1 The Cell Envelope External covering outside the cytoplasm Composed of two basic layers: –Cell wall and cell membrane Maintains cell integrity 8/18/12Mickey Dufilho

2 Insert figure 4.12 Comparative cell envelopes 8/18/12Mickey 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 8/18/12Mickey Dufilho

Peptidoglycan 48/18/12Mickey 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 8/18/12Mickey Dufilho

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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 8/18/12Mickey Dufilho

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Comparison of Gram-Positive and Gram-Negative 98/18/12Mickey 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 8/18/12Mickey Dufilho

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