Bacteria are microbes with a cell structure simpler than that of many other organisms. Their control centre, containing the genetic information, is contained in a single loop of DNA. Some bacteria have an extra circle of genetic material called a plasmid rather than a nucleus. The plasmid often contains genes that give the bacterium some advantage over other bacteria. For example it may contain a gene that makes the bacterium resistant to a certain antibiotic.

Bacteria are classified into five groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios) or corkscrew (spirochaetes). They can exist as single cells, in pairs, chains or clusters.

Bacterial shapes
© ttsz / iStock Different bacterial shapes.

Bacteria are found in every habitat on Earth: soil, rock, oceans and even arctic snow. Some live in or on other organisms including plants and animals including humans. There are approximately 10 times as many bacterial cells as human cells in the human body. A lot of these bacterial cells are found lining the digestive system. Some bacteria live in the soil or on dead plant matter where they play an important role in the cycling of nutrients. Some types cause food spoilage and crop damage but others are incredibly useful in the production of fermented foods such as yoghurt and soy sauce. Relatively few bacteria are parasites or pathogens that cause disease in animals and plants.

E. coli
© gaetan stoffel / iStock 3D illustration of Escherichia coli

How do bacteria reproduce?

Most bacteria reproduce by binary fission. In this process the bacterium, which is a single cell, divides into two identical daughter cells. Binary fission begins when the DNA of the bacterium divides into two (replicates). The bacterial cell then elongates and splits into two daughter cells each with identical DNA to the parent cell. Each daughter cell is a clone of the parent cell.

When conditions are favourable such as the right temperature and nutrients are available, some bacteria like Escherichia coli can divide every 20 minutes. This means that in just seven hours one bacterium can generate 2,097,152 bacteria. After one more hour the number of bacteria will have risen to a colossal 16,777,216. That’s why we can quickly become ill when pathogenic microbes invade our bodies.

Survival mechanism

Some bacteria can form endospores. These are dormant structures, which are extremely resistant to hostile physical and chemical conditions such as heat, UV radiation and disinfectants. This makes destroying them very difficult. Many endospore-producing bacteria are nasty pathogens, for example Bacillus anthracis, the cause of anthrax.


Learn more about bacteria

  • Microbiology Today: Mycobacteria

    The organisms that cause tuberculosis in humans and animals, Mycobacterium tuberculosis and Mycobacterium bovis, are featured in this edition of Microbiology Today alongside Mycobacterium leprae, the cause of leprosy, and Mycobacterium ulcerans, which causes Buruli ulcer.

  • Building bacterial bridges

    Often, the first things that come to mind when we think about microbes in the built environment are damage, decay, discolouration and staining to building materials and their surfaces. What we don’t often consider is their ability to act as ‘bioengineers’.

  • Tuberculosis explainer

    Tuberculosis (TB) is a debilitating multi-organ disease caused by the bacterium Mycobacterium tuberculosis. The most important form of the disease is pulmonary TB, an infection of the lungs and respiratory tract.

  • Antimicrobial resistance (AMR)

    The threat of antimicrobial resistance (AMR) has now been recognised globally and it is estimated that 10 million people a year will die due to antimicrobial resistance by 2050 if no urgent action is taken.

  • Pseudomonas - friend and foe

    Species within the genus Pseudomonas are amongst the most researched bacteria in the scientific community. Bacteria in this genus are widely used as model organisms in microbial research, and include a range of important species in fields such as plant pathogenicity, bioremediation, and environmental microbiology.

  • Streptomyces – nature’s solution to AMR

    As well as being hugely important in the medical and pharmaceutical industries, Streptomyces also play a significant environmental role; contributing to the decomposition of organic matter, and fertility of the soil.




Microbiology Today: Mycobacteria. Science Photo Library.
Building bacterial bridges. B. Reeksting.
Tuberculosis Explainer. iStock/Dr_Microbe.
Antimicrobial resistance. digicomphoto/Thinkstock.
Pseudomonas - friend and foe. iStock/Dr_Microbe.
Streptomyces – Nature’s Solution to AMR. Thinkstock.
Podcasts. Roger Harris/Science Photo Library.