Question

Identify the bacterium that appears violet after gram staining.
a) Salmonella enterica
b) Escherichia coli
c) Mycobacterium tuberculosis
d) Rhizobium meliloti

Answer 1

Hint:The gram-positive bacteria appear deep purple or deep violet in colour and the gram-negative bacteria appear colourless after the final step (treatment with a de-staining agent) of the gram-staining method. Some bacteria are also there which are intermediate between gram-positive and gram-negative bacteria.

Complete answer:
A Danish bacteriologist, Hans Christian Gram devised a differential staining technique for microorganisms in 1884. The technique is called Gram-staining technique. The primary stain of this method is crystal violet, which forms the crystal violet-iodine complex after addition of iodine. The microorganisms that hold the crystal violet-iodine complex look purple under microscopic examination and are called as Gram-positive. The microorganisms that are not stained by crystal violet are referred to as Gram-negative.

Gram-staining technique is based on the ability of bacteria to retain the crystal violet stain in their cell walls. The two features that play a key role in this technique are – the thickness of the peptidoglycan layer and presence or absence of the outer lipid membrane in the bacterial cell wall.
In this staining method the bacterial smear (thin film of bacterial culture) is stained by crystal violet. Addition of iodine solution makes the smear appear deep purple in colour. This step is followed by the treatment with a de-staining agent (alcohol). Since gram-negative bacteria have relatively high lipid content in their cell walls. The de-staining agent dissolves the lipid layer, which allows the leakage of crystal violet-iodine complex. The gram-positive bacteria with less lipid content in their cell walls are less susceptible to the action of de-staining agents. Also, the peptidoglycan present in high concentration in the cell walls of gram-positive bacteria traps the crystal violet-iodine complex in their many cross-linkages, thus they appear deep violet in colour. The gram-negative bacteria having less peptidoglycan and fewer cross-linkages loose the stain readily, and appear colourless.
Salmonella enterica, Escherichia coli and Rhizobium meliloti are gram-negative bacteria, thus, they appear colourless after gram-staining.
Mycobacterium tuberculosis is a gram-variable bacteria. It takes gram-stain unevenly because it has the characters of gram-positive as well as gram-negative bacteria.

Thus, the correct answer is C, i.e., Mycobacterium tuberculosis.
Note:Mycobacterium tuberculosis has a waxy coating on its cell surface that consists of mycolic acid. This waxy coating makes the cell impenetrable to gram-staining. That is why the bacterium does not respond in the same manner to the gram-staining. Such bacteria are called gram-variable bacteria.