Question

Identify the bacterium that appears violet after Gram staining.

• A. Salmonella enterica
• B. Escherichia coli
• C. Mycobacterium tuberculosis
• D. Rhizobium meliloti

Answer 1

The correct option is C Mycobacterium tuberculosis

Gram staining includes staining of both gram positive and gram negative bacterial cell purple with crystal violet followed by stabilization of dye in peptidoglyacan layer with mordant Gram’s iodine and application of alcohol to dissolve outer membrane and dye from peptidoglycan layer. Presence of thick peptidoglycan layer in gram positive cells make them retain the dye after alcohol treatment, while gram negative bacteria become colorless.

Salmonella enteric, Escherichia coli and Rhizobium meliloti are Gram negative bacteria and remain colorless with Gram stain, which makes options A, B and D incorrect.

Mycobacterium tuberculosis is Gram indeterminate bacteria and stains unevenly due to presence of characters of both Gram positive and Gram negative bacteria. This makes option C correct.