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Bacteria are found all around us; in the air we breathe, in the soil and water, and inside and on our bodies. They are tiny single-celled organisms, only a few micrometers in size, and the individual cells can only be seen under a microscope.

However bacteria can also form colonies when grown on agar plates. These can be made up of several hundred thousands of cells and are visible to the naked eye.

Electron microscopy image of individual Salmonella bacteria (left) and colonies of Salmonella growing on agar (right).
Figure 1. Salmonella typhimurium cells visualized by scanning electron microscope (left) and colonies of S. typhimurium growing on agar (right). Photo: Courtesy of Prof. D.I. Andersson (left) and Dr. M. Pränting (right).

Generation time of bacteria in relation to man

Bacteria multiply fast. Some fast growing species like Clostridium perfringens and Escherichia coli can double every 15 to 20 minutes in favorable environments. In the table below, it is visualized how fast bacteria grow in comparison to man*:

Bacteria Man
100 generations ≈ 24 hours 100 generations ≈ 2,000 years
1000 generations ≈ 10 days 1000 generations ≈ 20,000 years

*Calculation based on a generation time of 15 minutes for bacteria and 20 years for man.

Gram-positive and Gram-negative bacteria

Bacteria can be broadly categorized into two groups: Gram-positives and Gram-negatives. Staphylococcus aureus and Streptococcus pneumoniae are examples of Gram-positive bacteria, while E. coli and Salmonella typhimurium are examples of Gram-negative. The categorization is based on the structure and composition of the cell wall and the membranes surrounding the bacterial cell. Depending on how different colored stains enter and remain in the cell wall, bacteria are either stained purple (Gram-positive bacteria) or pink/red (Gram-negative bacteria). The structure of the cell wall is one of the factors that influences bacterial susceptibility to antibiotics.

Schematic image of gram-positive bacteria surrounded by a plasma membrane and a thick mesh-like peptidoglycan cell wall
Figure 2. Cell envelope structure of Gram-positive and Gram-negative bacteria. Peptidoglycan: mesh-like cell wall layer; cytoplasm: area inside the cell membrane; LPS: lipopolysaccharide, molecule found in the outer membrane of Gram-negative bacteria; Porin: pore in the membrane; OM: Outer membrane; IM: inner membrane; Periplasmic space: area between OM and IM. Courtesy of Dr. E. Gullberg.

DNA and genes

All the information the bacterium needs to carry out its normal life and function is stored as DNA in the bacterial chromosome. Bacteria usually have a single, circular chromosome. Within the chromosome, there are genes instructing the cell on how to build proteins. Proteins carry out many of the cell’s functions such as transporting nutrients and harvesting energy. A typical bacterium has a couple of thousand genes, as compared to about 20,000-25,000 genes in humans. Sometimes the function of a gene can be disrupted or altered due to mutations in the DNA. Mutations may occur due to mistakes when bacteria are producing copies of their chromosomes, and could in some cases give rise to antibiotic resistance. Read more about how mutations can lead to antibiotic resistance here: Antibiotic resistance – Mutations and selection.

Selected Resources

Resource Description
What is Microbiology/Introducing microbes Facts sheets Learn more about microbiology, bacteria and other microbes in these online facts sheets.
Bacteria, Life-form (Encyclopaedia britannica) Article about bacteria; what they are, what the bacterial cell looks like, and differences between bacterial cells and human cells.
Eric’s Medical Lectures: Classification of Bacteria Video on the classification of bacteria, introductory slide show lecture (22 min, narrated).
Genes vs. DNA vs. Chromosomes – Instant Egghead #19 Video on the organization of genomes in humans: What are DNA, genes and chromosomes? (3 min).
Video with analogies comparing the sizes of bacteria, viruses and human cells (2 min).
Video explaining the structure of bacteria (9 min).
Microbiology Society. What is Microbiology? [Internet]. [cited 2021 Oct 29]. Available from:
Bacteria | Cell, Evolution, & Classification [Internet]. Encyclopedia Britannica. [cited 2019 Aug 15]. Available from:
Armando Hasudungan. Microbiology - Bacteria (Structure) [Internet]. 2013 [cited 2017 May 16]. Available from:
Finlay, Ganem. Size Analogies of Bacteria and Viruses [Internet]. [cited 2015 Feb 21]. Available from:
Genes vs. DNA vs. Chromosomes - Instant Egghead #19 [Internet]. 2012 [cited 2015 Feb 9]. Available from:
Classification of Bacteria (Antibiotics - Lecture 1) [Internet]. 2013 [cited 2014 Dec 4]. Available from: