Bacterial pathogenesis Flashcards
Where are our normal microbiota normally found and what are their functions? (4)
- Restricted to our body surfaces (external facing surfaces and membranes)
- Provide nutrient
- Protective against infection from other microorganisms by competition
- Acquired at birth
Difference between normal microbiota and pathogen? (2)
- Pathogens express virulence factors, normal microbiota don’t.
- Pathogens can mediate adhesion, anti-phagocytic, damage
When was helicobacter pylori discovered and what does it cause? (2)
- First cultured in 1982 by Barry Marshall and Robin Warren
- Associated with gastritis and is leading factor in pathogenesis of peptic ulcers and gastric cancer.
What must pathogens do to cause disease? (5)
- Enter the body
- Colonise the host
- Evade host defences
- Multiply and disseminate
- Cause damage to host
What are some physical barriers pathogens must overcome? (5)
- High salt, fatty acids, dryness of skin
- Acid in stomach
- Mucous and cilia in lower respiratory tract
- Peristalsis of GIT
- Soluble mediators like lysozyme in tears
What are some barriers of the innate immune system pathogens must overcome? (3)
- Complement system
- Phagocytosis (neutrophils/macrophages)
- Inflammation (triggered by TLC, cytokines via recognition of bacterial products)
What does colonisation of the host involve? (2)
- Pathogens establish themselves within the host by overcoming physical barriers and innate immune response, outcompeting normal microbiota.
- Normally requires adherence to mucosal surface (host specific and tissue specific).
What are the steps of bacterial adherence? (3)
- Loose attachment to mucosal surface via pili or fimbrae
- Close adhesion to epithelial cells via afimbrial adhesins
- Invasion of epithelial cells
Describe the structure of pili/fimbriae and how it functions to adhere to host cell. (3)
- Rod shaped, hollow cylinder, ordered helical array of protein subunits of pilin
- The tip of the pilus (pilin or other protein) mediates adhesion
- May be all over cell (peritrichous) or polar
What can pathogenic E.coli cause?
- Normal microbiota of human digestive system that can cause digestive infections (diarrhoea), UTI, septicaemia or meningitis.
How does enterotoxigenic E.coli (ETEC) produce disease? (5)
- Colonisation mediated by specific pili on ETEC called colonisation factor antigen (CFA).
- It allows bacteria to attach to host cell but not invade epithelial cells.
- ETEC then produces heat-liable toxins (LT) or heat-stable toxins (ST) or both types of enterotoxins which target epithelial cells of small intestine.
- The toxins stimulate secretion of electrolytes into the lumen of the gut and H20 follows cuasing diarrhoea.
- Toxins are plasmid encoded so can undergo horizontal gene transfer.
Describe an afimbrial adhesin. (3)
- Establish close association with host cell
- Usually membrane-embedded proteins (typically cell envelope component-gram positive or outer membrane proteins- gram negative or a capsule)
- Can bind to protein or carbohydrate receptors on host cell
Describe a capsule and its function in adherence. (3)
- Made of polysaccharide (or sometimes protein), well organised, difficult to wash off, occur in both types of bacteria.
- Can help prevent desiccation and can help avoid phagocytosis.
- It is sticky so aids in adherence
What is dental plaque and how does it form? (3)
- Biofilm that develops on the surface of teeth.
- Formation initiated by attachment of salivary proteins followed by attachment of bacteria.
- Provide anaerobic environment for some bacteria to flourish.
Reasons to invade host cell? (4)
- Obligate intracellular parasite
- Hide from immune system
- escape from normal microbiota
- disseminate to deeper tissues
What are invasins and what is their function? (3)
- single proteins or complexes of proteins
- Binding to host cell receptor causes internalisation of the bacteria
- Can be passive or active uptake
How does listeria monocytogenes cause disease? (2)
- Through zipper mechanism, binding of internalin to host cell receptor (cadherin) causes clustering of host receptor and formation of phagocytic cup.
