Q.(a) List out the differences between Gram positive and Gram negative bacteria. OR

Gram positive bacteria have a thick, single-layered peptidoglycan wall and stain purple; Gram negative bacteria have a thin peptidoglycan layer plus an outer lipid membrane, and stain pink/red.

The Gram staining technique, devised by Hans Christian Gram, differentiates bacteria into two major groups based on differences in their cell wall structure and the resulting ability (or inability) to retain a crystal violet-iodine dye complex when treated with alcohol/acetone as a decolouriser. The key differences between the two groups are:

Cell wall structure: Gram positive bacteria have a relatively thick, multi-layered peptidoglycan (murein) wall directly external to the plasma membrane, without an outer membrane. Gram negative bacteria have only a thin, single-layered peptidoglycan wall, but this is surrounded by an ADDITIONAL outer membrane containing lipopolysaccharides (LPS) and lipoproteins.

Staining reaction: In Gram positive bacteria, the thick peptidoglycan layer traps and retains the crystal violet-iodine complex even after alcohol treatment, so these bacteria appear purple/violet under the microscope. In Gram negative bacteria, the alcohol/acetone treatment dissolves the outer lipid membrane and washes the crystal violet-iodine complex out of the thin peptidoglycan layer, so these bacteria lose the purple stain and instead take up the pink/red counterstain (safranin) applied afterward, appearing pink/red under the microscope.

…