An Overview of Microbiology

Definition, History, Classification and Fun Facts

Microbiology is defined simply as the study of microorganisms, with "micro" meaning small and "biology" referring to the study of living things. The microorganisms studied vary widely, and the field of microbiology is broken down into many subfields of study.

Petri dish containing bacterial culture being examined with inverted light microscope in microbiology lab
Rafe Swan / Getty Images

The field of microbiology is critical to human beings not only due to the infectious diseases caused by these microbes but because "good" microorganisms are necessary for us to live on the planet. It's often been said that the bacteria in and on our bodies outnumber our own cells. Research shows they at least equal the number of cells if not outnumber so this field of study could be considered one of the most important areas of knowledge and study.

Types of Microorganisms

Microorganisms, or "microbes," are small living things. Most of these organisms cannot be seen by the naked eye, and until the invention of the microscope and germ theory, we had no idea how plentiful they are.

Microbes are found nearly anywhere on Earth. They are found in boiling pools of water in Yellowstone and in volcanic vents at the lowest depths of the sea. They can live in salt flats and some thrive in saltwater (so much for using salt as a preservative). Some need oxygen to grow and others do not.

The world's "toughest" microorganism is a bacteria called Deinococcus radio trans, a bacteria that can withstand radiation to a phenomenal degree, as its name implies, but can also survive without water, exposure to strong acids, and even when placed in a vacuum.

Classification of Microorganisms in Microbiology

There are many different ways in which scientists have classified, and in doing so tried to make sense of, the millions of microbes in our midst.

Multicellular vs. Unicellular vs. Acellular 

One of the ways microbes are classified is by whether or not they have cells, and if so, how many. Microorganisms may be:

  • Multicellular: Having more than one cell
  • Unicellular: Having a single cell
  • Acellular: Lacking cells, such as viruses and prions; prions are usually referred to as "infectious proteins" rather than microbes.

Eukaryotes vs. Prokaryotes

Another way in which microorganisms are classified has to do with the type of cell. These include eukaryotes and prokaryotes:

  • Eukaryotes are microbes with "complex cells" that have a true nucleus and membrane-bound organelles. Examples of eukaryotes include helminths (worms), protozoa, algae, fungi, and yeasts.
  • Prokaryotes are microbes with "simple cells" that do not have a true nucleus and lack membrane-bound organelles. Examples include bacteria.

The Major Classes of Microorganisms

The different types of microbes can also be broken down into:

  • Parasites: Parasites are sometimes more frightening than other microorganisms, at least when they can be viewed with the naked eye. Parasites include helminths (worms), flukes, protozoa, and others. Examples of parasitic infections include malaria, giardia, and African sleeping sickness. Ascariasis (roundworms) are known to infect 1 billion people worldwide.
  • Fungi and yeasts: Fungi are microorganisms that are in some ways similar to plants. Yeasts are a type of fungus. Examples include athlete's foot or other types of yeast infections, which all fall under the class of fungal infections. This category also includes mushrooms and molds. Like bacteria, we also have many "good fungi" that live on our bodies and do not cause disease.
  • Bacteria: We have as much bacterial in and on our bodies as we do cells, and the vast majority of these bacteria are "healthy bacteria." They protect us against infection from bad or pathologic bacteria and play a role in digesting our food. Examples of infections caused by bacteria include tuberculosis and strep throat.
  • Viruses: Viruses are abundant in nature, though the ones most people are familiar with are those that cause human disease. Viruses can also infect other microorganisms such as bacteria, as well as plants. Immunizations have decreased the risk of some frightening diseases, but others, such as Ebola and the Zika virus, remind us that we haven't begun to conquer these miniature menaces.
  • Prions: Most scientists at this time do not classify prions as microorganisms, but rather as "infectious proteins." That said, they are often studied by virologists. Prions are essentially a piece of abnormally folded protein and may not appear frightening at first. Yet prion diseases such as mad cow disease and kuru are some of the most feared infectious diseases.

History of Microbiology

What we now know about microorganisms, which will be discussed further, is relatively new in history. Let's take a brief look at the history of microbiology:

First microscope/first microorganisms visualized: The first major step in microbiology came about when Antonie van Leeuwenhoek (1632-1723) created the first single lens microscope. Through a lens that had a magnification of roughly 300X, he was able to visualize bacteria for the first time (from scrapings off of his teeth).

Development of germ theory: The human body was recognized as a source of infection by three scientists:

  • Dr. Oliver Wendall Holmes found that women who gave birth at home were less likely to develop infections than those who delivered in a hospital.
  • Dr. Ignaz Semmelweis linked infections with physicians who went directly from the autopsy room to the maternity ward without washing their hands.
  • Joseph Lister introduced aseptic techniques, including both hand-washing and using heat for sterilization.

