Lecture 3 - bacterial morphology & cell envelope structure Flashcards
Morphologies that bacteria can take
- bacillus - rod shaped
- coccus - spherical
- spirochetes and spirilla - spiral
Example of bacilli
Lactobacillus lactus (used for milk cultures)
- Borrelia burgdorgeri (causes Lyme disease)
- Leptospira interrogans (cause of leptositosis)
Spirochetes
Examples of cocci
Streptococcus pneumoniae (cocci in pairs aka diplococci), anabaena spp (filaments of cyanobacteria)
What group of bacteria did mitochondria arise from?
Alpha-proteobacteria
Relationship between bacterial phylogeny and shape
There is not a clear relationship. Phylogenetically distant bacteria can take the same shape.
- Stella vacuolata
- Prosthecomicrobium
- Ancalomicrobium adetum
Alpha-proteobacteria
Morphology of caulobacter
Curved with a holdfast stalk. Two differentiated cells before full division. Stalk cell has the holdfast. Other cell has a flagellum.
Morphology of streptomyces
Branched filamentous
Myces meaning
Myces = “fungi”
Strepto- meaning
Strepto = “chain of”
Therefore, streptococcus is a chain of cocci
Staphylo - meaning
Staphylo = “bunch of grapes”
What is one reason that bacterial morphology differs?
Can differ based on how the bacteria makes its cell wall
Significance of lack of bacterial intracellular compartments for DNA replication
Transcription and translation are not spatially or temporally separated like in eukaryotic cells.
Relationship between ribosome content and growth rate of bacteria
Higher ribosome content = higher growth rate
Ribosomes are generally the rate-limiting material in cell division
Common features of eubacterial cells
- gel-like viscous cytoplasm; in dilute solutions, water wants to enter cell
- circular chromosome in cytoplasm (nucleoid); takes up most of the cell’s volume
- cell membrane of phosphoglycerolipids and protein
- cell wall of peptidoglycan
- organelles (ex: flagella, microcompartments)
Why do some bacteria have microcompartments?
Sequestration of enzymes or other material
Why is peptidoglycan highly conserved amongst bacteria?
Good for resisting osmotic pressure (prevents water from flooding into cell in dilute solutions)
Importance of lipopolysaccharides in bacterial outer membranes
Primary mechanism of bacterial interaction with host
Polyamines found in bacteria
Positively charged and protonated, act as counter-ions for DNA packing purposes
- Putrescine (smells like rotting fish due to same decaying amines)
- Spermine (discovered in sperm)
Why are two DNA molecules found per E.coli on average?
E. coli can replicate every 30 min, but the chromosome takes approx. 40 min to replicate
Gram-negative bacterial envelope structure
Two lipid membranes surrounding a thin layer of peptidoglycan cell wall
Gram positive bacterial envelope structure
One cell membrane under a multi-layered peptidoglycan wall. Can have an S-layer (surface layer). Glycosyl chains on exterior surface.
What do teichoic acids do?
Located in the peptidoglycan layer of Gram-positive bacteria. Contributes to wall stability and is responsible for retention of Gram stain
What are the two lipid membranes made of in Gram-negative bacteria?
Outer layer = lipopolysaccharide (LPS)
Inner layer = phospholipid
Major component of E. coli membrane (aside from phospholipid)
Proteins for material transport through the membrane
Composition of phosphoglycerides
Glycerol backbone + two fatty acids + phosphoryl head group through ester linkages.
Type of head group and fatty acid can be swapped out.
How do bacteria mediate their membrane fluidity?
Can change the fatty acids produced for their phospholipid membranes. Warmer temp –> increased saturation of fatty acid –> combat increased membrane fluidity
Why do saturated fatty acids in bacteria membranes fight against membrane fluidity?
More saturated = more straight –> tighter packing
What is cardiolipin?
Dimeric phospholipid (four fatty acid tails) discovered in cardiomyocytes.
Why was cardiolipin discovered in cardiomyocytes?
Cardiomyocytes require large amounts of energy for muscle contraction, therefore they have high amounts of mitochondria. Mitochondrial membrane is about 20% cardiolipin.
