Bacteria are prokaryotic microorganisms found in nearly every habitat on Earth, including within and on the human body. As prokaryotes, their genetic material (DNA) is not housed within a true nucleus. Most bacterial cells feature cell walls containing peptidoglycan. While many bacteria are harmless or beneficial, some act as pathogens and cause diseases in humans and other animals.

Bacteria

Viruses are acellular microorganisms, meaning they are not composed of cells. A virus fundamentally consists of proteins and genetic material (either DNA or RNA, but never both) and remains inert outside of a host organism. To multiply and infect other hosts, viruses must incorporate themselves into a host cell and co-opt its cellular mechanisms. They can infect all types of cells, from human cells to other microorganisms, and while some are responsible for diseases like the common cold or Ebola, many do not cause disease.

Viruses

Which of the following are considered types of microbes?

Which of the following statements about microbes is true?

Which of the following microbes is correctly matched with its description?

Which of the following types of microbes are correctly paired with an example?

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

University of Michigan - Ann Arbor

Austin Community College

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

https://openstax.org/details/books/microbiology/

Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

Microbiology is produced through a collaborative publishing agreement between OpenStax and the American Society for Microbiology Press. The book aligns with the curriculum guidelines of the American Society for Microbiology.

OpenStax Microbiology Textbook

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

Microbial ecology is the study of microbial populations and how they interact with their environment.

Microbial Ecology

What is a microbe?

Which of the following best describes a microbe?

Which of the following is NOT considered a microbe?

Which of the following environments can microbes be found in?

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.

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