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What is the morphology of the bacterial cells Corynebacterium diptheriae and Staphylococcus aureus? What is the relevance of the Gram staining procedure as a diagnostic tool?

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The morphology of a bacteria refers to the external characteristics of the bacteria. Most commonly this refers to its size, its shape, and how its components are arranged. Let’s look at each individually.

Corynebacterium diphtheriae

Shape: Rod-shaped (can be straight or slightly curved)

Size: 1-8 micrometers in length

Arrangement: The rods tend to group together in a “V” or “Y” shape.

Staphylococcus aureus

Shape: Spherical

Size: 0.6-1 micrometers in diameter

Arrangement: The spheres tend to group together in clusters.


The best way to observe the morphology of a cell is to apply a stain/dye, which is absorbed by the bacteria and allows its characteristics to be differentiated when viewing it under a microscope. The Gram stain has become a valuable method of differential staining. When a bacteria undergoes a Gram stain, it’s treated with a crystal violet stain, and it either comes out Gram positive or Gram negative, depending on the type of cell wall it has. (Gram positive cells have a thick peptidoglycan cell wall which retains the crystal violet dye; Gram negative cells have thinner peptidoglycan walls, so the stain is easily washed out.) Both of the above examples are Gram positive.

What is the benefit of differentiating cells according to whether they’re Gram positive or negative? It’s not really a comprehensive diagnostic tool – we have tests that allow us to more accurately identify a bacterial organism. But the diagnostic benefit of a Gram stain is that it quickly narrows down the unknown bacteria. Because there are certain antibiotics that work for Gram positive bacteria, and certain ones that work for Gram negative, a Gram stain allows an immediate medical response – which is valuable, because it’s crucial to begin antibiotics as soon as possible. For example: a bacteria can be sent out to be identified, and is also treated with a Gram stain. Before the identification results come back, we can see that the bacteria is Gram positive. Therefore, the doctor will be able to immediately prescribe vancomycin (an antibiotic against Gram positive bacteria).

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