5 & 6 -Intro to Bacterial cell envelope Flashcards
What three things must a pathogen do to colonise/infect a host?
1) Gain access to hose
2) Adhere to host surfaces
3) Evade Host defences
What is the difference between the inner and outer membrane of gram-negative bacteria?
Inner membrane, both faces made of phospholipid bilayer
Outer membrane, inner face = phospholipids, outer face = LPS (lipopolysaccharide)
Give 3 functions of the outer membrane in gram-negative bacteria
Structure - mechanical stability, (needed as gram-negative don’t have thick peptidoglycan wall)
Defense - Protects against antibiotics, bacteriophages, antimicrobial peptides
Permeability Barrier - Stops things permeating cell envelope, like detergents, needed as no thick peptidoglycan wall
What are the 3 key areas that form LPS?
Lipid A - embedded in membrane
Core oligosaccharide region
O-antigen region - important for virulent response
What are three functions of LPS?
Barrier Function -> repels hydrophobic agents, detergents, bile and antibiotics
Forms tightly packed layer -> due to strong lateral interactions between LPS molecules
Pro-inflammatory -> Interacts with receptors on macrophages, and B-cells leading to cytokine release.
Which part of LPS is also called endotoxin and why?
Lipid A, due to role in proinflammation
What short sugars is the core oligosaccharide made of?
D-galactose or D-glucose
Occasionally heptose / keto deoxyoctanate
Is O-antigen region highly conserved or highly variable?
Highly variable
Composition can change between strains of a pathogen
E.coli has approx 160 different O-antigen structures
What is rough LPS?
No O-antigen / O- polysaccharide present
Only core oligosaccharide attached to lipid A present.
What is smooth LPS?
All three regions are present ; Lipid A, Core Oligosaccharide, O-Polysaccharide/Antigen.
Why would losing the O-antigen be beneficial to bacteria?
O-antigen causes virulence and triggers and immune response.
Losing it allows the bacteria to maintain chronic infection and go undetected by the immune system
Give two examples of bacteria that alter their LPS
Vibrio Cholerae
Helicobacter Pylori
What does the addition of a positively charged amino acid to LPS in bacteria such as Vibrio Cholerae do?
Removes negative charge on lipid A and increases resistance to cationic antimicrobial peptides.
Give two features found on the surface of gram-positve bacteria.
Teichoic acids
Covalently bound anchor proteins
Are Teichoic acids negatively or positively charged?
What molecules give them this charge?
Negatively charged
Phosphates in the polymer chain
What are the two types of teichoic acids?
Where are they found?
Wall teichoic acids -> embedded in cell wall
Lipoteichoic acids -> embedded in cytoplasmic membrane
What groups can bacteria add to teichoic acids?
Glycosyl groups
Positively charged amino acids
What are six functions of teichoic acids?
Important in bacterial growth and division
Negative charge protects from antimicrobials and antibiotics
Allow bacteria to bind to surfaces, (mucosal surfaces in host)
Prevent/block host from binding to peptidoglycan
Contribute to virulence
The molecules host cells recognise and trigger an immune response (modifications made to evade this recognition)
Is the polymer chain of teichoic acids very conserved or very variable in a given species?
Highly conserved
the modifications are highly variable however
What are the two types of modification teiochoic acids can have?
D-alanine
Glycosylation
What effect does the addition of D-alanine have on teiochoic acids?
Advantage:
Positively charged amino acid, infers increased resistance to; host defences, antimicrobial peptides and glycopeptide antibiotics (antibiotics that inhibit cell wall synthesis)
Disadvantage:
Can prevent adhesion to host cells and thus prevent establishing and infection
What effect does glycosylation of teiochoic acids have on bacteria?
Advantage:
Increases protection from the immune system
Disadvantage:
Increase susceptibility to phages as viruses can recognise the glycosyls on teiochoic acids and use it to infect bacteria
Are modifications permanent?
No they fluxuate through time and are not fixed
Are modifications more common in early infection of chronic infection?
More common in chronic infection
Where are covalently bound proteins attached to on the cell surface?
Anchored to amino acid cross-bridges in peptidoglycan of bacteria.
