Take-Home Assignment 1
Steven Foulis
Bio 212A
September 13, 2013
In every field of study, one can always identify a historical event that completely
revolutionizes the way the study is accomplished. For the field of microbiology, the importance
of the electron microscope cannot be overstated. It enables researchers every day to see the
smallest units of both life and matter, a necessity for a discipline that studies organisms generally
smaller than what can be viewed with the unaided eye.
Prior to the advent of the electron microscope, traditional light microscopes were
extensively utilized by microbiologists.
These light microscopes work via a convex lens
refracting light rays to a focal point, thus magnifying an image to be viewed. However, these
microscopes do have their limitations. The very nature of visible light is such that it has a
𝜆
wavelength of 550nm.1 Using the Abbé equation2, which describes resolution (𝑑 = 2𝑛𝑠𝑖𝑛𝜃), it is
determined that a light microscope, at best, can resolve two dots about 200nm apart.
Considering the human eye can resolve a 0.2mm speck, a light microscope at best can magnify
specimens up to only about 1,000X to still be resolvable by the human observer. Though a light
microscope can be used to view cells and many small bacteria, there is an obvious need for a
microscope that can resolve even smaller specimens.
In 1931, the German engineer Ernst Ruska, alongside Dr. Max Knoll, sought to utilize
electrons instead of light, whose wavelength in a vacuum is 1/100000 that of the latter.3 Thus, a
microscope that uses electrons instead of light to resolve an image would allow one to view far
smaller specimens. Though Ruska’s first prototype electron microscope could only magnify
17X, he was poised to make a significant contribution to the microbiological world. In just two
1
http://inventors.about.com/od/mstartinventions/a/microscope.htm
Willey, Joanne M., Linda Sherwood, and Christopher J. Woolverton. Prescott's Microbiology. Print.
3
http://inventors.about.com/od/mstartinventions/a/microscope_2.htm
2
Take-Home Assignment 1
Steven Foulis
Bio 212A
September 13, 2013
years, he was able to magnify specimens to a tenfold degree greater than any light microscope.
These microscopes work by “passing electrons through a thin slice of the specimen to be studied,
which were then deflected to a photographic film emulsion or projected onto a fluorescent
screen, generating an image at high magnification.”4
Thus, the far shorter wavelength of
electrons compared to visible light allows scientists to view the smallest of microorganisms and
viruses (such as Poliovirus), as well as proteins and other small biomolecules. Even something
as miniscule as a hydrogen atom is within the outer limits of that which can be magnified; useful
electron microscope magnification is over 100,000X.
Today, the recently developed atomic force microscope is of great benefit for
microbiologists. It works by scanning the surface of a specimen at constant distance while
exerting minimal force. Thus, for the first time, an electron microscope can be used to study
surfaces that do not conduct electricity well as well as protein interactions and the behavior of
living bacteria and cells. Such an undertaking was previously impossible, as specimens were
generally dead being exposed to the vacuum of an electron microscope as well as facing other
preparation and staining techniques.
To conclude, the historical importance of electron microscopy has obvious – and
immeasurable – relevance and importance to the microbial world. As a result of this innovation,
microbiologists can now study minute microorganisms previously thought of as inaccessible.
Very nice
4
http://micro.magnet.fsu.edu/optics/timeline/people/ruska.html