- This bacterium invades the intestinal epithelial cells
How does salmonella typhimurium cause disease? (2)
- Through trigger mechanism where it directly injects invasins into the host cell to activate membrane ruffling (SipA and SipC) and then uptake of bacteria by macropinocytosis and endomal trafficking occurs.
- It invades intestinal epithelial cells and intestinal macrophages.
How is the membrane attack complex formed and what does it do? (2)
- End result of complement activation/fixation.
- Kills bacteria by puncturing the cell membrane.
What are some ways bacteria avoid complement fixation? (2)
- Use of capsule prevents binding of complement proteins.
- Bind antibody by Fc end of antibody so complement can’t bind to it.
What is host mimicry and provide an example of a bacterium that uses it. (3)
- Displaying of similar surface sugars.
- N-meningitidis type B has a capsule made up of sialic acid which is a common component of our cell surface sugars.
- The bacteria flourishes and releases huge amounts of LPS causing septic shock
What is a privileged site and what happens when it is colonised? (2)
- A sterile site that has poor access to immune system.
- Colonisation here can form granuloma (M.tuberculosis) or a biofilm (Salmonella Typhi)
What does oxygen dependent killing involve? (2)
- Killing bacterium using reactive oxygen species generated by NADPH oxidase
- Occurs on the membrane of vacuole
What does oxygen independent killing involve? (5)
- Low pH (lysosome)
- Proteolytic enzyme
- Lysozyme
- Lactoferrin
- Membrane damaging proteins
What is major impediment of pathogen? (2)
- The normal microbiota poses as competition for space and nutrients.
- Since pathogenic bacteria are chemoheterotrophs, they require iron to survive
What does dissemination of pathogen mean?
- Escape from normal microbiota/immune system and travel to immune privileged or other body sites to open up new sources of nutrients
What aids in dissemination pathogen? (2)
- Virulence factors such as collagenase and hyaluronidase, elastase, protease, phospholipase
- Travelling in bloodstream inside phagocytes or just by itself (serum resistance bacteria)
What are some mechanisms of iron uptake? (4)
- Siderophores are high affinity iron chelators (hydroxamate or catecholates)
- Iron binding proteins (high molecular weight membrane proteins)
- Receptors for iron binding proteins of the host (transferrin and lactoferrin receptors)
- toxins ( haemolysins that release iron from intracellular stores).
What is an exotoxin and list a few examples. (2)
- A secreted toxin, usually protein (often in vaccines)
- A-B toxins (simple VS compound), membrane disrupting toxins (pore forming, enzymatic), superantigens (immunopathology)
What is A-B toxin? (3)
- A= catalytic active subunit
- B= binding subunit
- Linked by disulphide bond
What is an example of a simple AB toxin and how does it work? (2)
- Diphtheria toxin secreted by corynebacterium diphtheriae which colonise our upper respiratory tract and back of our throats.
- It has an ADP-ribosyltransferase as its catalytic subunit which transfers ADP-ribose groups onto target protein (EF2) and halts protein synthesis thereby killing the cell.
What is an example of a compound AB toxin and how does it work? (5)
- Cholera toxin secreted by vibrio cholerae which ultimately results in fluid lost causing diarrhoea.
- It binds to GM1 ganglio site receptors on surface of host cell membrane.
- Once in vacuole, low pH causes cleavage of A subunit into A1 and A2.
- A1 subunit (an ADP-ribosyltransferase) causes ADP-ribosylation of GTPase resulting in GTPase being locked into the “on” form.
- This causes increased levels of adenylate cyclase activity and increased levels of cAMP in cell leading to secretion of ions into lumen.
How does the simple neurotoxin botulinum toxin work? (3)
- Botulinum toxin secreted by clostridium botulinum binds to synaptotagamin 2 receptor on neuronal cells resulting in flaccid (no muscle tension) paralysis.
- The bacteria colonises anaerobic environment (eg. tinned food) and ingestion of active toxin causes disease.
- Causes cleavage of SNARE proteins so acetylcholine can no longer bind.
