Bacterial Structure - Schein 4/25/16 Flashcards
basic bacterial characteristics
prokaryotic: DNA in cyto
rapid division by transverse fission (division cycle as short as 20 min → potential for infections to spread v quickly)
structure determined by genes coding for cytoskeleton (actin homologues) → key for chromosome/plasmid segregation
- bacilli : rods
- cocci : spherical
- spirochetes (spiral)
bacterial envelope
bacterial envelope : cell wall outside of plasma membrane
- interface with environment (protection)
- interface with mammalian cells (virulence)
- target for antibodies/antibacterial drugs
main component: peptidoglycan (long chain polysacch) with peptide crosslinks
- critical to envelope integrity → damage leads to osmotic lysis
- no equivalent in mammalian cells → antibiotics can target it with very selective toxicity
- recognized by TLRs as a signal for bacteria
bacterial plasma membrane
much like typical euk plasma membrane…
- lipid bilayer + proteins
- solute import and export system
BUT also contains complexes of respiratory chains
- equivalent to inner mito membrane
bacterial envelope structure
peptidoglycan, made of repeating disacch-peptide units assembled in cyto on lipid carrier → exported across plasma membrane to form the bacterial envelope
disaccharide backbone : NAM/NAG repeating
- MurNaC (N-acetyl muramic acid) and GlcNaC (N-acetyl glucosamine)
with 5-aa peptide side chain for connecting layers of the disacch backbone → stability of the wall
- contains both L and D a.a.s
- last two a.a.s: D-ala, D-ala
- imp for cross-linking and for antibiotic therapy
how does penicillin work??
penicillin is a structural analogue of D-ala-D-ala section of peptidoglycan subunits
- when a bacterial cell is dividing and the enzyme responsible for crosslinking of peptide side chains goes to work, it irreversibly binds to penicillin
- bacteria end up with defective cell wall synth → weak wall due to absence of crosslinks → holes and death by osmotic lysis
in order for penicillin to work, need new peptidoglycan synthesis to be occuring!
- acts best on growing cells
peptidoglycan and bacterial staining
diffs in peptidoglycan thickness leads to diffs in bacterial Gram staining
- thick peptidoglycan (20-50 layers) → Gram+
- thin peptidoglycan (1-3 layers) → Gram-
difference in staining due to ability of thick peptidoglycan to retain dye that would be washed out of thin peptidoglycan
Gram staining (steps)
- heat-fix (heats carbs on bacterial surface, carmelizes them onto slide to prevent washoff)
- stain : add Gentian or crystal violet
- “bite on” : add iodine “Mordant” : complexes with dye
- decolorize with alcohol (washes dye/iodine complexes out of thin, Gram- bacterial peptidoglycans)
- counterstain : add red Safranin : picked up by all cells, allows us to see all and differentiate between Gram+/-
- Gram+ purplish blue
- Gram- pink
diffs between Gram+ and Gram- bacterial envelopes: basics
Gram+
- stain purple
- thick peptidoglycan (20-50 layers)
- no outer membrane
Gram-
- stain red
- thin peptidoglycan (1-3 layers)
- outer membrane
Gram+ envelopes
thick peptidoglycan with attached substances such as…
protein fibrillae
- adhere to mammalian cell membrane proteins and ECM
- covalently linked to peptidoglycan; many types per cell
- essential to virulence → potential target of antibiotics
teichoic acids : polymers of sugar-alcohols linked by P groups (unique to Gram+)
- set off TLRs, ID bacteria to immune system as Gram+ → important to immunity
‘group carbohydrate’ : linked to peptidoglycan, can be imp for ID
Gram- envelopes
thin peptidoglycan
outer membrane (second membrane comprising lipid bilayer and protein outside peptidoglycan)
- inner leaflet is normal PL; outer leaflet is primarily LPS (lipopolysacchs - disacch + attached FAs) → set off TLRs, indicate presence of Gram- bacteria
- FAs are unsaturated, pack v tightly → outer membrane is highly impermeable and serves as defense for bacterial cell, which means you need….
- porin proteins : req for solutes/nutrients to get in, can be co-opted for antibiotic transport
periplasm : space between inner and outer plasma membranes (incl peptidoglycan)
- not empty! filled with enzymes, polysacchs, carrier proteins
- traversed by protein assemblies which link inner and outer membranes
porin proteins
found in outer membrane of Gram- bacteria
trimers
- monomer is cylindrical; walls are beta sheets
arranged such that…
- outside of trimer is hydrophobic (allows porin to be embedded in lipid bilayer of outer mem)
- inside is hydrophilic (allows diffusion of solutes)
- pore diameter determines ability of antibiotics to enter cell
lipopolysacchs
LPS comprises 3 parts:
- lipid A : bound into outer membrane
- endotoxin (entrenched in bacterial cell membrane and toxic to humans)
- present in living and dead bacteria - NOT removed by sterilization → have to be removed before parenteral use
- antibiotics that kill bacteria can result in release of endotoxin and other infl components → fever, HTN, and/or systemic tox (via Jarish-Herxheimer rxn)
- core polysaccharide
- O antigen
lipid A toxicity
TLRs on monocytes bind LPS via lipid A → synthesis and release of inflammatory cytokines
- locally: imp in defense
- systemically: shock, organ dysfx, death
can result in…
- septicemia (bacteria in bloodstream → septic shock with 40% mortality)
- disseminated intravascular coag (DIC)
bacterial appendages
flagella : locomotion
- made of flagellin protein
- called “H antigen”
- locomotion via rotation
- counterclockwise rotation: steady direction
- clockwise rotation: tumble until desired direction achieved
common pili/fimbrae : adhesion
conjugation pili : DNA transfer between cells (ex. spread of virulence factors, antibiotic resistance factors)
capsule
found on many bacteria outside of cell wall
aka “K antigen”
- usually made of high MW polysacchs; can also be protein
- often have many antigenic types per species
- NOT visible in Gram staining → need special stain, EM
