Bacterial Structure (Schein) - 4/25/16 Flashcards
Describe the structure of bacterial cells (5).
Typical bacterium characteristics:
SIZE: 1 -2 micrometer vs. 10-20 micrometer (mammalian cells)
CELL WALL: Bounded by a complex envelope (3 layers: plasma membrane, cell wall, structures outside wall) | NOTE* Target of antibodies
ORGANELLE: No membrane-bound organelles (no separation into nucleus and cytoplasm - DNA found in cytoplasm)
SHAPE: spherical (cocci) or rod-shaped (bacilli)
CYTOSKELETON: Contain proteins (mreB, parM) homologous to actin and tubulin - required for chromosome/plasmid segregation
Describe the process by which bacteria divide.
Binary Fission:
- Grow by elongation in one dimension –> Development of transverse septum of new cell wall that divides elongated cell into two
- Rapid division: each division takes approximately 20-30 min
Describe bacterial population growth.
Exponential Increase
Time vs. Relative Number of Cells:
Sigmoid curve
Cells added to fresh medium -
1. Lag Phase (before growth resumes)
2. Log or exponential phase (cells dividing at constant rate)
3. Stationary phase (cells exhaust nutrients, stop growth)
Time vs. Log (Cell Number):
Positive linear curve
Terms to know:
- Clone
- Strain
- Isolate
- Serotype
- Clone: Population derived from a single cell (i.e. bacterial colony, humans)
- Strain: Clone that is genetically different from other clones of the same species
- Isolate: Clone cultured from a patient with an infection
- Serotype: Clone characterized by specific forms of important surface structures recognized by the immune system
Describe the bacterial plasma membrane.
- Equivalent to IMM (lipid bilayer + proteins)
- Permeable to water
- Contains complexes of bacterial respiratory chain and F1 ATP synthetase (since no mitochondria)
- Transport proteins (permeases) in membrane: required to transport small molecules across plasma membrane into cell / Other transport systems pump small molecules out
Describe the function of bacterial cell walls.
Function: protect cells against osmotic lysis
PM = permeable to water
- [Solute] inside bacterium > [Solute] in surrounding medium –> Water moves into cell
- Inflow needs to be blocked or then cell will swell –> membrane rupture
- Water flows into cell –> internal pressure rises –> cell swells until restraint provided by cell wall stops expansion
What is the main component of bacterial cell walls? Describe its importance and structure.
Peptidoglycan:
- Long-chain polysaccharide w/ peptide cross-links
- Essential to integrity of envelope –> damage leads to osmotic lysis
- No equivalent to mammalian cells –> TARGET FOR ANTIBIOTICS
- Toll-like receptors (peptidoglycan = bacteria)
Structure:
- Disaccharide: N-acetylmuramic acid + N-acetylglucosamine (NAM-NAD)
- 5 amino acid side chain off of NAM (tail consists of D-ala, D-ala-COOH) –> important for connecting NAM-NAD to another NAM-NAD –> cell wall cross-linking | NOTE* Target for Penicillin (D-ala-D-ala) –> irreversible binding –> cell wall can’t be made –> osmotic lysis (only works when cell wall is growing)
- Contain both D and L amino acids* - L amino acids set off our immune system since we only have D’s
Describe peptidoglycan assembly.
- Disaccharide + peptide units assembled in cytosol on lipid carrier
- Exported across PM to outside of cell
- Disaccharides assembled into long chains
- Chains cross-linked by peptide bonds between side chains - essential for strength of wall (3D)
Discuss how differences in peptidoglycan thickness can cause bacterial cells to stain differently.
Procedure = Gram Stain
Step 1 in bacterial ID-ing: look at cell wall (thick = 20-50 layers of peptidoglycan = gram (+); thin = only 1-3 layers of peptidoglycan = gram (-)
Thick/gram (+) retains more dye
Describe the Gram-Stain Procedure.
- Heat-fix bacterium to slide
- Add a little bit of gentle heat so that any carbs on the outside of the bacterial cell caramelize gently to the slide –> this prevents them from washing off in subsequent steps (keeps bacteria on the slide) - Stain with Gentian or Crystal violet
- Stains all bacterial purple - Add Iodine “Mordant”
- Complexes with dye –> result in strong violet color - Add alcohol (decolorize)
- Wash out dye… allows differentiation between gram (+) and gram (-) - Counterstain with red dye (safranin) - pink dye
- Stain both pink… if you don’t wash this out… gram (-) will retain pink stain - this is important so that you can see structures under the microscope
Gram (+) = purple-ish blue
Gram (-) = pink
Compare and contrast gram (+) and gram (-) cell wall structures.
In addition to differences in peptidoglycan thicknesses, gram (-) have an outer membrane.
But both have structures protruding out of cell wall - important for ID-ing bacteria and/or virulence factors
Describe the envelopes of gram (+) bacteria.
- Plasma membrane
- Thick peptidoglycan
- Protein fibrillae - adhere to mammalian cells and ECM; many types per cell
- Teichoic acids* - polymers of sugar alcohols linked by phosphate groups; virulence factors that set off TLR - unique to gram (+)
- ‘Group carbohydrate’ - linked to peptidoglycan - in some groups, important for ID
Describe the envelopes of gram (-) bacteria.
- Plasma membrane
- Thin peptidoglycan
- Outer membrane - second membrane (lipid bilayer + protein) outside peptidoglycan
- Periplasm = space between outer and inner (plasma) membranes [not empty space, filled w/ enzymes, solute carrier proteins, polysaccharides, protein assemblies traverse periplasm and link inner and outer membranes]
Structures outside wall:
- LPS (lipopolysaccharides) –> set off TLR for gram (-) [similar to teichoic acid for gram (+)]
More details on outer membrane:
- Inner leaflet = typical phospholipids
- Outer leaflet = composed almost entirely of LPS (disaccharide with attached fatty acids)
- Fatty acids = unsaturated, pack tightly
- Outer membrane is highly impermeable - major defense against toxic small molecules (including antibiotics)
- Porins = ways that nutrients, etc.. enter and exit cell… cell is impermeable otherwise
More details on porins:
- Trimer: monomer = cylinder; walls of beta-sheet
- Outside is hydrophobic b/c embedded in phospholipids, inside is hydrophilic so that solutes can pass through
- Pore diameter determines ability of antibiotics to enter cell (3 barrel structure)
Describe the structure of lipopolysaccharide (LPS).
Lipid A (bound to outer membrane) + Core polysaccharide + O-antigen (in outer leaftlet)
More details from outside to inside:
- O-antigen - polymer, repeat of 3-5 sugars
- Core - phosphorylated complex oligosaccharide; adjacent units bonded via divalent cations
- Lipid A - disaccharide + 6-8 saturated fatty acids. TOXIC. Endotoxin b/c physically part of the cell wall and released by lysis | NOTE* Exotoxins = proteins deliberately released by some bacteria
More details on Lipid A = ‘Endotoxin’
- Present in both living and dead bacteria - not removed by sterilization
- Antibiotic treatment kills bacteria –> release of endotoxin and other inflammatory components –> cytokine storm –> fever + hypotension (Jarish-Herxheimer reaction)
What are the bacterial appendages and their functions?
- Flagella - locomotion (counterclockwise movement for steady movement; clockwise = random tumbling); driven by proton pump | bacterial chemotaxis (when swimming up concentration gradient of attractant, tumbling is suppressed… when swimming down concentration gradient, tumbling and re-orientation is promoted
- Pili (fimbrae) - adhesion
- Conjugation pili - transfer of DNA between cells
