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A domain consisting of prokaryotic organisms. Bacterial characteristics include the ability to adhere to and invade host cells, being toxic and being able to avoid the hosts immune system. Bacteria that cause disease are pathogens.

Bacteria were one of the first life forms on earth, and even though they are microscopically small, of only a few micrometres in length, they are very good at adapting to their environment around them, leading them to inhabit most habitats on Earth. There are many different names for bacteria depending on where they inhabit, such as Physcrophile, Mesophile, Thermophile, Hyperthermophile.

Bacteria can also be classified on whether they are gram positive or gram negative, which is dictated by their cell wall composition. This can be tested by performing a gram stain procedure. Some bacteria like Mycobacterium contain large amount of lipids substances within their cell wall called mycolic acid which resist gram staining. Instead Ziehl-Neelsen stain which is also known as acid-fast stain is used to identify these organism.

Gram-Positive bacteria

Gram Positive and Gram Negative bacterial cell wall cross section

The cell wall of Gram-Positive bacteria consists of a multilayer of murein, a special type of peptidoglycan, which is significantly thicker compared to Gram-Negative peptidoglycan layer. The cell membrane is located right underneath the peptidoglycan sheets. Additionally, there are molecules called teichoic acids and lipids that form lipoteichoic acids which are situated perpendicularly along the peptidoglycan layers and can be found exclusively in Gram-Positive cell walls. Due to the high content of peptidoglycan in their cell wall, Gram-Positive bacteria possess the ability to retain crystal violet dye, found in gram stain, and thus appear purple/blue after being washed with alcohol and water[1][2].

Gram-Negative Bacteria

Alternatively, Gram-Negative bacteria have a thin, single layer of peptidoglycan which is surrounded by an external and an internal membrane. The outer membrane is a porous structure and consists of high amounts of lipopolysaccharides. There is a space located between the peptidoglycan layer and inner cell membrane called the periplasmic space. Teichoic acids or lipoteichoic acids are absent. Unlike Gram- Positive bacteria, Gram-Negative bacteria cells which have a thin peptidoglycan layer become decolourized when washed with alcohol and acetone as they lose the gram stain and appear red/pink due to counterstain Safranin or Fuchsine.


  1. Kenneth Todar, PhD. (2012). Structure and Function of Bacterial Cells. Available: Last accessed 26/11/2014.
  2. Shawn Becker. (2010). CELL WALL COMPONENTS. Available: Last accessed 27/11/2014.
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