Microbiology is an important discipline for all physicians. Microbes have the ability to thrive in many extreme conditions. Microorganisms exist in every habitat on Earth and even within the human body. Many microorganisms are beneficial and perform multiple functions, including decomposition. However, some microscopic species have the ability to cause disease. Physicians need to understand which microbes cause disease, how they infect cells, and the means to treat them.

Image: "Amoeba from the Collection Pénard MHNG" by Thierry Arnet, Project Pénard. License: CC BY-SA 3.0

What is a Microbe?

Microbes are organisms that are visible only under a microscope. The field study of microbes is called microbiology. Bacteria, archaea, fungi and protozoa are all different types of microbes.


Escherichia Coli NIAID

Image: “Escherichia coli: Scanning electron micrograph of Escherichia coli, grown in culture and adhered to a cover slip.” by Rocky Mountain Laboratories, NIAID, NIH. License: Public Domain

  • They are prokaryotes they lack nuclei and membrane-bound organelles.



  • They exist in a series of shapes: cocci, bacilli, and spirals, among others.
  • There are about 5 x 1030 bacteria in the world today.
  • Bacteria are found in all of the environments on Earth.


  • Halobacteria

    Image: “Cluster of cells of Halobacterium sp. strain NRC-1” by NASA. License: Public Domain

    Developmentally closer to eukaryote cells than bacteria.

  • They lack nuclei and membrane-bound organelles like prokaryotes.
  • Found in extreme environments, like hot springs.
  • Archaea can also be found in soils, marshlands and oceans.


Trichoderma fertile

Image: “Conidiophores of Trichoderma fertile with vase-shaped phialides and newly formed conidia on their ends (bright points)” by Strobilomyces. License: Public Domain

  • These are eukaryotes, similar to human cells.
  • Are heterotrophs they obtain energy from organic substances.
  • They are significant for having chitin in their cell wall, which is unique to this species.
  • They also have sterols in their cell membrane, like humans.
  • Can exist in two forms. The first one is a filamentous form, with hyphae and mycella.
  • The other form of fungi are yeasts, which are unicellular organisms.


  • Protozoa are a type of eukaryote cell.
  • They consist of a diverse group of unicellular organisms.

    Colpoda inflata

    Image: “Colpoda inflata” by Dr. Eugen Lehle. License: CC BY-SA 3.0

  • These organisms can be further divided into flagellates, amoebas, cillates, and
  • Apicomplexas.
  • Some protozoa can act as pathogens.

The Concept of Microorganisms

Compared to other branches of science, like physics, microbiology is a relatively recent field of research, originating in the middle of the 17th century. There were three major discoveries that created the field of microbiology.

The first discovery was that microorganisms existed and were first seen by Antonie Van Leeuwenhoek, a Dutch lensmaker. He built his own microscope and was able to see microbes in the late 17th century.

Louis Pasteur

Louis Pasteur

The next great discovery in microbiology was made by Louis Pasteur in the mid-1800’s. It was assumed that all organisms were spontaneously generated. Pasteur’s experiments with broth showed that microorganisms can be growth and are not spontaneously created.

The last discovery was made by Robert Koch. Around the same time as Louis Pasteur, he discovered that some microbes can cause diseases in humans.

Surviving the Extremes of Life

  • Unlike many cells, microbes show the ability to live in the most extreme conditions on Earth.
  • Scientists can grow microbes at pH ranging from 0 to 11.4.
  • Microbes have been documented living at temperatures from -15 to 121 ºC.
  • Bacteria have survived exposure to 5 megarads of radiation.
  • Microorganism can sustain life at pressures of 117,000 pounds per square inch, which would destroy many human cells.

Metabolic Wizardry

Because of the wide range of environments that microbes can thrive, they have become wizards in terms of producing energy. Human cells that need oxygen and organic material to make energy.

Some organisms can use light to make energy (photosynthesis).

Others can derive energy from inorganic compounds, like hydrogen, iron, and sulfides.

Microorganisms play an important role on Earth in aiding in the maintenance of life.

  • Half of all the oxygen in the atmosphere is produced by bacteria.
  • Almost 100 % of natural nitrogen is made by microbes.
  • Microorganisms aid in the breakdown of plants and organic matter.

Why Do Doctors Care about Microbes?


Image: “This scanning electron micrograph (SEM) clearly showed the ventral surface of a Giardia muris trophozoite” by CDC/ Dr. Stan Erlandsen. License: Public Domain

Why do physicians need to know about microbes? Since many microbes make humans sick, it is essential that doctors know about microbial morphology, virulence factors and treatment methods. Bacteria like streptococcus and staphylococcus cause pneumonia and skin infection.

Protozoa, such as Giardia can enter the intestine and cause massive diarrhea. Fungi can cause systemic infections in the immunocompromised and viruses are responsible for many diseases and conditions, like AIDS.

Microbes are all around us and are part of our everyday life. While many organisms cause diseases, there are thousands of microbes that live in a close relationship with another organism.

If both parties benefit from the relationships, they are called mutual symbiotic relationships. Humans have many symbiotic relationships with microorganisms. A good example is the human intestinal tract has millions of bacteria in it that aid in the digestion of food. They also prevent pathologic bacteria from colonizing the intestinal lumen by crowding it out of space.

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