Bacteria roughly 1/100th the volume of a typical Escherichia coli have been found in groundwater.
Jillian Banfield at the University of California, Berkeley, Luis Comolli of Lawrence Berkeley National Laboratory in California, and their colleagues filtered groundwater through a mesh with holes around 0.2 micrometres in diameter and collected a variety of extremely small bacteria (around 0.009 cubic micrometres) that have never been cultured. Under the electron microscope, the microbes seemed to have tightly packed DNA, few of the protein-making structures called ribosomes, and structures that might allow the cells to connect and communicate with one another.
The researchers suggest that these bacteria had not been cultured before because they depend on other microbes to grow.
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Ultra small bacteria spotted. Nature 519, 133 (2015). https://doi.org/10.1038/519133a