Size of Bacteria: Giant, Smallest, and Regular Ones

Bacteria are too small to see through the naked eye. This does not necessarily mean that bacteria are of uniform size and shape, in fact, bacteria come in a great many sizes and several shapes. 

Most bacterial size range from 0.2 to 2.0 μm in diameter and 2 to 8 μm in length.  The ubiquitous Escherichia coli is about 1 μm in diameter and 1-2  μm long. The world’s biggest bacteria, Thiomargarita magnifica is up to 2 cm long and is visible to the naked eye, whereas the smallest bacteria, Mycoplasma, is about the same size as the largest viruses (0.2 to 0.3 μm).

Unit of Measurement 

The measurement used for microorganisms is micrometer (μm); 1 μm = 0.001 millimeter; 1 nanometer (nm) = 0.001 μm. To illustrate how small a bacterium is, consider that 500 bacteria of one μm in length could be placed end-to-end across the period at the end of this sentence.

For example, the dimensions of an average rod-shaped bacteria, E. coli, are about 1×2 μm, typical of most prokaryotes.  For comparison, average eukaryotic cells can be 10 to more than 200 μm in diameter. Though we often use diameter to specify cell size, many cells are not spherical in shape.

Bacteria vary in their cell size, depending on the nature of bacterial cells.  

• Cocci are approx. 0.5 to 1 μm in diameter.
• The vast majority of rod-shaped bacteria cultured in the laboratory are between 0.5 to 4  μm wide and less than 15  μm long.
• Spirochetes are longer (up to 20 μm) and narrower (0.1 to 1.0 μm). 

The size of bacteria also varies with the medium and growth phases. Bacteria are usually the smallest in the logarithmic phase of growth.

Typical Size of Representative Bacteria 

Observation of Bacterial Cells 

Experts believe that an unaided eye with normal vision can see objects ≥200 μm, so most bacteria are too small to be seen without a microscope.  Humans can see only a handful of giant bacteria without a microscope. 

In some cases, even a light microscope fails to see bacteria such as spirochetes. The highest resolution possible in a compound light microscope is about 0.2 μm (i.e., two objects closer together than 0.2 μm cannot be resolved as distinct and separate). Because of this reason, it is hard to see spirochetes in a light microscope. For example, Leptospira, a delicate spiral-shaped bacteria (spirochete), is only about 0.1 μm in diameter and is barely discernible under a darkfield microscope.

Cell size and volume of some prokaryotic cells, from the largest to the smallest

Giant Bacteria

Thiomargarita magnifica is the world’s biggest single-cell bacteria. It is up to 2 cm long and is visible to the naked eye· It is 50 times bigger than any other known bacteria. Thiomargarita namibiensis, which means “sulfur pearl of Namibia,” is another largest known prokaryote. This sulfur chemolithotroph can be 750 μm in diameter and nearly visible to the naked eye. This gram-negative coccid proteobacteria is about 100 times bigger than the average bacterial cell.

Epulopiscium fishelsoni is another very large prokaryote with cells longer than 600 μm (0.6 millimeters). This bacterium is phylogenetically related to the endospore-forming bacterium Clostridium and is found in the gut of the surgeonfish.

Smallest Bacteria 

The smallest bacteria (Mycoplasma) are about the same size as the largest viruses (poxviruses) and are the smallest organisms capable of existing outside a host. 

Mycoplasmas are the smallest known bacteria that can grow and reproduce outside living host cells. Because of their size and because they have no cell walls, they pass through most bacterial filters and were first mistaken for viruses.

Cell-size Comparison

• Eukaryotic cells are known with diameters as small as 0.8 μm or as large as several hundred micrometers. 
• Cells of yeast, Saccharomyces cerevisiae measures 8 μm in diameter.
• Borrelia is longer than a human blood cell. It is 10 μm long, whereas RBCs are 7 μm in diameter.
• Viruses vary in size, with the smallest known viruses being only about 10 nm in diameter.

Lower Limits of Cell Size

Small cells have a higher surface-to-volume (S/V) ratio. A Higher S/V ratio of smaller cells supports a faster rate of nutrient exchange per unit of cell volume compared with larger cells. Thus, smaller cells generally grow faster than larger cells, and a given amount of resources will support a larger population of small cells than large ones. Smaller cells also increase the possibilities of evolutionary possibilities due to the larger pool of mutations in a population.

Though smaller bacteria have a more tremendous selective advantage in nature, a cell needs to have a specific size to accommodate biomolecules essential for its growth. Cell size > 0.15 um diameter is needed to house the essential components of a free-living cell, such as proteins, nucleic acid, and ribosomes.

References and further readings

1. Microbiology: An Introduction. Gerald J. Tortora, Berdell R. Funke, and Christine L.Case. Pearson Education.
2. Madigan, M., Martinko, J., Stahl, D., & Clark, D. (2012). Brock Biology of Microorganisms (13th ed). Pearson Education.
3. Pelczar Jr., M., Chan, E., & Krieg, N. (2007). Microbiology (5th ed). Tata McGraw-Hill.