How small can they go? Ultra-small bacteria discovered

An international team of scientists, co-led by Dr. Luis Comolli of the Lawrence Berkeley National Laboratory and Professor Jillian Banfield of the University of California at Berkeley took the first detailed images of the smallest bacteria known to date. Although the existence of ultra-small bacteria has been heavily debated by scientists for decades, the associated study, published in Nature Communications this past February, provides evidence of their existence through a combination of morphological characterization and DNA sequencing.

Just how small are these cells?

The cells have a median volume of 0.009±0.002 cubic microns with an associated diameter of 0.254±0.025 microns.

How small is this exactly and how small are these cells compared to other microorganisms we already know about? Check out the graphic below.

The size of these ultra-small bacterial cells compared to other cells we are much more familiar with. The units shown are nanometers, which represents the billionth of a meter or ~3 ft. Although very small compared to bacteria we know a lot about, viruses still hold the record for smallest life forms but not by much. Credit: Kenneth Flynn.

Although several articles have highlighted this study, an important point needs to be made: these ultra-small bacteria are not the smallest known life forms. They are, however, the smallest known cells.

As you can see in the graphic above, viruses still maintain the record for smallest known life form. In fact, the researchers even published an image of one of these ultra-small cells under attack by bacteria-targeting viruses called bacteriophages.

Cryogenic transmission electron microscopy images of ultra-small cells. Left, small appendage structures that are pili-like possibly used for movement and the transfer of required cellular components not encoded by the small genomes. Right, an ultra-small cell with three bacteriophage attaching to the outside. Credit: Berkeley Lab.

Why is proving the existence of such ultra-small bacteria difficult?

Previously, the same group of researchers at Berkley sequenced DNA from water samples acquired from an aquifer at Rifle, Colorado. Using this data, they constructed several near-complete genomes of organisms belonging to bacterial branches of life that lack isolated representatives that microbiologists can grow and work with in a lab called Candidate phyla. Most notably, these genomes are very small and appear to lack genes involved in important cellular functions that should be necessary for life. Although the researchers suspected that these genomes could belong to small organisms, direct evidence was lacking.

Seeing is believing.

Candidate phyla often do not have known representative isolates because we have not yet figured out how to grow them outside of their natural environments. They are a huge challenge for microbiologists known as unculturables. To isolate and take photos of these bacteria without being able to grow them in the lab, the researchers had to get creative.

First, they used a series of filtration steps to exclude cells and debris larger than 0.2 microns and smaller than 0.1 microns. In addition to DNA sequencing, filtered samples were flattened onto a copper plate and flash frozen in liquid ethane. This method freezes the sample so fast that ice crystals cannot form and remain suspended in ‘vitreous’ ice. Finally, cryogenically frozen samples are imaged using a transmission electron microscope (TEM) in a process referred to as Cryo-TEM. Check out the video below for an explanation.

This is a very brief introduction to how transmission cryo-electron microscopy is used to solve the structures of protein complexes. ...

Bacteria are already pretty tiny, right? What’s the big deal? 

Currently, the researchers have no idea what these bacteria do yet but the potential importance is immense.

Accompanied with the bacteria’s genome sequences, the researchers examined DNA sequencing data from other studies and found that similar bacteria likely exist in a wide variety of different environments and could play important roles in microbial communities and ecosystems. Now that we know they exist, researchers can begin asking more specific questions and start testing hypothesizes related to this modified view of the microbial world.

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