximately 4-10 gray (Gy), while these organisms can survive radiation of at least 1000 (Gy) (100x more than humans!) The most extreme example is Thermococcus gammatolerans, rightly named after its ability to survive 30,000 (Gy)! 
And last, but certainly not least, there are astronaut microbes! When the unmanned lunar lander Surveyor 3 returned to earth, NASA scientists were surprised to find living Streptococcus mitis from Earth that had survived on the lander for 31 months in the vacuum of space . Since then several microorganisms have been identified as having the ability to survive in space, and these include one of my favorite organisms (micro or macro): Tardigrades! Also known as “water bears”, because they literally look like little bears, these little guys are the ultimate extremophiles. Not only can they survive in space, they are also radio-resistant and can survive radiation levels up to 5,700 (Gy), as well as in temperatures below freezing and above boiling. To top it off, they can survive more than 10 years without food or water . Basically, Tardigrades will outlive us all.
Some other not-so-common places that microbes are commonly found: bubbling tar, steam vents, boiling water, in soil and ice miles underground, and most likely in areas that humans have been unable to discover thus far .
Humans and Microbes: A love-hate relationship
The term “human microbiota” is becoming increasingly well-known as we learn more about our close interactions with microorganisms, but as a short description: the human microbiota, or microflora, is the collective of microbes that live on the surface and in layers of the skin, the saliva and oral mucosa, in the conjunctiva (lines the inside of the eyelids), in the gastrointestinal tract, in the respiratory system, and in the vagina . The interactions between the human body, the microbiota, and the environment are so complex that I’m not even going to go there in this post. What I do want to discuss though, is how microbes are actually helping us!
Bacteria in our body play vital roles in keeping us healthy – they interact with and boost our immune systems and even combat pathogenic microbes (ones that cause disease). The bacteria on our skin act as an extra layer of protection against any bad guys getting in or on us. In a nut-shell, having these communities of good bacteria in and on our bodies helps prevent bad communities from moving in . In fact, bacteria play such an important role in our health that there are more bacterial cells in our bodies than our own actual cells .
Microbes are also used to improve our health and combat diseases in more targeted ways. An example of this is the use of microbes as vehicles, or carriers, for medicine. Some bacterial strains are commonly used as “delivery capsules” for drugs that are normally toxic when taken alone. E. coli has been engineered to transport an enzyme specifically to cancer cells, without targeting and harming other cells .
Viruses are also being used to treat cancer, as a team from Massachusetts General Hospital and Harvard Medical School used engineered Herpes Simplex Virus Type 1 (HSV-1) as vectors for targeting cancer cells . Using viruses or other microbes as vehicles could eliminate more dangerous types of treatment such as chemotherapy. Microorganisms can also be genetically engineered to target tumors and specific areas of the body that are under attack from disease.
Other ways that microbes are helping us range from food production to toxic cleanup. Yeasts are commonly used for bread and beer production and various bacteria are used for cheese production. Microbes are also exploited for the compounds they make: including enzymes, vitamins, and antibiotics. For example, penicillin was originally isolated from the fungi Penicillium, and lactic acid is used as a food preservative. Microbes are also essential to agriculture and nutrient recycling. Microbes living in the soil break down nutrients that are found in their environment, and provide things like nitrogen to plants in a process called mineralization  (side note: bacteria also break down nutrients in the GI tract to help humans digest them better). As mentioned, microbes are also used for cleaning up environmental toxins, including oil spills. Some microorganisms actually use oil as fuel, so that they can be released into a contaminated area and, given enough time, the oil will be removed . This is a promising approach to cleaning up other toxins found in soil and water in a relatively safe and hands-off way.
I could honestly go on and on about the wonders and curiosities of microorganisms, but for the sake of keeping at least some readers I’m going to stop here. I hope you enjoyed reading about these amazing organisms, and have a new-found or refreshed appreciation for the invisible world of microbes. Now I dare you to look at anything around you, even in the mirror, and not think about how many microorganisms are there…good luck, and welcome to my world.