What are Bacteria?

Bacteria (or singular bacterium) create a large domain of prokaryotic (single-celled) microorganisms that have cell walls, but lack organelles and an organized nucleus. Usually only a few micrometres long, the five groups of bacteria are classified according to their basic shapes: bacillus (rod), coccus (spherical), spirillum (spiral), spirochaete (corkscrew), or vibria (comma). They can exist as single cells, in pairs, chains or clusters.

Previously, researchers estimated that the number of bacteria on Earth to be five million trillion trillion, or a five with thirty zeroes. To compare, astronomers estimate that there are one billion trillion (one with 21 zeroes after it) stars in the observable universe. This means that there may be more bacteria on earth than there are stars in the universe. Newer research decreases previous estimates, but “their still gigantic number means that they play a role in global biogeochemical cycles,” says Jen Kallmeyer, a Geomicrobiologist at the University of Potsdam in Germany. (Lougheed) Many bacteria are unknown and have not been categorized, and only about half of the phyla of bacteria have species that can be grown in a lab. (Rappe)

Among the oldest life forms on Earth, bacteria are extremely adaptable and can be found in nearly every habitat, including soil, water, acidic hot springs, radioactive waste, within the Earth’s crust, and arctic snow. They have even been known to thrive in space.

Bacteria live in parasitic or symbiotic relationships with their hosts, which include both plants and animals. Bacteria that live in soil or on dead plant matter play a vital role in the production of nutrients. Food spoilage or crop damage can occur with some types of bacteria; however, many are exceptionally valuable in fermented food production, including yogurt, kimchi, soy sauce, and kombucha. Despite the common belief that bacteria are often pathogenic, relatively few are parasites or pathogens that cause disease in animals and plants.

Although recently up for debate, we believe there may be up to nearly 10 times as many bacterial cells as human cells within or on the human body. Many of these bacterial cells can be found within the gastrointestinal (GI) tract or on the skin and aid in bodily functions like digestion and boosting immunity.

 The vast majority of the bacteria in the body are commensal, and many are now considered mutualistic. However, several species are pathogenic and can cause infectious diseases such as syphilis, gonorrhoeae, C. diff colitis, Lyme disease, anthrax, tuberculosis, and bubonic plague.

Today, antibiotics are often the used to treat bacterial infections and are also used in farming, making antibiotic resistance a rising dilemma. Antibiotics have saved lives since the 1940s, significantly reducing illness and death from infectious diseases; however, these drugs have been used so widely and for so long that the bacteria have adapted to the antibiotics, rendering them less effective.

According to the Centers for Disease Control and Prevention (CDC), at least 2 million people become infected with antibiotic resistant bacteria (like Clostridium difficile) each year and at least 23,000 people die each year from these infections.


"Antibiotic / Antimicrobial Resistance." Centers for Disease Control and Prevention. 2015. http://www.cdc.gov/drugresistance/.

"Bacteria." Microbiology Online. 2016. http://www.microbiologyonline.org.uk/about-microbiology/introducing-microbes/bacteria.

Lougheed, Kathryn. "There Are Fewer Microbes out There than You Think." Nature, 2012. http://www.nature.com/news/there-are-fewer-microbes-out-there-than-you-think-1.11275.

Rappé MS, Giovannoni SJ. 2003. "The uncultured microbial majority". Annual Review of Microbiology 57: 369–94. doi:10.1146/annurev.micro.57.030502.090759.

Woese CR, Kandler O, Wheelis ML 1990. "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya". Proceedings of the National Academy of Sciences of the United States of America 87 (12): 4576–9. doi:10.1073/pnas.87.12.4576.

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