Microbiology Basics

Microbial kingdoms: Bacteria

Post by Hazel Silistre, Alumna of Institut Pasteur
Inspired by her international name, Hazel has left Turkey in 2008 to get her university education in Germany and has lived in multiple countries ever since. Her interest to understand life by deciphering bacterial mechanisms led her to pursue a PhD in microbiology. After finishing her PhD at the University of Nottingham, she has left the U.K. and continued her research at Institut Pasteur in Paris. The creative aura of Paris has given her courage to leave the laboratory and follow her passion for writing full-time. She is committed to communicating science to the public and aspires to be a science journalist.

Illustration by Laura Salavessa, Alumna of Institut Pasteur and new member of Institut Curie
Laura finished her university studies in Portugal. Nudged by her nomad spirit to move from one city to another frequently, she has had a chance to explore biomedical research in several institutions where she developed a great interest in cell biology. In 2016, she moved to Paris to do her PhD -that will soon be finished when the post-pandemic regulations allow her thesis committee members to travel!-. She always enjoyed drawing and believes that illustrations constitute a major input in science communication for both an academic and a general audience. She prepares scientific illustrations for research presentations and papers.

Bacteria are unicellular prokaryotic microorganisms. The word prokaryote comes from the Greek πρό (pro) “before” and καρυόν (karyon) “nut or kernel”. “Before kernel” reflects the fact that prokaryotes don’t have a distinct nucleus enclosing the hereditary information of the cell, DNA, that is found in a compact form called chromosome (carrying genes, segments of DNA that encode full proteins or regulatory RNA molecules essential for cell function). Prokaryotes also lack membrane-bound organelles that are specialized compartments where cellular functions such as respiration and protein packaging are carried out. These two major features help us distinguish prokaryotes from eukaryotes (living organisms that have eu “true, genuine” nucleus).

A bacterial cell is defined by a thin, elastic and semi-permeable cell membrane, also called the cytoplasmic or plasma membrane (a bilayer composed of lipids), and in most cases also a porous cell wall (but there are also bacterial cells without cell walls such as Mycoplasma) that gives the cell rigidity and plays an important role in cell division. Outside the cell wall, there may also be a dense, well-defined protective sheath called a capsule. The required nutrients and other substances enter, waste materials and other cell products exit through the cell membrane.

Bounded by the cell membrane is a fluid portion called the cytoplasm including ribosomes (a cytoplasmic particle that is part of the protein-synthesizing machinery of the cell), macromolecules (proteins, nucleic acids, lipids, polysaccharides) small organic molecules that form the macromolecules and some inorganic ions. Bacterial chromosome is not compartmentalized and is loose in the cytoplasm as an aggregated mass called the nucleoid.

In this figure, you are looking at a monotrichous bacterial cell having a single flagellum which is a long, thin, rotatable filamentous appendage protruding from one end of the cell. Bacteria use their flagella to swim towards nutrients and avoid toxic substances. The number of flagella per cell can be 1 to 20 depending on bacterial species, and when there are multiple flagella located in many places around the surface of the cell, we call this a peritrichous cell.

The bacterial cell above is covered in fimbriae that are short spikes of protein that enable bacterial attachment to surfaces or host cells. There are also extensions called pili which are involved in attachment and the transfer of genetic material. Pili are longer than fimbriae but shorter than flagella.

Bacteria are self-sufficient cells. Whereas animal and plant cells exist only as a part of a multicellular structure, a single bacterial cell can grow, reproduce (divide), and produce energy independently of other cells. However, they might also exist as communities of their own or co-exist with different bacterial species in an extracellular polysaccharide matrix of their own secretion. This is called a biofilm.

Bacteria are only a few micrometers (1 micrometer (µm) is 0.000001 meters) in size. They can be classified based on cellular shape, morphology: bacilli (rod), cocci (spherical), spirilla (spiral)

Bacteria and their microbial cousins, the Archaea are known as the earliest forms of life on Earth and scientists believe that they helped shape and change our planet’s environment and supported the formation of more complex living organisms.

Some bacteria live at extremely high temperatures whereas some exist in the blood-freezing water of the Arctic. Some break down starch, some can perform photosynthesis and produce oxygen, others utilize sulfur or consume spilled oil and thus, help clean up pollution. A species of bacteria called Deinococcus radiodurans can withstand blasts of radiation 1,000 times greater than what would kill a human being. Sooooo, do not be deceived by their small size and simplicity, bacteria happen to be much more complicated and powerful than you think…

You may enjoy the information below that I copied from a biology education website 🙂

If you were bacteria:
  • You have 0.001 times as much DNA as a eukaryotic cell.
  • You live in a medium which has a viscosity about equal to asphalt.
  • You have a wonderful “motor” to aid in your movement.
  • While you can “learn”, you divide every twenty minutes (subject to change depending on the species) and have to restart your education.
  • You can have sex, with males possessing a sexual apparatus, the pilus, for transferring genetic information to receptive females. However, if you are male, nature gave you a severe problem. Every time you mate with a female, she turns into a male!
  • Eukaryotes have enslaved some of your brothers to use as energy generating organelles such as mitochondria (organelles responsible for the respiration of the cell) and chloroplasts (organelles equipped with light-absorbing pigments).
  • You have spent three and a half billion years practicing chemical warfare. Humans thought that antibiotics would end infectious diseases, but their overuse has lead to antimicrobial resistance.



Greenwood, D., Slack, R., Peutherer, J. (2002). Medical Microbiology. A Guide to Microbial Infections: Pathogenesis, Immunity, Laboratory Diagnosis and Control, 16th edn. Edinburgh: Churchill Living Stone

Prokaryotes, Eukaryotes, & Viruses Tutorial