Gram Stain

Background info: A gram stain is one of the first diagnostic tests done when a swab from a patient arrives in the lab. It gives valuable information about the type of bacteria that is infecting the patient. The gram stain differentiates between bacteria with two different types of cell walls - gram-positive and gram-negative.

What is the purpose of a gram stain? (select all that apply)

• To distinguish between gram-positive and gram-negative bacteria.

• To make bacteria into an art project.

• To turn bacteria into gram-positive and gram-negative organisms.

• To help identify what type of infection a patient has.

Match the following reagents with their function:

• Counterstain

• Iodine

• Mordant

• Ethanol

• Decolorizer

• Crystal Violet

• Fixative

• Safranin

• Primary Dye

• Methanol

Label the following bacteria, based on their cell wall composition:​

a

b

Gram Negative

Gram Positive

What would cause a gram-negative bacteria to appear purple?

• Not using a decolorizer

• A correctly performed gram-stain

• Forgetting to use crystal violet

A ​ (a)   bacteria has a thick layer of peptidoglycan in its cell wall. A ​ (b)   bacteria has a thin layer.

A gram-negative bacteria will also have a second ​ (c)   .

• gram-positive
• gram-negative
• membrane
• nucleus
• infectious
• wall