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Bacterial classification


The simplest classification system of bacteria is based on their shape and arrangement. Bacteria come in sphere, rod, spiral, comma and filament shapes, and can be paired up in twos, strings or 3D shapes. Using the diagram, can you figure out what Staphylococcus aureus, Vibrio cholerae and Streptococcus sp. look like?

Gram test

Amongst bacteria, the cell wall composition is a key determinant of what type they belong to. This is important in terms of predicting their response to various antibiotics. Based on different bacteria species’ response to crystal violet stain, Gram positive bacteria are able to take up the stain and appear violet under a microscope, while Gram negative bacteria do not take the stain up and will appear pink if a counterstain is added after washing off the crystal violet stain (this will persist in the Gram positive bacteria).

Iodine is added after the stain in order to bind to it and trap it inside the bacteria. Acetone is used in the decolorisation step. Any free stain is washed off. In gram positive bacteria, the stain persists. The difference arises because different bacteria have different cell walls. The bacterial cell wall is one of the main targets of antibiotics.


Notice the difference in thickness of the murein layer in gram positive versus gram negative cells. This layer is what absorbs the violet stain. Hence gram positive bacteria turn violet, while gram negative bacteria lose the stain upon washing.

Penicillin is an antibiotic used against gram positive bacteria. It doesn’t work on gram negative bacteria because their outer membrane (cell envelope) protects against it. Penicillin works by interfering with the production of the cell wall component murein, and as gram positive bacteria have so much of it and at the outer surface, losing it kills them off. Gram negative bacteria have much less murein and an outer membrane, so penicillin doesn’t interfere with their function.

There are many different classes of antibiotics, some of which do work against both types of bacteria, for example by interfering with DNA synthesis.





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