University of Michigan - Ann Arbor

Microorganisms, or microbes, are small organisms that are generally too small to be seen without a microscope. They are highly abundant and can be found almost everywhere on Earth, from boiling thermal hot springs to deep beneath the Antarctic ice. While the vast majority of this invisible world remains unexplored, microbiologists continue to discover new ways in which microbes interact with the environment and humans.

Microbes

- Bacteria
- Viruses
- Fungi
- Prions
- Protozoans
- Protist
- Helminths

Types of Microbes

Microbes are too small to be seen with the naked eye, so humans have developed techniques and technology to observe and study them.

Visualizing Microbes

Microbial genetics is the study of the microbial cell genetic storage in the form DNA and its mechanisms of replication and expression.

Microbial Genetics

Microbial growth is indicated by the increase in number of cells by division of parent cells into daughter cells. 

Microbial Growth

Epidemiology

What is a microbe?

Which of the following best describes a microbe?

Which of the following is NOT considered a microbe?

Most microorganisms are harmless and many provide significant benefits. They form the backbone of many food webs, playing fundamental roles in ecosystems globally. Additionally, humans utilize microbes in various ways, including the production of biofuels, medicines, and foods such as bread, cheese, and beer.

Ecological and Human Uses of Microbes

Long before microscopes were invented, humans possessed a basic awareness of microbial life. Historical evidence indicates that since prehistoric times, humans have utilized microorganisms to create foods, as well as to prevent and treat diseases.

Prehistoric Knowledge of Microorganisms

Even prior to the invention of the microscope, various ancient civilizations recognized that diseases could be transmitted by invisible entities, which are now understood to be microbes. This early awareness of communicable diseases led to the implementation of public health measures, such as quarantining the sick and building comprehensive sanitation systems, to mitigate the spread of infections.

Ancient Understanding of Disease Transmission

The vast majority of microbes are unicellular and require artificial magnification to be observed. However, there are important exceptions: some unicellular microbes are visible to the naked eye, and some multicellular organisms are microscopic. While an object must measure approximately $${}100$$ micrometers (µm) to be visible without a microscope, most microorganisms are significantly smaller. For perspective, a typical microscopic animal cell is roughly $${}10$$ µm across, bacterial cells average about $${}1$$ µm, and viruses can be up to $${}10$$ times smaller than bacteria.

Size of Microorganisms

Microorganisms are highly diverse and differ from one another in various characteristics, including size, structure, habitat, and metabolism. While many microbes are unicellular, there are also multicellular organisms that are microscopic. Additionally, certain microbes, such as viruses, are acellular, meaning they are not composed of cells at all.

Diversity of Microorganisms

Microbial ecology is the study of microbial populations, their interactions with one another, and their relationships with their surrounding environment.

Microbial Ecology

Eukaryotic microbes are a highly diverse group of microscopic organisms that display a wide range of life cycles, nutritional needs, and morphological specializations. While most infectious diseases are caused by bacteria and viruses, eukaryotic microorganisms are responsible for several diseases of major public health significance.

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