16 Which of the following are generally true about Gram nega

16. Which of the following are generally true about Gram negative cell envelope? a. Lipopolysaccharide is covalently attached to the N-acetylmuramic acid. b. There is periplasmic space. c. Peptidoglycan is linked with B-1,3-glycosidic bonds. d. Interbridge strengthens the peptidoglycan layer.

Solution

The cell envelope of Gram-negative bacteria has three principal layers: the outer membrane (OM), the peptidoglycan cell wall, and the cytoplasmic or inner membrane (IM).

The OM is a lipid bilayer, not a phospholipid bilayer. It contains lipopolysaccharides, lipoproteins, proteins and phospholipids. The outer leaflet of the OM is composed of glycolipids, predominantly lipopolysaccharide (LPS).

Peptidoglycan cell wall:. The gram-negative bacteria has a thin (2-7 nm) peptidoglycan layer in their cell wall. Peptidoglycan is a polymer made up of repeating units of the disaccharide N-acetyl glucosamine (NAG) and -N-actyl muramic acid (NAM), cross-linked by -1,4 glycosidic bond. NAM is NAG with a lactic acid linked through ether group. A tetrapeptide chain of four alternating D- and L- aminoacids (L -alanine, D-alanine, D-glutamic acid and L-lysine/ meso diamino-pimelic acid) is connected to the carboxyl group of the NAM. The two tetrapeptide chains lying side by side may be linked together directly to each other or indirectly by a peptide interbridge, of short chain aminoacids through transpeptidation. In addition to the peptidoglycan layer the gram-negative cell wall also contains an additional outer membrane composed by phospholipids and lipopolysaccharides.

The OM is covalently joined to the underlying peptidoglycan by the abundant lipoprotein, Lpp/murein lipoprotein, or Braun\'s lipoprotein. In addition, proteins such as OmpA bind with peptidoglycan noncovalently.

The OM and IM define an aqueous cellular compartment/space called the periplasm with densely packed proteins such as RNAse , alkaline phosphatase, periplasmic binding proteins (for sugar and amino acid transport and chemotaxis).

The IM is a phospholipid bilayer, comprising mainly phosphatidyl ethanolamine and phosphatidyl glycerol. The proteins that function in energy production, lipid biosynthesis, protein secretion, and transport are located in the IM.

So based on the above inference, the true statements are:

b. There is periplasmic space

d. Interbridge strenghtens the peptidoglycan layer.