Introductory Microbiology

Outer membrane

• The peptidoglycan of the gram -ve bacteria displays a rather low degree of cross linkages within the glycan strands. The fine structure of the outer membrane, like the cell membrane, consists of a lipids bilayer in which both phospholipids and lipopolysaccharides are present. The peptidoglycan of the wall has specific types of lipoproteins on its outer surface, which are linked by peptide bonds to some of the Dipicolonic acid residues in the peptidoglycan. The lipoproteins thus serves as the bridge from the peptidoglycan to the outer wall layer.
• The lipopolysaccharides form the major component of the outer membrane and is are extremely complex molecule varying in chemical composition. The lipopolysaccharides are the major antigenic determinants and also the receptors for adsorption of many bacteriophages. The outer membrane of gram -ve bacteria acts as a barrier to the passage of substances such as antibiotics, dyes and bile salts into the cell.

  Sl.No.	Characters	Gram positive	Gram negative
  1.	Grams staining	Retain crystal violet and appear dark violet.	Pass crystal violet and counter stained by safranine and appear red
  2.	Outer membrane	Absent	Present
  3.	Peptidoglycan	Several layers (thick)	Single layer (thin)
  4.	Lipid and lipo-proteins	Low	High
  5.	Lipopolysaccharides	Absent	High
  6	Techoic acid	Mostly present	Absent
  7	Periplasmic space	Absent	Present
  8	Flagella	2 rings at basal body	4 rings
  9	Toxin production	Exotoxin	Endotoxin
  10	Strength	High	Low
  11	Resistance to drying	High	Low
  12.	Susceptibility to streptomycin, tetracycline and chloramphenicol	Low	High

  Table: Differences between cell walls of gram positive and gram negative bacteria
• The lipid found are mostly phosphatidyl ethanolamine and to a small extent phosphatidyl-serine and phosphatidyl-choline.

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