Microbiology Basics

Microscopes: instruments that turn invisible into visible

Post by Hazel Silistre, Alumna of Institut Pasteur
Inspired by her international name, Hazel has left Turkey in 2008 to get her university education in Germany and has lived in multiple countries ever since. Her interest to understand life by deciphering bacterial mechanisms led her to pursue a PhD in microbiology. After finishing her PhD at the University of Nottingham, she has left the U.K. and continued her research at Institut Pasteur in Paris. The creative aura of Paris has given her courage to leave the laboratory and follow her passion for writing full-time. She is committed to communicating science to the public and aspires to be a science journalist.

Illustration by Charlotte Brancaz, Alumna of Université Paris Saclay
Motivated by her interest in life sciences, Charlotte has obtained a biology degree at university. Combining her love for science as well as drawing with science communication, she sketches scientific illustrations that can be found on her Instagram page.

The global scientific language is English now, however scientists were using Latin and Greek in ancient times. Did you know that microscope emerged from the combination of the following two words in ancient Greek?

μικρός (mikrós) “small” and σκοπεῖν (skopeîn) “to look, see”

A microscope is a magnifying instrument that is used to look at objects that are too tiny to be seen by the naked eye. The first person to build a microscope and start observing small living organisms has been said to be Robert Hooke. He has included illustrations of plants and fauna in his book Micrographia published in 1665 [1]. However, the first one to observe microbes in detail was the Dutch merchant Antoni van Leeuwenhoek in the 17th century [2]. He built light microscopes that could magnify objects 200x – 300x in comparison to others (30x) on the market. He could observe cells having a diameter as small as 1 μm. He managed to document eukaryotic cells such as blood and sperm cells, furthermore, he could see in detail many microorganisms present in a river water sample [3]. Can you imagine the sort of excitement he must have felt? 😍 Watch this wonderful video by the Howard Hughes Medical Institute to get a feeling:

And if you would like to put yourselves in his shoes for a while or are looking for a nice children’s activity, click here to get creative and build a Leeuwenhoek microscope 🙂

I can sort of resonate with his feelings when he first saw the “animalcules” and continued to observe them for 50 years. Of course I knew what I was looking at but what I was looking at with only 100x magnification was mesmerizing… I was infecting human lung cells with bacterial cells and just within 3 hours, I could witness the incredible power of these tiny creatures (the damage depended on the type of toxins these bacteria released). The healthy, stretched out lung cells were gone and instead there were tiny spheres!

Light microscopes (LM) are commonly used to look at intact cells in the laboratories, they produce images by projecting light beams on the specimen surface and magnifying the image through the use of optical lenses. However, they are not powerful enough to image extremely small organisms such as viruses (about 10x smaller than bacteria) or zoom into the interior of cells. Therefore, nowadays we have much more complex microscopes such as the electron microscopes (EM). In EMs, electron beams are projected onto the specimens and images are magnified with the use of electromagnetic fields. Whereas transmission EM (TEM) allows the study of the cell’s interior, scanning EM (SEM) suffice to investigate the surface of the cells [4, 5].

Several types of LMs exist, differing in the manner that the light gets in. For instance, in dark-field LMs, the light reaches the specimen from the sides only, whereas in the bright-field LMs it is the contrast between the specimen and their surroundings that allow visualization. In cases where the contrast is negligible, dyes can be used to stain the specimen and increase the contrast, but staining kills the cells. For this reason, phase contrast LMs were developed in order to improve the contrast and study cells without staining them [4].

Moreover, there are fluorescence microscopes that are used to visualize specimens that emit light, but of different color, when light is projected on them. Cells may be naturally fluorescent such as the Vibrio fischeri bacteria or they may have been treated with a fluorescent dye [4].

Evolution of microscope; on the left are light microscopes starting with Leeuwenhoek’s version, followed by fluorescence (2nd from right) and electron microscopes (last one on the right)


  1. Hooke, R. (1665). Micrographia, or some physiological descriptions of minute bodies made by magnifying glasses, with observations and inquiries thereupon. Published by the Royal Society, London, UK
  2. Lane, N. (2015). The unseenworld: reflections on Leeuwenhoek (1677) ‘Concerning little animals’. Phil. Trans. R. Soc. B370: 20140344
  3. Leeuwenhoek, A. V. (1676). Concerning little animals observed in rain-well-sea- and snow water; as also in water wherein pepper had lain infused. Phil. Trans. R. Soc. 12821 – 831
  4. Madigan, Martinko, and Parker (2003). Brock Biology of Microorganisms, 10th edn. Prentice Hall/Pearson Education Upper Saddle River, NJ, USA
  5. What is Electron Microscopy?