Identify the bacterium that appears violet during Gram staining.

The correct option is A Mycobacterium tuberculosis
Gram staining technique was developed by Christian Gram to differentiate between gram positive and gram negative bacteria.

• In this technique a small amount of culture is taken on the slide. The bacteria in the slide is appropriately stained using the primary stain (crystal violet), mordant (iodine), decolourised with alcohol and then counterstain (safranin) is used.
• Some bacteria will have a thick layer of peptidoglycan and low lipid content in their cell wall.
• They will take up the primary stain-mordant complex (crystal violet-iodine).
• Decolourisation with alcohol will lead to this thick wall shrinking and the closure of the pores in the cell wall.
• The crystal violet is retained and the bacteria is stained violet.
• These bacteria are called gram positive bacteria. For example Mycobacterium tuberculosis
• Some bacteria will have a thin peptidoglycan layer and more lipids.
• As a result the cell wall will dissolve when alcohol is used and the crystal violet will come out.
• They will then take up the counterstain (safranin) staining pink. These bacteria are called gram negative.
• For example Salmonella enterica, Escherichia coli, Rhizobium meliloti.