What are Bacteria? Characteristics & Overview

Denise DeCooman, Angela Hartsock
  • Author
    Denise DeCooman

    Denise DeCooman was a teaching assistant for the General Zoology course at California University of Pennsylvania while she earned her Master's of Science in Clinical Mental Health Counseling from fall semester of 2015 and spring of 2017. She also has a Bachelor's of Science in Biological Sciences from California University. She has been writing instructional content for an educational consultant based out of the greater Pittsburgh area since January 2020.

  • Instructor
    Angela Hartsock

    Angela has taught college microbiology and anatomy & physiology, has a doctoral degree in microbiology, and has worked as a post-doctoral research scholar for Pittsburgh’s National Energy Technology Laboratory.

Learn about bacteria. Understand what the characteristics of bacteria are and see how bacterial infection is caused. Find out the benefits of bacteria to humans. Updated: 12/22/2021

Table of Contents


Bacteria Overview

Bacteria are a classification of microorganisms that belong to the domain Bacteria. Bacteria, along with Archaea, were once grouped into the Kingdom Monera, but now both classifications are considered separate domains due to differences in both genetic and anatomical characteristics. Currently, the three different domains used to group living things are Bacteria, Archaea, and Eukarya. Archaea contain unicellular organisms which are known as extremophiles and live in environments with extreme temperatures, high salinity, or in certain substances where most other types of organisms would be unable to survive. Eukarya consists of unicellular and multicellular organisms that have true nuclei in their cells. These organisms include protists, fungi, plants, and animals.

Bacteria are prokaryotes, which means that their DNA (deoxyribonucleic acid) is not contained within a nucleus, as these organisms lack a nuclear envelope. Bacteria are microscopic, which means they are way too small to see with the naked eye. To provide perspective on size, bacteria are typically roughly one micron in length or circumference, depending on the bacteria's shape. One micron is the equivalent of a millionth of a meter and the smallest unicellular eukaryotes (including protists like Plasmodium and Euglena) are typically observed to be between 10 and 100 microns. Bacteria are unicellular as well, which means that a single bacterium is composed of a single cell. Due to being so small, bacteria can be found in the geothermal vents deep within the ocean and also on the skin surface of most animals.

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What are the Characteristics of Bacteria?

Bacteria are prokaryotes that lack a nuclear envelope, thus the DNA is found suspended in the cytoplasm. Bacteria are classified by their shape as well as how they react when treated with the Gram stain in the laboratory setting. Bacteria may be considered either Gram-negative or Gram-positive. Gram-negative bacteria do not react to the Gram stain due to their thin peptidoglycan layer and outer lipid layer. Gram-positive bacteria do stain because they have a thicker peptidoglycan layer and lack the outer lipid layer.

All bacteria have a plasma membrane, and some bacteria have projections, like flagella or cilia, on the outside of their bodies. These may help with their locomotion, depending on the structure and type of bacteria. Similar to plant cells, most bacteria have cell walls. Cell walls give the cell support and contribute to its structure and shape. The five most common types of shapes in bacteria are:

  • Sphere-shaped-referred to as cocci, and an example of a sphere-shaped bacteria is "Streptococcus pneumoniae"
  • Rod-shaped-bacilli, with the example of "Escherichia coli"
  • Comma-shaped-vibrios, with an example of "Vibrio cholerae"
  • Spiral-shaped-spirilla, with an example of "Helicobacter pylori"
  • Corkscrew-shaped-spirochete, with an example of "Treponema pallidum"

Bacteria are able to metabolize or break down, many different organic (carbon-based) and inorganic chemical substances which makes it easy for them to live in such diverse habitats. Some bacteria need oxygen in order to survive, and these bacteria are said to be aerobic. Bacteria that thrive in conditions where atmospheric oxygen is not present are known as anaerobic bacteria. Another type of bacteria that does not need oxygen but can survive in its presence are called facultative anaerobes.

Bacteria reproduce via the process of binary fission. The process of binary fission is somewhat similar to mitosis because both processes involve a parent cell splitting and giving way to two daughter cells. Some bacteria complete the process of replication via binary fission in just a few minutes while others take up to several years. Some bacteria form what are called endospores, which are spores that can enter a cryptobiotic, or dormant, stage until environmental conditions are favorable for their growth. These spores can withstand stressors such as extreme temperatures and toxic chemical substances where most other life forms would die off.

Bacterial Cell Diagram

Bacterial Cell Diagram

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Frequently Asked Questions

What is bacteria and what are its characteristics?

Bacteria are prokaryotic, unicellular organisms which belong to the domain Bacteria and are not Archaea. Most bacteria have no known affect on humans but some are pathogenic and cause disease while others actually benefit human beings.

What disease is caused by bacterial infection?

Many different bacteria are capable of causing infections in humans. The bacteriumm, Treponema pallidum is responsible for the sexually transmitted infection known as syphilis and Haemophilus influenzae can cause pneumonia or bacterial meningitis in humans.

What are the 5 characteristics of bacteria?

Five characteristics of bacteria include being unicellular, prokaryotic, microscopic, lacking a nucleus, and having a plasma membrane. These traits are shared by all bacteria.

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