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Classification of microbes

This article introduces antimicrobial mechanisms, such as bactericidal and bacteriostatic.
A pile of pill blister packs

In basic terms, microbes belong to one of the three ‘domains of life’ [image] All life can be divided into Prokaryotes and Eukaryotes. In the table below we explore the differences between these two groups:

Prokaryotes Eukaryotes
More simple More complex
Prone to mutation Resistant to mutation
Circular chromosomes anchored to cell wall Chromosomes in the nucleus
Virulence plasmids No extra DNA elements
No organelles Cellular organelles
Prokaryotic ribosomes Eukaryotic ribosomes
Small size and simplicity allows rapid growth Slow cell division

We will now discuss classification of bacteria.

Between bacteria, many variances exist and can be used to assist with classification of bacteria. This includes their shape, the way they cluster into groups, the type of cell wall (Gram staining), metabolic processes, and resistance to antibacterials.



The table above details the differences in bacterial shape, and what these look like. Shape can be one way to classify bacteria [explain further how shape relates to function?]


The way in which bacteria arrange themselves into groups can be used in classification. These groups may be clusters, such as S aureus: [image]

Or they may be ‘beads on a string’, such as strep pyogenes: [image]

Cell wall

The Gram Staining method was developed at the end of the 19th century to distinguish one bacterium from another, based on the chemical and physical properties of their cell walls. This includes detecting the presence or absence of peptidoglycan in the bacterial outer layer, and the stain colour reveals this information. The Gram-staining technique is carried out through the sequential use of 2 stains – first a crystal violet (purple), followed by safranin (dark pink). Gram-positive bacteria retain the purple stain, whereas Gram-negative do not.

Gram-positive Gram-negative
Presence of peptidoglycan layer Absence of peptidoglycan layer
Stains purple Stains pink-red

If you would like to learn more about Gram-negative bacteria, you may be interested in the FutureLearn course [Challenges in Antibiotic Resistance: Gram Negative Bacteria].

Metabolic Processes

Oxygen use: Aerobes vs Anaerobes

Aerobic – bacteria require oxygen to grow Anaerobic – bacteria grow in the absence of free oxygen Bacteria can be classified into four groups according to their requirement for oxygen: – Obligate {strict) anaerobes – grow only in the absence of oxygen – Faculative anaerobes – grow in the presence or absence of oxygen – Microaerophilic – grow best in low oxygen concentrations – Obligate aerobes – grow only in the presence of oxygen


The presence of the enzyme catalase can be used to differentiate between streptococci and staphylococci – staphylococci are catalase positive.


The ability of bacteria to lyse red blood cells (haemolysis) or convert haemoglobin to a green pigment on a culture plate can be used to classify bacteria.

  • Alpha-haemolytic bacteria – no lyses but does convert haemoglobin to a green pigment
  • Beta-haemolytic bacteria – lyse RBCs
  • Gamma-haemolytic bacteria – do not lyse RBCs

Historically haemolysis has been important, but in the present day it is not as commonly used in classifying bacteria. [fact check]

Now we have explored a variety of techniques to classify bacteria, the next step will provide an example of how these may be used in practice.

Let us know in the comments below: do you use any of the above techniques in your setting? Are there any different methods of classifying bacteria you are aware of?

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Introduction to Practical Microbiology

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