Which of the given statements are correct? (i) Bacillus subtilis is a Gram-positive bacteria (ii) Escherichia coli is a Gram-negative bacteria (iii) Washing of the Gram’s stain in gram-negative bacteria is due to the high lipid content of the cell wall, which gets dissolved in organic solvents like acetone. A. (i) and (ii) B. (ii) and (iii) C. (i) and (iii) D. (i),(ii) and (iii)
Hint: Gram staining is a special technique, which is used to classify bacteria into two groups, viz. Gram-positive and Gram-negative bacteria. Those bacteria that take up the Gram stain are Gram-positive and the others that do not are called Gram-negative bacteria.
Complete answer: First let the bacterial smear dry, after drying stained the smear with crystal violet/gentian violet for thirty seconds. After staining bacteria get bluish-purple stain then washed with water, after this again stain the smear with iodine (0.5 percent solution), this results in the formation of Crystal Violet Iodine (CVI) complex. Iodine works as a mordant. Now treatment with alcohol if the bacteria retain the stain then it is Gram-positive bacteria but if it does not retain stain then it is Gram-negative bacteria. Bacillus subtilis retains its stain after treatment with alcohol so it is Gram-positive but Escherichia coli (E.coli) does not retain its stain after treatment with alcohol so it is Gram-negative bacteria. Washing of stain in Gram-negative bacteria is due to high lipid content in the bacteria wall. The lipid content gets dissolved in the alcohol solution or any organic solvent like acetone.
So the correct answer is option (D).
Note: In gram-positive bacteria the outer membrane is absent. The wall of gram-negative bacteria is smooth. Murein content is 70-80 percent in Gram-positive bacteria. In Gram-negative bacteria the outer membrane is present. The wall is wavy and comes in contact with plasmalemma only at a few loci. Murein content is only 10-20 percent.