Types of Bacteria

Find out more about how we categorise bacteria below, and please – towards our work today. We educate the public, support patients and fund research into antibiotic resistance, a global health catastrophe that puts millions at risk of death each year.

Bacteria in petri dish

Biology of bacteria

Bacteria are very small (usually single-cell) organisms. They are one of the first life forms to exist on Earth and have been around for approximately three billion years. Various sources suggest there may be as many as one trillion species – although the vast majority of these have not yet been discovered. Most individual bacterial cells – an individual ‘bacterium’ – are too small to be seen with the human eye, although the colonies they form are visible.

Bacteria are prokaryotic, which means that they exist as single cells (although they form groups known as colonies), and they do not have a nucleus. This means that their genetic material is ‘loose’ within each cell. Each cell is generally contained within a cell membrane, which itself is contained within a protective cell wall. Some types also have a third outer layer called a capsule.

Bacteria can be separated into categories based on various different features. The first way, which is particularly relevant in medicine, is related to the outer covering of the cell and involves a simple staining procedure.

Different types of bacteria

Gram-positive and Gram-negative bacteria

Bacteria are either classified as Gram-positive or Gram-negative based on the outcome of a test called the ‘Gram stain’ named after the inventor Danish scientist Hans Christian Gram (1853–1938). The cells are stained with a purple dye (‘crystal violet’), which only binds to a substance called peptidoglycan. The cells are then rinsed, and stained with a red dye (safranin). If the cells take on the purple colour, they are considered to be ‘Gram positive’, and have a high proportion of peptidoglycan in their cell wall. If the cells take on the red colour, they are ‘Gram negative’.


What does it mean if bacteria are Gram positive?

The substance that retains the purple colour in Gram-positive bacteria is not usually found in the human body, so they are more easily recognised and targeted by the immune system. They are also more susceptible to beta-lactam antibiotics like penicillin, as well as the action of detergents, drying and physical disruption.

This does not mean that Gram-positive bacteria are not harmful. The examples below all pose a risk to human health, and Gram-positive bacteria can still acquire antibiotic resistance (for example, the superbug MRSA is a staphylococcus species).

Gram positive examples

  • Streptococcus
  • Staphylococcus
  • Clostridium botulinum

What does it mean if bacteria are Gram negative?

Gram-negative bacteria do not tend to take up the purple Gram stain as they have little peptidoglycan in their cell wall. Bacterial cells often have labels on their cell wall, called ‘antigens’, which allow the human body to recognise them. If the cell has a capsule or slime covering, as Gram-negative bacteria do, these antigens are hidden from the immune system, so the immune system cannot target the infective bacteria.

In addition to this, a substance called lipopolysaccharide that is found in the outer cell wall of Gram-negative bacteria causes inflammation, which can be severe enough to trigger sepsis. This cell wall also stops antibiotics penetrating the bacteria, making Gram-negative infections more difficult to treat than Gram-positive infections

Gram negative examples

  • Cholera
  • Gonorrhea
  • Escherichia coli (E. coli)
  • Pseudomonas aeruginosa
  • Acinetobacter baumannii
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Different shapes of bacteria

There are three primary shapes: spheres, rods and spirals. In some cases, you can tell the shape of the bacteria from the name: round often have ‘coccus’ in the name, rods ‘bacillus’, and spirals ‘spirillum’. The way that round bacteria join together as a colony can also be used to categorise them.

Other ways

There are plenty of other ways to categorise bacteria, based on how genetically similar they are, the environment they exist in and more.


Bacterial reproduction is complicated, but two key features that affect healthcare are:

  • Most bacteria can reproduce very rapidly, with one ‘parent’ cell dividing to create two identical ‘daughter’ cells in just 20 minutes in some cases.
  • Bacterial reproduction only requires one cell, which splits into two, which both then split again to make four and so forth. It is possible for individual cells to transfer genes to other cells in the colony, such as a gene for antibiotic resistance, or to transfer genes even into another species.


The type of bacteria causing an infection will usually influence the type of treatment given. Find out more about antibiotics, and about multidrug-resistant ‘superbugs’, which are very difficult to treat.