How is cardiolipin evidence for the bacterial origin of mitochondria?
In eukaryotic cells, mitochondria is the only source of cardiolipin. Cardiolipin is commonly found in bacterial membranes, thus mitochondria must have arisen from bacteria.
Why does passive diffusion of hydrophilic solutes across membranes require desolvation?
Activation energy for passive diffusion is very high for hydrophilic solutes. Without desolvation, it would take a very long time for it to diffuse across the membrane.
How are hydrophilic solutes moved across bacterial membranes?
Transporters interact with the solute to move it across without bringing water molecules with it.
How can weak acids and bases diffuse across membranes?
When not ionized, weak acids and bases are neutral and therefore are membrane-soluble
How do we know that weak acids and bases move through membranes?
Molecules like aspirin and penicillin are able to be taken up by cells for therapeutic purposes. We haven’t evolve receptors to transport these things, so they must be getting in to the cell through diffusion
Active vs passive transport
Passive transport: molecules move along concentration gradient
Active transport: energy is expended to move molecules against their concentration gradient (ex: glucose moves into cell, against its gradient, through primary or secondary active transport)
Primary vs secondary active transport
Primary: directly expends energy via ATP hydrolysis to transport molecule
Secondary: co-transports a molecule down it’s gradient to provide energy to move a molecule up its gradient
What does the lac operon encode?
Encodes a lactose transporter (lactose permease) which allows lactose to pass through the membrane via secondary active transport
ABC transporters
Ancient protein family found in all three branches of life. Uses ATP hydrolysis to facilitate primary active transport.
What can ABC transporters move?
- Vitamin B12, important for DNA synthesis.
- Toxins and chemotherapeutic drugs
How do human cells resist chemotherapy?
Upregulation of ABC transporters –> export of chemotherapeutic drugs from the cell
Primary component of bacterial cell walls
Peptidoglycan (aka murein)
What is peptidoglycan made out of?
Polymers of alternating N-acetyglucosamine and N-acetylmuramic acid sugars on a glycan strand.
How is crosslinking facilitated in PG cell walls?
N-acetylmuramic acid residues carry peptides of 4-6 residues.
Peptides found attached to N-acetylmuramic acid for crosslinking
L- alanine
D-Glutamic acid
m-Diaminopimelic acid
D-alanine
D-alanine
How are N-acetylglucosamine and N-acetylmuramic acid linked?
1,4 beta linkage, very resistant to enzymatic breakdown
How is cell wall rigidity regulated in bacterial cells?
Crosslinking of peptides
What is capable of breaking bonds between N-acetylglucosamine and N-acetylmuramic acid?
Lysozyme can break 1,4 beta linkages and therefore acts as an antibacterial agent.
Is lysozyme effective against all bacteria?
No. Lysozyme is only effective against Gram + bacteria due to their exposed PG wall. Gram - bacteria have an outer membrane protecting their PG
How does penicilin act as an antibiotic?
Blocks release of the terminal D-alanine, preventing PG crosslinkages
How does penicilin act as an antibiotic?
Blocks release of the terminal D-alanine, preventing PG crosslinkages
How does vancomycin act as an antibiotic?
Binds D-Ala D-Ala and prevents the release of the terminal D-Ala
Does vancomycin work on all bacteria?
Only works against Gram + with exposed PG because the molecule is too big to pass through the Gram - membrane
How do bacteria become resistant to antibiotics?
Development of enzymes that degrade the antibiotic
Beta-lactam antibiotics
Penicillins, cephalosporins, carbapenems
What is the function of the bacterial cell wall?
Confers shape and rigidity and helps the cell withstand turgor pressure.
Sacculus
Term for the single interlinked molecule that makes up the bacterial cell wall
How does growth occur in the bacterial cell wall?
Formation of new glycan chains at the center of the bacteria, expanding outward. Requires the formation and breakdown of crossbridges between strands to make a single interlinked molecule
Why are penicillins only useful against growing cells?