Germ theory: The two people most credited with the acceptance of the germ theory were Louis Pasteur and Robert Koch:

  • Louis Pasteur (1822-1895): Pasteur is credited with the theory of biogenesis, noting that all living things come from something rather than the prevailing view at the time of spontaneous generation. He claimed that many diseases were caused by microbes. He showed that microorganisms are responsible for fermentation and spoilage and developed the method called pasteurization still used today. He also developed the rabies and anthrax vaccines.
  • Robert Koch (1843-1910): Koch is the author of "Koch's postulates," the scientific series of steps that proved the germ theory and which has been used in scientific studies since (with some revisions). He identified the cause of tuberculosis, anthrax, and cholera.

Since that time, a few landmarks include the following:

  • 1892: Dmitri Iosifovich Ivanoski discovered the first virus.
  • 1928: Alexander Fleming discovered penicillin.
  • 1995: The first microbial genomic sequence was published.

Infectious Microorganisms

When we think of microorganisms, most of us think of disease, though these little "bugs" are overall more likely to help us than hurt us. (Read further to learn about "good microbes.")

Until less than a century ago, and currently in many places of the world, infections with microorganisms were the leading cause of death. The life expectancy in the United States improved dramatically over the last century not only because we are living longer, but mostly because fewer children die in childhood.

In the United States, according to the Centers for Disease Control and Prevention, heart disease and cancer are now the first and second leading causes of death. According to the World Health Organization, in low income countries worldwide, the leading cause of death is lower respiratory infections, followed by diarrheal diseases.

The advent of vaccinations and antibiotics, plus even more importantly clean water, has lowered our concern over infectious organisms, but it would be amiss to be arrogant. At the current time, we are facing not only emerging infectious diseases but also antibiotic resistance.

Microorganisms That Are Helpful to Humans

Though we seldom talk about it, microorganisms are not only helpful but necessary in nearly every aspect of our lives. Microbes are important in:

  • Protecting our bodies against "bad" microbes
  • Making food. From yogurt to alcoholic beverages, fermentation is a method in which the growth of microbes is used to create food.
  • The breakdown of wastes on the ground and recycling atmospheric gases above. Bacteria can even help with difficult waste such as oil spills and nuclear waste.
  • Producing vitamins such as vitamin K and some B vitamins. Bacteria are also extremely important for digestion.
  • Information storing. The field of cryptography is even looking at ways in which bacteria can be used as a hard drive to store information.

Not only do microbes perform many functions for us—they are part of us. It's thought that the bacteria in and on our bodies number our cells equally one to one.

You've probably heard the latest in healthy eating. In addition to eating broccoli and blueberries, most health experts recommend eating fermented foods daily, or at least as often as possible. Without bacteria, there would be no fermentation.

At birth, babies do not have bacteria in their bodies. They acquire their first bacteria as they pass through the birth canal. (The lack of bacterial exposure via the birth canal is thought by some to be the reason why obesity and allergies are more common in babies delivered by C-section.)

If you've read the news lately, it's even been postulated that the bacteria in our guts are responsible for our day-to-day moods. The study of the microbiome is now being used to explain many things, such as why antibiotics may lead to weight gain.

Fields of Microbiology

There are several different fields within the field of microbiology. Examples of some of these fields broken down by type of organism include:

  • Parasitology: The study of the biology of parasites and parasitic diseases
  • Mycology: The study of fungi
  • Bacteriology: The study of bacteria
  • Virology: The study of viruses
  • Protozoology: The study of protozoa
  • Phycology: The study of algae

Fields of microbiology can also be broken down by scope to include a wide range of topics. A few examples among many include:

  • Microbial physiology (growth, metabolism, and structure of microbes)
  • Microbial genetics
  • Microbial evolution
  • Environmental microbiology
  • Industrial microbiology (for example, wastewater treatment)
  • Food microbiology (fermentation)
  • Biotechnology
  • Bioremediation

The Future of Microbiology

The field of microbiology is fascinating, and there is more we don't know. What we have learned most in the field is that there is so much more to learn.

Not only can microbes cause disease, but they can also be used to develop drugs to fight other microbes (for example, penicillin). Some viruses appear to cause cancer, while others are being evaluated as a way to fight cancer.

One of the most important reasons for people to learn about microbiology is to have respect for these "creatures" that far outnumber us. It's thought that antibiotic resistance is increased due to improper use not only of antibiotics but of antibacterial soaps. And that is only when looking at the microbes we currently recognize. With infectious diseases emerging, and with our ability to travel almost anywhere in the world on three flights, there is a great need for microbiologists to be educated and prepared.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Ingrid Koo, PhD
 Ingrid Koo, PhD, is a medical and science writer who specializes in clinical trial reporting