Question

1.Explain characteristics and tests used by microbiologists to assign bacteria to different taxonomic groups.

2.Some prokaryotes play essential roles in the ecology of the biosphere. Give examples and describe their impacts.

Answer 1

There are many tests to characterize a microorganism which can be divided by possession or lack of certain structures such as peptidoglycan cell wall, spores, capsules among others. Other tests are based on the degradation capacity of different biomolecules such as sugars, proteins, lipids, etc. There are also tests that are based on the enzymatic capacity of the microorganism. Below I will write in more detail the most used tests for the taxomomic identification of microorganisms.

Microscopic morphology
By means of the morphology that the bacteria have, we can give an idea of which taxonomic group they belong to. They can be cocci, bacilli or spirochetes, it is necessary for a microscope to observe the form that each bacterium has.

Gram stain

This is a stain developed by Hans Christian Gram, which is a differential stain because it allows to identify bacteria that have a peptidoglin cell wall from those that do not. This stain requires a primary dye (Crystal violet) and a mordant (iodine) to fix the primary dye. At this point all the bacteria are stained, followed by a discoloration step (in this step the bacteria that do not have a wall lose their color), another coloration is made with the secondary or contrast dye. Those bacteria that were stained with the primary dye are identified as Gram positive while those that were stained with the secondary dye are Gram -. This is one of the most important taxonomic criteria because many times they ask if a bacterium is Gram positive or Gram negative

Zhiehl-Neelsen stain.

This stain allows to see cell walls formed of mycolic acids (Mycobacterium genus) and Nocardic acids (Nocardia Genus): In this stain uses fenicada fuchsin which when added to the preparation is heated to emission of vapors in such a way that increases the kinetic of the dye and permeate through those walls. A decolorization is made with acid alcohol. If they retain the dye it is classified as positive or alcohol-resistant acid. This staining is important for the classification of bacteria related to tuberculosis

Presence of catalase.
This is a simple test in which it is evidenced if an organism has this enzyme or not (catalase). A drop of oxygenated water is added to a pure culture bacterial colony. If they possess this enzyme, they will degrade oxygenated water (H2O2) to water (H2O) and molecular oxygen. This test tells us indirectly how tolerant are the bacteria to oxygen, because not all support it and die in contact with it.

Growth in anaerobic conditions.
For this, special systems that generate conditions of total absence of oxygen are required. If the bacteria grow in this jar, they can be classified as strict anaerobic or aerotolerant (that is, they do not require oxygen, but if they do not, they do not die instantaneously).

In the degradation of biomolecules

The Durham fermentation in which the medium has an inverted hood which traps fermentation gas (if it is generated) and allows us to demonstrate if they are gas-forming.
The Methyl Red tests.
In this test the medium contains glucose depending on the fermentation products (lactic acid, acetic acid, ethanol, CO2) is how the indicator will turn at pH lower than 4.4 the indicator turns to a deep red (positive result). if the pH rises to more than 6 it generates a yellow coloration (which gives a negative result).

There are many more tests such as casein fermentation, hemolysis, organic acid utilization, Vogues-Proskauer test, degradation of amino acids with Möeller-Hinton media, ensopores stain, identification by antibodies among others. however, the ones I described above are the most important and easy to do in a laboratory.