Question

Which of these components is/are not present in Gram-negative bacteria?
(a)Teichoic acid
(b)Pseudomurein
(c)Lipopolysaccharide
(d)Both (a) and (b)

Answer 1

Hint: Bacteria are morphologically differentiated into Gram-positive and negative depending on how they stain in the Gram staining procedure. The difference in staining occurs due to different components in their cell wall and membrane structures.

Complete answer: - Gram staining is a type of differential staining that distinguishes bacteria based on the composition of their cell wall, as Gram-positive and Gram-negative.
- In this technique, the bacteria on the slide is stained first with crystal violet, the addition of a mordant (fixative), then decolorized and re-stained with a red stain, safranin. Bacteria with thick cell walls do not decolorize and retain the crystal violet dye and are termed as Gram-positive. Bacteria with thinner cell walls get decolorized and are stained by safranin, being termed as Gram-negative.
- The Gram-negative cell wall is multilayered. It is composed of a thin peptidoglycan layer above the cytoplasmic membrane. It is succeeded by the outer membrane.
- Teichoic acids are present in the cell wall of Gram-positive bacteria and linked to peptidoglycan monomers.
- Pseudomurein, or pseudopeptidoglycan, is the major component of the cell wall in Gram-positive archaea. It is chemically different but structurally similar to the peptidoglycan of Gram-positive bacteria.

Additional Information: - The peptidoglycan is composed of cross-linked chains of peptidoglycan monomers (NAG-NAM-pentapeptides). NAG is N-acetylglucosamine and NAM is N-acetylmuramic acid. The monomers are joined together by transglucosidase enzymes.
- The outer membrane of Gram-negative bacteria is a lipid bilayer composed of phospholipids, lipopolysaccharides (LPS), lipoproteins, and other proteins.
- The LPS of the membrane is composed of a lipid portion known as Lipid A embedded in the membrane and the hydrophilic polysaccharide portion extending outwards.

So, the correct answer is ‘Both (a) and (b)’.

Note:
- Examples of Gram-negative bacteria are Salmonella, Shigella, Escherichia, Neisseria, Klebsiella, etc.
- Examples of Gram-positive bacteria are Bacillus, Streptococcus, Staphylococcus, Mycobacterium (except M. tuberculosis), Clostridium, Lactobacillus, etc.
- Some bacteria such as Mycoplasma and Ureaplasma cannot be stained by this technique as they do not have a cell wall.