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Bacteria Definition

Bacteria are single-celled microorganisms with prokaryotic cells, which are single cells that do not have organelles or a true nucleus and are less complex than eukaryotic cells. Bacteria with a capital B refers to the domain Bacteria, one of the three domains of life. The other two domains of life are Archaea, members of which are also single-celled organisms with prokaryotic cells, and Eukaryota. Bacteria are extremely numerous, and the total biomass of bacteria on Earth is more than all plants and animals combined.

Evolution of Bacteria

Bacteria first arose on Earth approximately 4 billion years ago, and they were the first forms of life on Earth. For 3 billion years, bacteria and archaea were the most prevalent kinds of organisms on Earth. Multicellular eukaryotes did not appear until around 1.6-2 billion years ago. Eukaryotic cells, which make up all protists, fungi, animals, and plants, also contain what was once bacteria; it is thought that the mitochondria in eukaryotes, which produce energy through cellular respiration, and chloroplasts in plants and algae, which produce energy through photosynthesis, both evolved from bacteria that got taken up into cells in an endosymbiotic (mutually benefiting) relationship that became permanent over time.

Bacteria Characteristics

Bacteria are single-celled organisms. They lack organelles such as chloroplasts and mitochondria, and they do not have the true nucleus found in eukaryotic cells. Instead, their DNA, a double strand that is continuous and circular, is located in a nucleoid. The nucleoid is an irregularly shaped region that does not have a nuclear membrane. Bacteria also have a cell membrane and a cell wall that is often made of peptidoglycan. Together, the cell membrane and cell wall are referred to as the cell envelope. Many bacteria need a cell wall in order to survive.

Reproduction occurs through binary fission, which is the splitting of a bacterial cell after it reaches a certain size. Bacteria reproduce asexually, so the two daughter cells that result from binary fission have the same DNA as the parent cell. However, some bacteria can also exchange genetic material among one another in a process known as horizontal gene transfer. This method involves two already existing bacteria; it is not a form of transmission from parent to child.

Bacteria Shapes

Bacteria come in a myriad of shapes. The three main shapes of bacteria are coccus, spiral, and bacillus.

  • Cocci are bacteria that are spherical or ovoid in shape. Some cocci remain attached after binary fission, even though separate cells have been formed. For example, diplococci are cocci in pairs, streptococci are chains, and staphylococci are clusters of multiple cocci. Tetrads are square arrangements of four cocci, while sarcinae are cubes of eight cocci.
  • Spiral bacteria are, as the name suggests, spiral-shaped. Spirillums are thick, tough spirals. Spirochetes are spirals that are thin and flexible. Vibrios are comma-shaped rods with a small twist.
  • Bacilli are rod-shaped bacteria. Like cocci, bacilli can be solitary or arranged together. Diplobacilli are two bacilli arranged next to each other, and streptobacilli are chains of bacilli.

Bacteria can also be other shapes such as filamentous (long and thin), square, star-shaped, and stalked. This diagram depicts the numerous shapes of bacteria.

Bacterial morphology diagram
Bacterial morphology diagram

Types of Bacteria

The cell wall also makes Gram staining possible. Gram staining is a method of staining bacteria involving crystal violet dye, iodine, and the counterstain safranin. Many bacteria can be classified into one of two types: gram-positive, which show the stain and appear violet in color under a microscope, and gram-negative, which only show the counterstain, and appear red. Gram-positive bacteria appear violet because they have thick cell walls that trap the crystal violet-iodine complex. The thin cell walls of gram-negative bacteria cannot hold the violet-iodine complex, but they can hold safranin. This makes gram-negative bacteria appear red under Gram staining. Gram staining is used for general identification of bacteria or to detect the presence of certain bacteria; it cannot be used to identify bacteria in any specific way, such as at a species level. Examples of gram-positive bacteria include the genera Listeria, Streptococcus, and Bacillus, while gram-negative bacteria include Proteobacteria, green sulfur bacteria, and cyanobacteria.

Examples of Bacteria

Escherichia coli is one example of a common species of bacteria. It is bacillus-shaped and found naturally in the intestines of many animals including humans, where it produces vitamin K and b-complex vitamins. E. coli is also often used in laboratory research since it reproduces quickly and is hardy. Most strains of E. coli are harmless to humans, but some can cause infection. E. coli infection can result in gastrointestinal problems like diarrhea, and in more severe cases, bacterial meningitis or pneumonia can occur.

Lactobacillus acidophilus is another bacillus-shaped species of bacteria naturally found in places like the intestines and vagina, where it protects against harmful bacteria. It is a probiotic, a bacterium found in certain foods like yogurt and other fermented foods that is consumed in order to help absorb nutrients and replenish the body’s supply of “good” bacteria. It can also be consumed in small amounts by people with lactose intolerance in order to help them consume lactose.

Some bacteria can be extremely harmful, such as Clostridium botulinum, the bacteria that causes botulism. C. botulinum produces the neurotoxin botulinum, which is responsible for the symptoms of botulism. Symptoms include blurred vision, nausea, trouble breathing, muscle weakness, and paralysis. Botulinum toxin is the deadliest known toxin; just one kilogram of botulinum would be deadly enough to kill the entire human population.

  • Prokaryote – An organism that has a simple prokaryotic cell; bacteria and archaea are prokaryotes.
  • Binary fission – The method by which bacteria reproduce asexually through dividing.
  • Probiotic – A bacterium that helps maintain a healthy digestive tract when consumed.
  • Horizontal gene transfer – Gene transfer between two organisms that are not parent and offspring.


1. How do bacteria reproduce?
A. Sexual reproduction
B. Horizontal gene transfer
C. Binary fission
D. Mitosis

Answer to Question #1
C is correct. Bacteria reproduce asexually through binary fission. They can also exchange genes with other bacteria through horizontal gene transfer, but this is not reproduction since it does not involve creating offspring. Mitosis is similar to binary fission, but mitosis only occurs in eukaryotic cells.

2. Which is not one of the three main shapes of bacteria?
A. Coccus
B. Bacillus
C. Spiral
D. Star

Answer to Question #2
D is correct. Star-shaped bacteria, such as those in the genus Stella, are not as common as cocci, bacilli, and spiral bacteria.

3. When did bacteria first begin to exist on Earth?
A. 4 billion years ago
B. 2 billion years ago
C. 1.6 billion years ago
D. 1 billion years ago

Answer to Question #3
A is correct. Bacteria first arose around 4 billion years ago. They are the oldest forms of life on the planet. Eukaryotes started to appear much later, around 1.6-2 billion years ago.

Cite This Article

MLAAPAChicago Editors. "Bacteria." Biology Dictionary,, 19 Mar. 2017, Editors. (2017, March 19). Bacteria. Retrieved from Editors. "Bacteria." Biology Dictionary., March 19, 2017.

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