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.
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.
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). |