1

The Cell Envelope

• External covering outside the cytoplasm

• Composed of two basic layers:

– Cell wall and cell membrane

• Maintains cell integrity

8/18/12 Mickey Dufilho

2

Insert figure 4.12 Comparative cell envelopes

8/18/12 Mickey Dufilho

3

Structure of Cell Walls

• Determines cell shape, prevents lysis

(bursting) or collapsing due to changing

osmotic pressures

• Peptidoglycan is primary component:

– Unique macromolecule composed of a

repeating framework of long glycan chains

cross-linked by short peptide fragments

8/18/12 Mickey Dufilho

Peptidoglycan

4 8/18/12 Mickey Dufilho

5

Gram-Positive Cell Wall

• Thick, homogeneous sheath of peptidoglycan

– 20-80 nm thick

– Includes teichoic acid and lipoteichoic acid:

– Some cells have a periplasmic space, between the

cell membrane and cell wall

8/18/12 Mickey Dufilho

6 8/18/12 Mickey Dufilho

7

Gram-Negative Cell Wall

• Composed of an outer membrane and a thin peptidoglycan layer

• Outer membrane is similar to cell membrane bilayer

structure

– Outermost layer contains lipopolysaccharides and lipoproteins (LPS)

• Lipid portion (endotoxin) may become toxic when released during infections

• May function as receptors and blocking immune response

• Contain porin proteins in upper layer – regulate molecules entering and leaving cell

– Bottom layer is a thin sheet of peptidoglycan

• Periplasmic space above and below peptidoglycan

8/18/12 Mickey Dufilho

8

8/18/12 Mickey Dufilho

Comparison of Gram-Positive and Gram-Negative

9 8/18/12 Mickey Dufilho

10

The Gram Stain

• Differential stain that distinguishes cells with a gram-positive cell wall from those with a gram-negative cell wall

– Gram-positive - retain crystal violet and stain purple

– Gram-negative - lose crystal violet and stain red from safranin counterstain

• Important basis of bacterial classification and identification

• Practical aid in diagnosing infection and guiding drug treatment

8/18/12 Mickey Dufilho

11 8/18/12 Mickey Dufilho

8/18/12 Mickey Dufilho 12