Bacteria are single-celled microorganisms found in nearly every environment on Earth, existing in vast numbers both inside and outside other organisms. They are microscopic, too small to be seen without magnification. While some bacteria can cause illness, many play beneficial roles, supporting various forms of life and participating in industrial and medicinal processes. These ancient life forms appeared on Earth approximately 4 billion years ago.
Typical Bacterial Dimensions and Measurement Units
Bacteria range in size from 0.5 to 5 micrometers (µm) in length. Their shapes vary, impacting precise dimensions; for example, spherical bacteria, known as cocci, are approximately 0.5 to 1 µm in diameter. Rod-shaped bacteria commonly fall between 0.5 to 4 µm wide and less than 15 µm long, while spiral-shaped spirochetes can be longer, up to 20 µm, but narrower, from 0.1 to 1.0 µm.
Scientists use the micrometer as the standard unit of measurement. A micrometer is one-millionth of a meter, or 0.001 millimeter. This unit helps quantify objects at a microscopic scale, invisible to the unaided human eye, which can discern objects around 100 µm or larger. Observing bacteria requires light microscopes, which magnify their tiny forms.
The Spectrum of Bacterial Sizes and Relative Scale
The size of bacteria spans a considerable range, from the smallest self-replicating organisms to those visible without a microscope. The smallest known bacteria are found within the Mycoplasma genus, with species like Mycoplasma genitalium measuring approximately 0.2 to 0.3 micrometers (200-300 nanometers) in diameter. These parasitic bacteria lack cell walls and possess a minimal set of organelles, making them compact. Other small parasitic bacteria, such as Chlamydia species, produce elementary bodies that can also be as small as 0.2 µm.
On the other end of the spectrum, some bacteria achieve remarkable sizes, exceptions to the microscopic rule. Thiomargarita namibiensis, called the “sulfur pearl of Namibia,” is a prominent example of a large bacterium. While its average diameter is between 0.1 and 0.3 millimeters (100-300 µm), some individual cells can reach up to 0.75 millimeters (750 µm). This makes Thiomargarita namibiensis large enough to be seen with the naked eye. Even larger, Thiomargarita magnifica has been described with an average length of 10 millimeters, further expanding the upper limits of bacterial size.
A human red blood cell is 7-8 micrometers in diameter, meaning many average bacteria are several times smaller. Viruses, by contrast, are significantly smaller than bacteria, ranging from 20 to 400 nanometers (0.02 to 0.4 µm), with some of the smallest viruses around 10 nanometers. Even the largest viruses are smaller than the smallest bacteria. A human hair, with a width ranging from 50 to 180 micrometers, is vastly larger than most bacteria, while a common dust particle can be around 1 micrometer, putting it in the typical size range of bacteria. The minute size of most bacteria allows them to inhabit nearly every niche on Earth, influencing global nutrient cycles and interacting with larger organisms.