Disrupts the formation of new cross-linkages at the center of the bacteria. Weakens cell wall and causes lysis at the midpoint due to increased pressure from cell growth
Function of the outer membrane of Gram - bacteria
Acts as a permeability barrier and contributes to the toxigenic properties of many pathogens (LPS is involved with the cell’s interaction with its environment)
What links the cell wall to the outer membrane of Gram - bacteria?
Murein lipoprotein links to the peptides on N-acetylmuramic acid
What is lipid A?
Lipid A is the highly conserved lipid portion of lipopolysaccharide (LPS) which is a toxic component of Gram - bacterial cell walls. Unlike other phospholipids, lipid A doesn’t have a glycerol phosphate backbone.
How does the immune system recognize lipid A?
TLR-4 on innate immune cells recognizes lipid A to monitor for Gram - bacteria
When does lipid A become problematic on its own?
If too much lipid A is released upon bacterial cell lysis, it can cause an overreaction from the immune system
What are the components of LPS?
From bottom to top: lipid A, inner core, outer core, O antigen
O antigen
Highly variable hydrophilic portion of LPS. Can be used to identify particular strains of bacteria
Why is it important that O antigen is hydrophilic?
Blocks entrance of bile acids that would break down lipid membrane of Gram - bacteria
Polymyxins
Detergent-like antibiotics that disrupt the outer membrane of Gram - bacteria
How do polymyxins act as antibiotics?
Part cationic (hydrophilic) and part hydrophobic –> acts as a detergent
What does O antigen dictate?
Host range of Gram - bacteria. Doesn’t have any control over toxicity.
Why did E. coli K-12 lose its O antigen?
E. coli K-12 is a lab strain of E. coli that doesn’t live in a human host. Therefore, it didn’t need the O antigen to protect against bile acids and other enzymes. Loss of O antigen allowed for faster growth
Structure of most porins
Made of beta barrels
Function of E. coli OmpC
Cation-selective porin that allows passage of materials under 600 Daltons
Examples of material moved by OmpC
Penicillin and beta lactam
What was the origin of the name OmpC?
Outer membrane protein C
What can induce the loss of OmpC and other porin expression?
Exposure to small antibiotic molecules. Loss of porin prevents transport of molecule into the cell
PhoE
Trimeric porin that transports anions like phosphate into cell
Tsx
Transporter of nucleosides. Receptor discovered in T6 phage
Nucleoside
Dephosphorylated nucleotide
LamB
Trimeric porin that transports maltose into the cell. Receptor discovered in lambda phage
BtuB
“B twelve uptake”
Gated channel that transports vitamin B12
TolC
Beta barrel protein that helps mediate export of antibiotics and toxins. Works with multiple different pumps
TolC + HlyD and HlyB
HlyD and B uses primary active transport to move hemolysine to the periplasmic space for TolC uptake.
Result: export of hemolysine, molecule that lyses RBCs for bacterial iron uptake.
TolC + AcrA and B
AcrA and B use secondary active transport (proton gradient used) to transport acridine and other hydrophobic drugs into the periplasmic space for TolC uptake. Result: export of acridine and other drug resistance
What is a capsule (cell envelope)
Polysaccharide matrix that can be found surrounding Gram + bacteria. Usually, production of a capsule is stress-induced.
What is an S-layer (cell envelope)
Surface protein layer that can be found on the surface of Gram + (and occasionally Gram -) bacteria. Proteins exported to surface and self-assemble into regular arrangements to protect against predation. Pores present at vertices of the arrangement to allow material transport.
Teichoic acid structure
Polymers made of glycerol or ribitol linked by phosphodiester bonds. Amino acids and sugars can be found attached. Anionic.
Teichoic acids vs lipoteichoic acids
Teichoic covalently linked to PG. Lipoteichoic linked to cell membrane and wall
Mycobacterial cell envelope
Waxy and hydrophobic. Include unusual lipids and sugars (mycolic acids and arabinogalactans).
Mycolic acids
Long, waxy lipids ~ 100 carbons long. Make tight permeability barriers for mycobacterial species. Causes slow growth of bacteria.
Relationship between phylogeny and cell envelope structure
Unclear. Mycobacteria are closely related to Gram + species but use a very different envelope
Average volume of a bacterial cell
1 femtolitre (fL0