What are 6 functions of covalently bound anchored proteins?
Bacterial adhesion Invasion of host cell Bind to plasma protein Immune evasion Induce inflammation Biofilm formation
What enzyme attaches anchor proteins to cross-bridges?
Sortase
What is the phenotype of SrtA mutants?
How does this effect the bacteria?
No cell wall anchored proteins
Much less virulent -> cell wall anchored proteins increases virulence.
Name three structures beyond the cell wall that are important in adhesion and colonisation.
Pili
Capsule
Flagella
What are EPS?
When is it produced?
Extra cellular polysaccharides
Secreted into the environment, NOT retained on the cell surface.
During biofilm formation.
What part of bacteria is a type of glycocalyx?
The capsule
What are the two types of capsules?
What are their differences?
‘True Capsule’ - distinct layer and gelatinous
Slime layer - irregular/ diffuse layer (slimy!)
What does hyaluronic acid in a capsule do?
Retains water
In what way are capsules important for virulence?
Resist phagocytosis, ‘hide’ surface structures from immune cells
Give an example of an encapsulated bacteria
What disease does it cause?
Bacillus anthracis
Anthrax (only encapsulated strains cause disease)
What are 4 functions of a capsule?
Barrier to toxic hyrdrophobic molecules, e.g. detergents. This is due to high water content.
Water content protects against dessication
Prevents bacteriophage infection by hiding surface structures
Hides bacteria from host immune system
How does the capsule avoid destruction?
Prevents complement-mediated lysis
How does the capsule avoid detection?
Hide surface structures to resist phagocytosis
Give an example of bacteria that resist phagocytosis
streptococus pneumoniae
What are opsonins?
Bind to foreign microorganisms/ pathogens to make them more susceptible to phagocytosis
e.g. antibodies, complement produced by immune system
How do opsonins trigger phagocytosis?
White blood cells have opsonin receptors,
Bind to osponins on pathogens and then phagocytose the bacterium
What effect does the capsule have on opsonins?
Prevents white blood cells from detecting/binding to opsonins
What are the four stages of forming a biofilm?
1) Adhesion to surface
2) Cell- Cell Communication
3) Proliferation
4) Maturation
Where is the S-layer located?
Between the outer membrane and biofilm
Not the outermost layer, that’s the capsule.
What do pores in the S-layer allow for?
Selective permeation of different products
S-layers are commonly found in archaea. True or False?
True, they protect them from harsh environments
Are S-layers found in gram-positive or gram-negative bacteria or both?
Both and many don’t have one at all
Give 3 functions of the S-layer
Molecular Sieve (porous mesh like chain mail)
Protection (against bacteriophages, complement (opsonins), phagocytosis, extreme environments)
Adherence (proteins adhere to epithelial mucosal cells)
What protein are pili made of?
Pilin
What are sex pili used for?
Transferring genetic material from one bacterium to another.
What are ordinary pili used for?
Attach to surfaces and other bacteria
Motility - crawling/twitching
Are pili mostly found in gram-positive or gram-negative bacteria?
Gram-negative
What do major and minor pilin proteins do?
Major pilin forms the Shaft
Minor pilin forms the tip
Describe the role of Type 4 (IV) pili in P. aeruginosa
Adherence to surface in host - sense initial contact, triggering a virulence cascade
Biofilm formation (important for evading immune system)
Twitching & Motility
Switch from virulent to non-virulent strains
How many pili and how many fimbriae are found on a typical bacterium?
1-2 pili
100’s - 1000’s fimbriae
Are fimbriae found in gram-positive, gram-negative bacteria or both?
Both
What is the function of fimbriae?
Attachment to surfaces / tissues
Clinging to other bacteria
Adhesion molecule at tip of fimbriae allows attachment to specific host molecules
List the 4 main components of a fimbriae structure
Tip (adhesion protein)
Fibrillium
Rod
Pore
What two virulence factors are found in uropathogenic E. coli (UPEC)?
Fim H (Type I fimbriae) - recognise elements of epithelial cell membrane to attach
P fimbriae - recognise receptor proteins on cell membrane & form contact
