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Microbiology Lec 5

Microbiology Lecture 5

QuestionAnswer
Prokaryotic Cell (Cell Envelop) - Cell envelope: outer covering of the cell - Consists of: semi-permeable cell membrane, rigid cell wall -In Gram negative bacteria, extra layer outer membrane
Plasma Membrane (structure, function) Regulate the passage of molecules and ions. In bacteria that lack cell walls it provides rigidity (Mycoplasma) Structure: 60-70% protein, 40-30% lipids. In bacteria, lipids have ESTER linkages b/n fatty acids and glycerol. Archaea ether linkage,
Plasma Membrane (Structure contd) - possess a negative charge. A molecule's passage through it depends on lipid solubility. Transport systems in bacterial cells are important. Certain large molecules are degraded by hydrolytic enzymes first before they are transported.
Cell Wall -specific cell shapes of certain bacteria is due to their cell walls, ie. cocci, rods. -barrier to the environment, survive in environments. Consist of polysaccharides linked by chemical bridges to form a rigid structure. - Bacteria have peptidoglycan
Cell Wall Contd. Peptidoglycan consists of repeating units of 2 carbohydrate derivatives. NAG and NAM. Both are linked to each other via beta 1,4 linkages.
NAM - Only found in the peptidoglycan and no where else in nature - Attached to the NAM are 4 amino acids a tetrapeptide. NAM is not found in Archaea
Peptidoglycan -Mechanical strength is due to the beta 1,4 linkages between NAM and NAG. Major strength comes from the cross linkages b/n 3rd and 4th amino acids of adjacent tetrapeptides.
Cell wall of Gram + bacteria Tetrapeptides are linked via a pentaglycine bridge. 75% of the tetrapeptides are cross linked. 90% of cell wall is peptidoglycan. The remainder is teichoic acids. These are repeating ribitol or glycerol units linked via phosphodiester linkages
Cell wall of Gram - bacteria -has cross linking between the D-ala and meso diaminopimelate tetrapeptides threfore cross linking occurs at a 25% frequency. Thinner peptidoglycan layer, representing 5-20% of the cell envelope.
S- Layer additional protective layer seen in Gram + bacteria
Penicillin -mode of action is to inhibit bacterial cell wall synthesis. It inhibits the transpeptidation step in cell wall synthesis. Non-growing cells are not impacted by penicillin nor cells that lack cell walls such as mycoplasma.
Outer membrane Part of the cell wall in Gram - bact. Consists of a phospholipid bilayer interspersed with proteins and lipoproteins. Lipoproteins anchor to peptidoglycan layer. LPS molecules are arranged w/in the external phospholipid layer and make up 40% cell surface
LPS (endotoxin) three components: 1.) O-specific side chain [O antigen, polysaccharide, up to 2200 variants in Salmonella]; 2.) core polysaccharide of constant composition; 3.)lipid A protein, glycophospholipid, toxic portion of the Gram - bacteria.
LPS (pyrogen) Lipid A, is responsible for fever/shock seen with Gram negative infections
Gram Stain the cell wall differences are responsible for the different color reactions
Bacteria w/o Cell Walls Derived chemically with treatment of lysozyme, hydrolyzes beta 1,4 linkages b/n NAG and NAM. The cell in sucrose is called a protoplast. L forms, bacteria w/o cell walls when cells are grown in presence of penicillin Mycoplasma causes atypical pneum
Periplasm Gap that separates the plasma membrane from the outer membrane. Many hydrolytic enzymes are located in this space. Seen ONLY in Gram - bacteria, since Gram + bacteria lack the outer membrane 50 diff enzymes, protein binding occurs here via chaperones
Capsules Additional layer external to the cell, found on pathogenic bacteria, protects against ingestion of phagocytes. Composed of polysaccharide or glycoprotein. Demonstrated by staining or on medium. Dextrans (Gram + bacteria, dental plaque)& food thickeners
Created by: clewis3
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