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Veterinary bacteriology: information about important bacteria
Veterinary bacteriology

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Gram staining


Gram staining is a so called differential staining techniques, since one can distinguish two major groups of bacteria by this method. These two groups are gram positive and gram negative bacteria, which are stained purple and pink to red, respectively.


Gram-positive bacteria have a thick cell wall (peptidoglycan), which consists of several layers and can be likened to a network. Gram negative bacteria have a much thinner cell wall and also an outer membrane. Crystal violet (CV+), which is the primary dye binds to the negatively charged groups on the bacteria and stain them purple. Then iodine (I-) will be used to form a large complex (CV-I) with CV and thereby bind the stain to the bacterium. When Gram-positive bacteria are treated with the decolourizing solution (ethanol-acetone), the bacteria will be dehydrated and the colour retained. When Gram-negative bacteria are treated with the decolourizing solution the outer membrane will be dissolved and the thin peptidoglycan exposed, so that the CV-I complex is washed out. Then a counterstaining with safranin or basic fuchsin is performed to stain gram negative bacteria pink or red.


  1. Divide the slide with the help of the diamond pen.
  2. Disperse some colony material in a drop of NaCl, air dry (in the incubator, if necessary).
  3. Fix the specimen by moving the slide, with the preparation side up, 6-8 times through the burner flame.
  4. Add chrystal violet, wait for 1 minute.
  5. Flood gently with Lugol's solution.
  6. Add Lugol's solution (which contains iodine), wait for 1 minute.
  7. Flood gently with acetone-ethanol solution.
  8. Flood gently with water.
  9. Add safranin, wait for 20 seconds.
  10. Flood gently with water.
  11. Remove the excess of fluid with a paper towel and allow to air dry until the specimen is completely dry.

Gram-positiva bacteria

Members of the phyla Bacillota and Actinomycetota (exception: genus Mycobacterium).

Gram-negativa bacteria

Members of the phyla Proteobacteria (exception: some members of the order Rickettsiales), Cyanobacteria and Spirochaetota.

Updated: 2023-03-08.


The taxonomy of chlamydias

Species within the family Chlamydiaceae were previously divided into two genera Chlamydia and Chlamydophila. However, the differences between these two genera were not that great and many research groups have not accepted this division. Therefore, the genus Chlamydophila has been returned to the genus Chlamydia and this change has now been incorporated in VetBact

Published 2023-03-15. Read more...
New names of bacterial phyla

The taxonomic category phylum was previously not regulated by the International Code of Nomenclature of Prokaryotes (ICNP), but now this has changed and it was decided to revise the names of bacterial phyla. All phyla must be written in italics (which has been done on VetBact also before) and have the ending -ota.

Published 2023-03-01. Read more...

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