Adhesins, Toxins, and Bacterial Secretion Systems Flashcards
What is the purpose of adhesins?
Allows bacteria to adhere to host tissue.
The human body expels invaders in many ways (Mucosa, dead skin constantly expelled, Liquid expelled from bladder, Coughing, cilia in lungs, Expulsion of intestinal contents).
Bacteria hang on tightly so they are not expelled. Occurs through pili (fimbriae)- hollow fibres with tips of adhesin or lectin to bind to sugars on host cell, or adhesins- surface proteins bind host cells.
What types of pili are used in microbial attachment?
Type I pili- adhere to mannose (sugar found on human cell receptor proteins, static)- grows from outer membrane, new subunits added at base.
Type IV pili- grows from inner membrane, can grow and retract. Pilin subunits are removed from base. Can reel itself in to the host cell it is attached to.
Where are pilus assembly genes located?
What is the major pulis subunit?
Found in a single operon.
PilA.
How is the type I pilus assembled?
Components are made on cytoplasmic ribosomes.
Sec-A dependent export into periplasm- Subunit is secA. Channel is secY. SecB is chaperone.
PapD chaperones protein to OM- PapD lets pilin subunit fold, but without sticking to anything else. Pilus in periplasm would be toxic.
PapC forms channel through OM.
PapG protein binds to host cell membrane- this is the tip of the pilus. Then F, then two E, then A units.
Attached in order from tip to base- units sorted by usher, which is a beta barrel protein.
PapH is a termination pilin, doens’t have a sticky patch like the other units. Next unit can’t displace the previous chaperone- doesn’t fit, so growth is terminated.
Example organism is uropathogenic E.coli.
How is the type IV pilus assembled and disassembled?
This one grows and retract. Made in inner membrane.
Sec-A independent but requires SecY for PilA. PilA made in cytoplasm and inserted in the membrane. Ribosome provides the energy needed for unit to go in.
Pilins polymerised in inner membrane. Proteins are never free in the periplasm.
PilD (pre-pilin peptidase)- takes 5 amino acids off the end of PilA for polymerisation. Transmembrane part of PilA is sticky.
PilQ is a beta barrel, but not the same as type I usher. Makes a hole for the protein to pass through.
Generates gliding or twitching motility.
Another ATPase pulls subunits off pilus back into inner membrane for retraction.
Example organisms are V.cholerae, Neisseria.
What are some non-pilus (afimbriate) adhesins?
Streptococcus M proteins- bind to fibronectin and complement regulatory factor H. Gives cell specificity to bind to certain cells.
Yersinia YadA adhesin binds to collagen.
Bordetella pertactin binds to integrin.
What are some types of bacterial toxins?
Exotoxins- Gram positive and negative: Secreted by the cell, catergorised by mechanism of action.
Endotoxins- Gram negative lipopolysaccharide: Component of outer membrane- endotoxin in lipid A component, activates inflammatory response, can cause toxic shock.
(Endotoxin in not a secretd protein- way bacteria are made. Lipid A can be modified with sugars- all Gram negative bacteria have this. When they die, lipid A is released and activated immune response- this is endotoxin).
What are the five categories of bacterial exotoxins?
Based on mechanism of action:
Membrane disruption- most toxins do this.
Disruption of protein synthesis- lots of toxins affect this, as there are many steps to affect.
Second messenger pathway disruption (what bacteria wants to do rather than host)- not many do this.
Superantigenic toxins- induce cells to make antigen without antigen presenting cells- this is unregulated and can cause toxic shock syndrome.
Proteases that cleave host components.
Give an example of a toxin that disrupts the membrane, and how it does this.
Haemolytic alpha toxin of S.aureus.
Single polypeptide chain (33kD size).
7 copies in membrane form a beta barrel. Amphipathic. Makes hydrophilic channel which allows stuff through along the concentration gradient eg. ATP, glutathione let out, water let in. Cell lyses.
The toxin can liberate ions that the cell needs to grow.
Give an example of a toxin that disrupts protein synthesis, and how it does this.
Shiga toxin from Shigella flexneri or E.coli O157:H7.
Destroys kidneys.
Toxin made as a hexamer- 5 B units, 1 A units. Secreted into the environment and binds to human cells (can’t affect many animals).
Toxin attaches to ganglioside Gb3. A unit enters through receptor mediated endocytosis.
A goes to ribosome and attacks one of the RNAs in ribosome- cleaves a specific adenine base in 28S rRNA.
Stops translation, therefore cell renewal- if cells can’t renew, it causes problems.
This is an AB toxin.
Describe AB toxins.
A subunit and B subunit pentamer.
B is binding subunit- binds to host cell: activates receptor mediated endocytosis when it docks at receptor. Delivers A subunit to cell. Often 5 B subunits form a pore for A entry. Sometimes B is taken up by the cell as well, instead of being a pore for A.
A is activity subunit-has toxic activity: Many are ADP-ribosyltransferase enzymes eg. diptheria toxin, cholera toxin.
What do ADP-ribosyltransferase enzymes do?
Modify protein structure and function by sticking ADP ribose onto protein- changes activity of target protein.
How does Diphtheria toxin disrupt protein synthesis?
Goes for protein instead of RNA.
Whole AB unit taken up by receptor mediated endocytosis and kept in a vacuole.
Vacuole acidifies- change in pH causes A to separate from B and escape.
A sticks ADP ribose to elongation factor 2, therefore ribosylates it. This stops protein synthesis as ribosome function is blocked, and the cell dies. (A pseudomembrane forms over trachea).
Dip is an unusual bacteria- only lives in pharynx, and toxin spreads through the bloodstream. An inactive form is used in immunisation.
What are some examples of toxins that disrupt second messenger pathways and how do they work?
ETEC stable toxin- 20 amino acids with resistance to heat.
Works outside the cell. Binds to receptor adenylate cyclase and affects cGMP production- makes it produce a lot of cGMP, which interferes with ion flow. Results in altered electrolyte transport.
Cholera toxin and ETEC labile toxin- work inside the cell. Affect cAMP production, which also results in altered electrolyte transport.
What is the normal method of regulation of human adenylate cyclase?
Hormone attaches to receptor, which is associated with G-factor complexes- have common beta gamma subunits, but have a distinct alpha subunit (Gs, which stimulates, and Gi, which inhibits). G factor promotes signalling in humans. Both alpha subunits can associate with beta and gamma.
When hormone receptor is stimulated, GTP binds Ga subunit, Ga-GTP binds adenylate cyclase- either stimulated in Gs or inhibited in Gi. cAMP (second messenger) production goes up or down depending on Ga subunit.
Intrinsic GTPase degrades GTP to GDP.
Cholera toxin interferes with this pathway.
