Basics of Microbiology

Bacteria Classification by Shape

There are three main shapes of bacteria. These are: - Bacillus, which is rod shaped. - Cocci, which is spherical shaped. - Spiral or known as corkscrew shaped.

Gram Staining

Gram staining allows microbiologists to determine the gram-negative and gram-positive bacteria apart. The stains differ due to the cell walls containing different compositions. All bacteria are colourless before staining. After the procedure of staining, the Gram-positive bacteria get stained a violet colour while Gram-negative will turn the colour red. This procedure includes a process of counterstaining. Bacterium’s cell wall consists of molecules called polysaccharides and polypeptides forming compounds known as peptidoglycans. Within the structure, there are cross-linkages, which provides strength giving the cell its shape. Gram-positive bacteria possess this basic cell structure while gram-negative also possess this structure but with an added layer called lipopolysaccharide.

Gram Positive Bacteria

Once this type of bacteria has been stained, it retains a violet or purple colour under the microscope. This type of bacteria is more susceptible to antibiotic, lysosome enzyme and penicillin than that of Gram-positive bacteria.

Penicillin interferes with the peptide linking of the peptidoglycan. A cell without intact peptidoglycan cross-links are generally weak which are then prone to collapse and disintegrate when the bacteria attempts to divide. Since eukaryote cells have no cell wall, it is physically impossible to be damaged by penicillin.

Gram Negative Bacteria

The cell walls are more chemically complex. This is due to the peptidoglycan is supplemented by a layer of lipopolysaccharide which has a function to protect the cell. It is impossible for the cell wall to retain the dyes such as crystal violet but instead it retains a red or pink colour when viewed under the microscope. This type of bacteria is enzyme and penicillin resistant.


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