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Ultra-Small Bacteria

May 7, 2015

In my early audiophile days, bigger was always better. Since the best magnets of the time were Alnico, with magnetic strength far less than today's magnets, loudspeakers were inefficient. Many watts of power were required to drive these, and this meant large amplifiers. Coupling from the high impedance outputs of vacuum tubes to the low impedance loudspeakers was done by transformers, and these transformers were built with large, heavy cores to preserve low frequency response.

6L6 vacuum tubes

The 6L6 was the most popular type of vacuum tube in the output stage of power amplifiers.

The 6L6 is still used today in some guitar amplifiers.

(Photograph (cropped) by Art Brom, via Wikimedia Commons.)


Today's trend is towards smaller electronics with more functionality. The minimum feature dimension in transistors has reached 14 nanometers, and there are plans in place to bring this down to 5 nm in 2020. Since the distance between atoms in silicon is about half a nanometer, a five nanometer field effect transistor channel would encompass just ten atoms of silicon.

There are hopes that new materials such as graphene will aid in this "race to the bottom," but we can't help but think that a limit will be reached. While science fiction fans wait for positronic brains, computer scientists have been improving computational speed using multiple processors. It's likely that the device with which you're reading this article operates by dividing computing tasks among severtal processors.

AMD Opteron quad core die

Image of the die for the Advanced Micro Devices, Inc. quad core Opteron processor.

Notice the symmetry about a central point.

(Image ©2008 Advanced Micro Devices, Inc. with permissions for reuse, via Wikimedia Commons.)


Just as there's an apparent limit to how small a transistor can be, there's also a limit to how small a living cell can be. In assessing this limit, we must immediately eliminate viruses, which might appear to be the smallest living things. Viruses, however, are not alive. They're just pieces of RNA or DNA protected by a protein sheath. Since they have no inherent ability to replicate on their own, instead relying on the reproductive mechanisms of living cells to make copies of themselves, they do not meet the definition of "living."

While the size of the smallest cell is much larger than that of a transistor, they are difficult to image accurately, since high resolution imaging devices require a vacuum, and vacuum will modify or destroy the cell structure. A team of scientists from the University of California (Berkeley, California), Lawrence Berkeley National Laboratory (Berkeley, California), and the Joint Genome Institute (Walnut Creek, California), have developed techniques to accurately image small cells, and they applied their techniques to ultra-small bacteria cells in groundwater.[1-2]

Since imaging ultra-small bacteria has been difficult, there has been no consensus on the range of sizes for bacteria. This new study is the first comprehensive electron microscopy and genomic study of ultra-small bacteria.[2] As a reference to the size of these cells, which have an average volume of just 0.009 cubic micrometers, 150 of them can fit inside a cell of the typical bacterium, Escherichia coli, and 150,000 such cells would be needed to cover the tip of a human hair.[1-2]

These ultra-small bacteria were collected by repeated filtering of groundwater, collected at Rifle, Colorado, through filters down to 200 nanometers. The last filter size is typically used to sterilize water, but the presence of these bacteria showed that the water was not really sterile.[2] The bacteria were flash frozen to -272 °C using an unique device called a cryo plunger that ensured that they weren't altered in transit from the field to the laboratory.[2]

The bacteria were imaged in a cryogenic transmission electron microscope, and the imaging revealed some dividing cells. The cell division showed that the bacteria were healthy and not bacteria starved to an abnormally small size.[2] Genome sequencing showed a length of about one million base pairs, and that the majority of the bacteria were not Archaea, but bacteria from WWE3, OP11, and OD1 phyla.[1-2] Imaging of these groundwater-harvested ultra-small bacteria shows densely packed spirals that are likely the DNA, a few ribosomes, and hair-like appendages.[1-2] The photograph shows a cell with a complex cell wall, and a dense interior compartment, with darker spots at each end most likely ribosomes.[2]

Image of an ultra-small bacteria

Cryo-electron tomograph of an ultra-small bacteria cell.

The scale bar is 100 nm.

(Lawrence Berkeley National Laboratory image.)


Jill Banfield, an author of the study and a professor in UC Berkeley's department of Earth and Planetary Science, says that “we don't know the function of half the genes we found in the organisms from these three phyla."[2] One interesting finding was that some of the bacteria have thread-like appendages. These so-called pili seem to enable inter-organism connections that aid in cellular processes.[2] This might be essential to their survival, since the limited genome would lack many functions.[1-2]

Image of an ultra-small bacteria showing pili-like structures

Cryo-transmission electron microscope image of an ultra-small bacteria cell showing pili-like structures at the surface.

The scale bar is 100 nm.

(Lawrence Berkeley National Laboratory image.)


As Banfield explains,
"These newly described ultra-small bacteria are an example of a subset of the microbial life on earth that we know almost nothing about... They're enigmatic. These bacteria are detected in many environments and they probably play important roles in microbial communities and ecosystems. But we don't yet fully understand what these ultra-small bacteria do."[2]

Birgit Luef of the Norwegian University of Science and Technology (Trondheim, Norway), a former postdoc in Banfield’s group, and an author of the paper, says that "there isn't a consensus over how small a free-living organism can be, and what the space optimization strategies may be for a cell at the lower size limit for life."[2]

This research was supported by the U.S. Department of Energy.[2]

References:

  1. Birgit Luef, Kyle R. Frischkorn, Kelly C. Wrighton, Hoi-Ying N. Holman, Giovanni Birarda, Brian C. Thomas, Andrea Singh, Kenneth H. Williams, Cristina E. Siegerist, Susannah G. Tringe, Kenneth H. Downing, Luis R. Comolli, and Jillian F. Banfield, "Diverse uncultivated ultra-small bacterial cells in groundwater," Nature Communications, vol. 6, article no. 6372 (February 27, 2015 ), doi:10.1038/ncomms7372.
  2. Dan Krotz, First Detailed Microscopy Evidence of Bacteria at the Lower Size Limit of Life, Lawrence Berkeley Laboratory Press Release, February 27, 2015.

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