Bacterial Cell Structure: Cell Membranes PDF

Bacterial Cell Structure 1: The cell envelope Readings Section 2.1-2.5 pp 39-51; Section 8.5 pp 243-245 Figure 2.2 The Cytoplasmic Membrane Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Bacterial cytoplasmic membrane General structure i...

Bacterial Cell Structure 1: The cell envelope Readings Section 2.1-2.5 pp 39-51; Section 8.5 pp 243-245 Figure 2.2 The Cytoplasmic Membrane Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Bacterial cytoplasmic membrane General structure is phospholipid bilayer containing embedded proteins Contain both hydrophobic (water-repelling) and hydrophilic (water-attracting) components hydrophobic = fatty acids hydrophilic = glycerol + phosphate and another functional group (e.g., sugars, ethanolamine, choline) Fatty acids point inward to form hydrophobic environment; hydrophilic portions remain exposed to external environment or the cytoplasm Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.1 Phospholipid Bilayer Membrane Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Bacterial cytoplasmic membrane Semi-permeable- allows nutrients in and wastes out Nonpolar molecules and small weakly polar molecules (e.g. H2O, ethanol, nonpolar solvents, CH4, O2, NH3) will pass through membranes Membranes do not allow the passage of ions, sugars or amino acids (large or strongly polar molecules). These must be transported. Has the fluidity of a light oil (Fluid mosiac) Fatty acids usually 12-20 C atoms long, most C-C bonds are single but a few are double (called an unsaturation) Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Adaptations of thermophiles: Lipid membranes IMAGE MDougM Wikipedia, Public domain Archaeal membranes –Ether linkages in membrane lipids of Archaea –Bacteria and Eukarya have ester linkages in phospholipids –Archaeal lipids lack fatty acids; have isoprenes instead –Major lipids are glycerol diethers with phytanyl C20 side chains and diglycerol tetraethers with biphytanyl C40 side chains, which can form lipid monolayers. –Archaeal membranes exist as lipid monolayers, bilayers, or a mixture of both Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.3 Major Lipids of Archaea and the Architecture of Archaeal Membranes Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Adaptations of thermophiles: Archaeal membranes Ether linkages are more thermostable than ester linkages Archaeal lipid membranes can exist as a monolayer Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.4 The Major Functions of the Cytoplasmic Membrane Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Membrane proteins Types: – embedded proteins: integral membrane proteins – transmembrane proteins: extend completely across membrane – peripheral membrane proteins: loosely attached Major functions: – Transport (Nutrient uptake, Osmotic balance, Protein secretion) – Environmental sensing – Electron transport, Respiration – Membrane and cell wall assembly Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Active transport – how cells accumulate solutes against a concentration gradient Transporters – three mechanisms simple transport: transmembrane transport protein group translocation: series of proteins ABC system: three components (binding protein, transmembrane transporter, ATP- hydrolyzing protein) – energy-driven (proton motive force, ATP, or another energy-rich compound) Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.6 The Three Classes of Transport Systems Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved The Cell Envelope Cell envelopes are composed of – Cell membranes (1 or 2) – Cell wall – S-layers There are two main cell envelope arrangements in bacteria – Gram negative: Cytoplasmic membrane, thin cell wall, periplasm, outer membrane – Gram positive: Cytoplasmic membrane + thick cell wall Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.7a-b Cell Envelopes of Bacteria Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Most bacteria are Gram - Firmicutes, Actinobacteria, and some Chloroflexi are Gram + Figure 13.9 Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved ducation, Inc. All Rights Reserved The Cell Wall Cell walls: – Maintains cell shape and rigidity – Need to withstand osmotic/turgor pressure to prevent cell lysis Bacterial cell walls contain peptidoglycan – Peptidoglycan: rigid polysaccharide layer that provides strength – Not found in Archaea or Eukarya – Glycan tetrapeptide contains Sugar backbone (alternating modified glucose (N-acetylglucosamine and N-acetylmuramic acid) joined by β-1,4 linkages Short peptide attached to N-acetylmuramic acid – Amino acids vary between species – L-alanine, D-alanine, D-glutamic acid, and either L-lysine or diaminopimelic acid (DAP) Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.8 Structure of the Glycan Tetrapeptide, in a typical Gram-negative bacterium Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved  Bacterial Cell Walls – Peptidoglycan strands run parallel around cell circumference – Cross-linked by covalent peptide bonds – Gram-negative crosslinks between DAP and D- alanine carboxyl on adjacent glycan strands – Gram-positive crosslinks often contain peptide interbridges (e.g., five glycines in Staphylococcus aureus) Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.9 Peptidoglycan Structure in the Cell Wall Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.10 Gram-Positive Cell Wall Gram-positive cell walls can contain up to 90% peptidoglycan, 15 or more layers thick (Gram negatives often have only a single layer) Contain teichoic acids Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.10b: Teichoic Acid Acidic, negatively charged Found in certain Gram + Not been found in Gram – Strongly antigenic Covalently bound to the peptidoglycan Lipoteichoic acids: teichoic acids also covalently bound to membrane lipids Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.10c Structure of the Gram-Positive Bacterial Cell Wall  Functions of teichoic acid (G+) Maintain porosity of cell wall Anchor cell wall to cell membrane Help maintain cell shape Capture essential cations (eg. Ca2+, Mg2+) Can be a reservoir of phosphate Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.12 The Gram-Negative Bacterial Cell Envelope Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved  Features of the G- cell envelope space located between cytoplasmic and outer Periplasm: membranes ~15 nm wide Houses many proteins Porins: channels for movement of hydrophilic low-molecular-weight substances through the OM Lipopolysaccharide -LPS instead of phospholipids in the outer half of the outer membrane --Facilitates surface recognition, important virulence factors, and add strength -polysaccharides covalently bound to lipids Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.13 Structure of Bacterial Lipopolysaccharide Lipid A is an endotoxin - can cause endotoxic shock... Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Figure 2.11 Pseudomurein found in Archaeal cell walls Figure 8.15 Bactoprenol (Undecaprenol Diphosphate) Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Steps in Peptidoglycan Synthesis 1. Autolysins break the b1-4 bond between M-G sugars in the peptidoglycan chains 2. Assembly of Lipid II, which is transported across the membrane via flippase 3. Transglycosylases insert the precursors into the broken peptidoglycan chain (inhibited by vancomycin) 4. Transpeptidation (inhibited by penicillin and vancomycin) Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved Agents that destroy peptidoglycan Lysozyme –Found in tears and egg whites –Hydrolyses peptidoglycan by breaking the b1-4 bond between M-G sugars Antibiotics –e.g. Vancomycin, Beta-lactam antibiotics (penicillin, etc.) –Inhibit formation and/or cross-linking of the glycan strands Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved -Bacteria quickly figure out how to outsmart antibiotics via enzymatic degradation (e.g. Beta- lactamases) and membrane alterations -Beta-lactam antibiotics are a large family containing penicillin, ampicillin, cephalosporin, and derivatives – the most widely used antibiotics -Beta lactamase is a common component of antibiotic resistance plasmids (R-plasmids) that Penicillin are increasing in clinical settings --Before 1946, 10% of Staphylococcus aureus isolates were resistant to penicillin, 75% resistant by 1952 , now >90% (NEJM 337:491) --until the 1990s, no antibiotic-resistant Salmonella typhimurium (typhoid fever) strains were known, Beta lactam ring now 78% of strains are resistant (WHO) --Streptococcus pneumoniae before 1990s no known resistance to penicillin, now widespread (WHO) --Practically all major pathogens now resistant Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved