What pathways and structures dictate cell shape?
What structures provide cell strength?
How do bacteria respond to environment?
• Example E.coli:
→ SEM: via light reflection
→ TEM: light penetrating trough object
→ Rod-shaped, uniform (+ some cells longer than others = dividing cells)
→ What determines the mean size? Is there a ‘ruler’? Or a timer (like an average growth rate)?
→ Dens cytoplasm + dens boundaries (phospholipid membrane + outer membrane) + less dens periplasm
→ A flagel
• Cell shape can depend on physiological state
• Cell shape is functionally important
• Cells often adopt multicellularity, i.e. microcolonies, biofilms
→ Eg. E.coli can bind on urotherial cells (bladder) and form intracellular cell
communities → to spread, they have to come out of the urotherial cell (lysis):
forming elongated cells (picture)
o Why elongate? To survive macrophages = a protective phenomenon
• But what regulation is behind this?
Sizes & shapes
• Spherical bacteria: + 0.5 – 2.0 µm
• Rod shaped or filamentous bacteria: + 1-100µm x + 0.25-1.0µm
• Eukaryotic cells: 10-100µm
• Viruses: 20-500 nm
Eg. spiral Helicobacter in the stomach:
• Normally a virulent bacterium
• Stomach = pH 2 → would kill bacterium
• Solution: lives in stomach mucosa → spiral shape to stay attached to
mucosa → no motility anymore → not virulent anymore
CELL SIZE & SHAPE
• Cells without boundary = dead → many drugs target cell membrane of pathogens
• Why are they not all a sphere?
• What dictates their shape?
• Does shape matter?
• How does a cell expand in size and split during fusion?
• How do so safely without compromising cell integrity?
• What regulates all these phenomena?
• How to transport and communicate across the barrier? Eukaryotic shapes
Eukaryotes: shape is regulated from the inside: cytoskeleton, actin!
Bacteria: mostly organized from the outside: an exoskeleton
1
,LIFE IN AN UNBUFFERED ENVIRONMENT
• Osmosis = water movement from low to high concentration of
molecules via a semi-permeable membrane
• Aquaporines let water pass through the membrane
• In a confined boundary like a cell, there is pressure = ‘turgor pressure’
→ Turgor pressure in bacteria: 0.5-3 mPa (5-30 Bar)
→ In hypotonic environment: higher osmolytes outside: water flows out → plasma membrane collapse →
BUT: due to a cell wall, it retains its shape
→ (isotonic = … ; hypertonic = …)
MICROBIAL CELL ENVELOPES
Role of the cell envelope
• Rigid & protects cytoplasmic membrane
• A selective barrier that allows entry of nutrients, while reducing entry of toxic compounds, or leakage of cell
content
• Determines & supports cell size and shape
• Provides scaffold for attachment of cell appendages
• Prevents cell rupture due to osmotic challenge
• Contains functions for cell wall assembly and growth
Role of cell (cytoplasmic) membrane
• Not rigid!
• Permeability barrier
→ Even water will cross with low efficiency!
• Energy conversion; protein motive force & electron transport
→ Hydrolysable energy sources are exclusive to cytoplasm
Bacteria – Archaea – Eukarya
➔ Mycobacteria: sugar chain with fatty acids on top to make a pseudomembrane → difficult to combat
BACTERIAL CELL ENVELOPES
2
,GRAM-POSITIVE CELL ENVELOPES
• Gram positive or monoderm bacteria
• Thick 20-80 nm
• Peptidoglycan layer, + 50% of cell envelop
• No lipids
• Secondary cell wall polymers: teichoic acid and/or teichuronic acid
→ Covalently linked to PG / anchored in the cytoplasmic
membrane
• PG-bound surface proteins: eg. pili, cell recognition proteins…
→ Eg. if they live on plant material: polymers to degrade lignine/cellulose/… (= large polymers) = eg.
cellulases → why anchored? Otherwise other bacteria could make advantage of it
• Good permeability
TEICHOIC ACIDS
• Covalently attached to PG (phosphodiester to MurNAc)
• Account for as much as 60% of total cell wall mass
• ManNac(β1→4)GlcNac disaccharide with one to three glycerol phosphates attached to the C4 hdyroxyl of
the ManNac residue (the ‘linkage unit’) followed by a much longer chain of glycerol- or ribitol phosphate
repeats (main chain)
• So, a linkage to sugar units and a polymer of eiter glycerol or ribitol P → so quite negatively charged
• Virulence associated, i.e. adherence and biofilm formation (eg. MRSA)
• You don’t have to be able to draw it
LIPOTECHOIC ACIDS
• 4 types, lipid anchored techoic acids found
→ Eg. polyglycerolphosphate (type I LTA): best-characterized LTA
o In phylum Firmicutes: eg. Staphylococcus aureus, Bacillus subtilus, Listeria monocytogenes
• Plays important role for bacterial growth and physiology
• Contributes to membrane homeostasis and virulence
→ So this makes LTA a target for vaccines and novel antimicrobial drugs
• LTA modification can protect against antimicrobial peptides
GRAM-NEGATIVE CELL ENVELOPES
• Gram negative or diderm bacteria
• Thin 8-11 nm
• PG monolayer
• Second lipid bilayer: outer membrane
→ Asymmetric with inner phospholipid layer + outer lipopolysaccharide layer (LPS)
→ Essential
• Periplasm < periplasmic proteins (up to 10% of proteome)
→ Oxidizing environment
→ No hydrolysable energy source there (like NAD etc. = inside cytoplasm)
→ So no trivial functions there
• Lipoproteins: inner & outer membrane
→ Specific sorting system → LOL pathway
• Outer membrane proteins → beta barrels
• Dedicated secretion and import systems (see Chap.3) Because molecules have to pass 2 membranes
• Less permeable → less susceptible to antibiotics
3
, OUTER MEMBRANE
• An additional permeability barrier
• Multitude secretion/import system (C3)
• Lot of proteins in outer membrane: mostly porines
→ Porin-based passive passage: + 500 Da molecules
• No ion gradients
• LPS remodeled in function of antibiotic pressure
→ O-antigen variation to escape immune response
→ LPS anchored on membrane via lipid A
o Lipid A = endotoxin → recognized by toll like
receptor 4 (innate IMS)
• Intrinsic resistance to antibacterials
→ Many AB’s are lipophilic → but LPS is negatively
charged so often AB’s cannot pass through it
→ Polymyxin, bile salts
→ Antibacterial peptides: bind membrane and makes pores → bacterium can modify lipids/phosphates so
that they are less attacked
• Mg and Ca2+ in outer membrane
2+
→ Many stressors remove these (eg. during fagocytosis)
o Sometimes, the bacterium modifies itself so that it does not need Mg for example anymore
OUTER MEMBRANE BIOSYNTHESIS
• Pathways are needed to get the proteins/molecules/… in
the membranes
• LPS synthesis at the inner membrane via Lpx pathway
→ Eg. LPS flipase: transporting LPS from IM to OM
• Transport to outer membrane via Lpt pathway
• Pathway is essential
• Outer membrane phospholipids ?
→ Not known how they get there
→ Has to be very coordinated: one fault = symmetry gone = membrane broken
• No outer membrane = cell dies
→ But why? All functions are in inner membrane, so? Not known
→ Good drug target
OUTER MEMBRANE STABILITY
• PG and outer membrane to give cell stability → °turgor pressure
• Loss of OM stability = cell lysis under hypotonic conditions
• 2 component system
→ Sensor kinase (stress senor) + response regulator
→ Transcription regulation
• Can sense destabilizing things in the OM: eg. Mg2+ taken away → but cell can take away the need for Mg2+
→ Attack by antimicrobial peptides (IMS) → bacterium itself will
take away Mg2+’s → antimicrobial peptides can no longer
detect Mg2+
o How: via LPS remodeling enzymes: replacing the LPS by
phospholipids
→ Plus: overproduction of SlyB = an OM guard protein, giving
rigidity to membrane → turgor pressure
4
Les avantages d'acheter des résumés chez Stuvia:
Qualité garantie par les avis des clients
Les clients de Stuvia ont évalués plus de 700 000 résumés. C'est comme ça que vous savez que vous achetez les meilleurs documents.
L’achat facile et rapide
Vous pouvez payer rapidement avec iDeal, carte de crédit ou Stuvia-crédit pour les résumés. Il n'y a pas d'adhésion nécessaire.
Focus sur l’essentiel
Vos camarades écrivent eux-mêmes les notes d’étude, c’est pourquoi les documents sont toujours fiables et à jour. Cela garantit que vous arrivez rapidement au coeur du matériel.
Foire aux questions
Qu'est-ce que j'obtiens en achetant ce document ?
Vous obtenez un PDF, disponible immédiatement après votre achat. Le document acheté est accessible à tout moment, n'importe où et indéfiniment via votre profil.
Garantie de remboursement : comment ça marche ?
Notre garantie de satisfaction garantit que vous trouverez toujours un document d'étude qui vous convient. Vous remplissez un formulaire et notre équipe du service client s'occupe du reste.
Auprès de qui est-ce que j'achète ce résumé ?
Stuvia est une place de marché. Alors, vous n'achetez donc pas ce document chez nous, mais auprès du vendeur lunawillems1. Stuvia facilite les paiements au vendeur.
Est-ce que j'aurai un abonnement?
Non, vous n'achetez ce résumé que pour €8,99. Vous n'êtes lié à rien après votre achat.
