Bacterial Cytoplasmic Membrane Quiz

Bacterial Cell Structure: The Cell Envelope

• The cell envelope is composed of cell membranes (one or two), a cell wall, and S-layers.
• Bacteria have two main cell envelope arrangements: Gram-negative and Gram-positive.
• Gram-negative bacteria have a cytoplasmic membrane, thin cell wall, periplasm, and outer membrane.
• Gram-positive bacteria have a cytoplasmic membrane and a thick cell wall.

Gram-positive vs Gram-negative

• Gram-positive bacteria possess a thick peptidoglycan layer outside of the plasma membrane, as well as cytoplasm and a membrane
• Gram-negative bacteria possess a periplasm, cytoplasm, cytoplasmic membrane, outer membrane, and a thin peptidoglycan layer between the cytoplasmic and outer membranes

Bacterial Cytoplasmic Membrane

• The general structure of the bacterial cytoplasmic membrane is a phospholipid bilayer that contains embedded proteins and is 6-8 nm thick.
• The cytoplasmic membrane contains both hydrophobic (water-repelling) and hydrophilic (water-attracting) components.
• Hydrophobic components are fatty acids.
• Hydrophilic components are glycerol, phosphate, and another functional group such as sugars, ethanolamine, or choline.
• Fatty acids point inward to form a hydrophobic environment, while hydrophilic portions remain exposed to the external environment or cytoplasm.
• The bacterial cytoplasmic membrane is semi-permeable, allowing nutrients in and wastes out.
• Nonpolar molecules and small, weakly polar molecules like H2O, ethanol, nonpolar solvents, CH4, O2, and NH3 can pass through the membrane.
• Membranes do not allow the passage of ions, sugars, or amino acids (large or strongly polar molecules), which must be transported.
• The membrane has the fluidity of a light oil.
• Fatty acids are usually 12-20 carbon atoms long, with most C-C bonds being single, but a few being double (called unsaturation).

Cytoplasmic Membrane Functions

• It is a permeability barrier that prevents leakage and it also functions as a gateway for transport of nutrients into and wastes out of the cell
• It is a protein anchor and the site of proteins that participate in transport, bioenergetics, and chemotaxis
• It functions for energy conservation and is the site of generation and dissipation of the proton motive force

Membrane Proteins

• Types of membrane proteins consist of:
• Integral membrane proteins that are embedded.
• Transmembrane proteins that extend completely across the membrane.
• Peripheral membrane proteins that are loosely attached.
• Major functions of membrane proteins:
• Transport (nutrient uptake, osmotic balance, protein secretion).
• Environmental sensing.
• Electron transport and respiration.
• Membrane and cell wall assembly.

Active transport

• Active transport is how cells accumulate solutes against a concentration gradient
• Transporters use three mechanisms: simple transport, group translocation, and the ABC system
• Simple transport uses a transmembrane transport protein
• Group translocation uses a series of proteins
• ABC system requires three components: a binding protein, a transmembrane transporter, and an ATP-hydrolyzing protein
• Active transort is energy-driven by a proton motive force, ATP, or another energy-rich compound.

Bacterial Cell Walls

• Cell walls maintain cell shape and rigidity.
• They are needed to withstand osmotic/turgor pressure to prevent cell lysis.
• Bacterial cell walls contain peptidoglycan.
• Peptidoglycan is a rigid polysaccharide layer providing strength.
• It is not found in Archaea or Eukarya.
• Glycan tetrapeptide includes:
• A sugar backbone of alternating modified glucose (N-acetylglucosamine and N-acetylmuramic acid) joined by B-1,4 linkages.
• A short peptide attached to N-acetylmuramic acid.
• Amino acids that vary between species, L-alanine, D-alanine, D-glutamic acid, and either L-lysine or diaminopimelic acid (DAP)

Bacterial Cell Walls Makeup

• Peptidoglycan strands run parallel around the cell circumference.
• They are cross-linked by covalent peptide bonds.
• Gram-negative crosslinks occur between DAP and D-alanine carboxyl on adjacent glycan strands.
• Gram-positive crosslinks often contain peptide interbridges (e.g., five glycines in Staphylococcus aureus).

Gram-Positive Cell Walls

• Gram-positive cell walls can contain up to 90% peptidoglycan, 15 or more layers thick; Gram-negative walls have only a single layer.
• Gram-positive cell walls contain teichoic acids.
• These are acidic and negatively charged.
• They are found in certain Gram-positive bacteria.
• Teichoic acids are strongly antigenic.
• They are covalently bound to the peptidoglycan.
• Lipoteichoic acids are teichoic acids also covalently bound to membrane lipids.
• Functions of teichoic acid (G+):
• Maintain porosity of cell wall
• Anchor cell wall to cell membrane
• Maintain cell shape
• Capture essential cations like Ca2+ and Mg2+
• Function as a reservoir of phosphate

Gram-Negative Cell Envelope

• The periplasm is a space located between the cytoplasmic and outer membranes, and is about 15 nm wide while housing many proteins
• Porins exist as channels to move hydrophilic, low-molecular substances through the outer membrane
• Lipopolysaccharide is consists of LPS instead of phospholipids in about half of the outer membrane.
• It facilitates surface recognition, important virulence factors, and adds strength
• Polysaccharides covalently bound to lipids

Lipopolysaccharide

• LPS and the O-specific polysaccharide, the core polysaccharide are the building blocks of the Lipid A endotoxin.
• Lipid A, as an endotoxin, can trigger endotoxic shock.

Archaeal Membranes

• Archaeal membranes have ether linkages in membrane lipids.
• Phospholipids have ester linkages in Bacteria and Eukarya.
• Archaeal lipids lack fatty acids, they have isoprenes instead.
• Major lipids in archaea consist 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.
• Ether linkages are more thermostable than ester linkages.
• Archaeal lipid membranes can exist as a monolayer.

Peptidoglycan Synthesis Steps

• Autolysins break the B1-4 bond between M-G sugars in the peptidoglycan chains.
• Lipid II is assembled, which is transported across the membrane via flippase.
• Transglycosylases insert the precursors into the broken peptidoglycan chain (inhibited by vancomycin).
• Transpeptidation is inhibited by penicillin and vancomycin.

Agents That Destroy Peptidoglycan

• Lysozyme
• It is 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.
• 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 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, 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