How does the neurotoxin tetanus toxin work? (2)
- Tetanus toxin secreted by clostridium tetani causes spastic (rigid/unable to release muscle contraction) paralysis.
- It binds to inhibitory interneurons preventing the release of glycine and relaxation of muscle which causes diregulation in the release of acetylcholine into the synaptic junction.
How does an enzymatic lysis work? (2)
- Membrane disrupting toxin
- Anaerobic bacteria, clostrisium perfringens release alpha toxin (phospholipase C) to hydrolyse phosphatidylcholine in target host membrane.
- This leads to tissue necrosis and production of gas and swelling in tissues as well.
How does cholesterol-dependent cytolysins work? (2)
- Membrane disrupting toxin of gram positive pathogens.
- Inserts beta barrel pore into host cell membrane and functions similarly to membrane attack complex.
What is an example of small-pore forming toxin and how does it work?
- alpha haemolysins from staphylococcus aureus inserts into host cell membrane resulting in tissue necrosis and pus.
What pathogen produces superantigens and how do they work? (3)
- S.aureus and S-pyogenes
- It links MHC 2 and TCR non-specifically (no antigen or presentation where there normally should be) causing massive overactivation of the immune system. eg. release of cytokines increases
- Results in organ failure and shock
What is an endotoxin? (2)
- Structural component of pathogen that is embedded in outermembrane (not free floating)
- In gram negative cell envelope (LPS)
What does LPS cause?
- Fever, massive inflammatory response, release of IL-1 and TNF.
What results from severe infection? (2)
- endotoxic (septic) shock
- can result in vascular collapse and multiple organ failure
What are some examples of indirect damage to host cells? (4)
- Allergic reactions (mediated by IgE)
- Formation of immune complexes
- Autoimmunity (host cell mimicry)
- Cell mediated damage (cell death, granulomas)
What are some ways bacteria become pathogens? (4)
- Plasmids
- Bacteriophages
- Transposons
- Pathogenicity islands
What does enterohemorrhagic E.coli (EHEC) cause and how does it cause disease? (2)
- Cause food-borne diarrhoea, bloody diarrhoea and haemolytic uraemic syndrome (HUS)
- Encodes virulence factors including Shiga toxin (Stx) which causes the bloody diarrhoea/HUS
Difference between bacteriocidal vs bacteriostatic.
- Kills bacteria
- Prevent bacterial growth
What are properties of magic bullets?
- Ideal properties of antimicrobials include: selectively toxic for microbes, broad spectrum of activity (target both gram +/-), kills the microbe, non-toxic to host, long half life, good tissue distribution/penetration, oral and parenteral forms, no interference with other drugs
What are ways to test for minimum inhibitory concentration? (3)
- disc diffusion
- broth serial dilutions
- e-tests
What do antibiotics target? (4)
- cell wall synthesis
- protein synthesis
- DNA synthesis
- cell membrane function
What microbe does penicillin come from and how does it work? (5)
- Produced by penicillium chrysogenum
- Inhibits peptidoglycan synthesis by binding to the penicillin binding proteins (good against gram + but less active against non-dividing cells)
- Not active against bacteria without peptidoglycan (mycoplasma) or who have an impenetrable cell wall (mycobacteria)
- Bactericidal
- Toxicity through allergy
What antibiotic inhibits cell wall synthesis? (2)
- B-lactam antibiotics
- penicillins, cephalosporins, carbapenems
What are ways which pathogens can resist B-lactam antibiotics? (3)
- Alteration in target sites (PBPs)
- Alteration in access to target site (porins)
- Production of B-lactamases (but can be inhibited by clavulanic acid which prevent bacteria form destroying amoxycillin)
Which microbe produces aminoglycosides and what does it do? (5)
- Streptomyces spp a gram positive filamentous soil bacteria
- They are amino acid/sugar complexes which inhibit protein synthesis. eg. streptomycin, kanamycin, neomycin
- They interfere with fmet-tRNA binding to ribosome (30S subunit function)
- Bactericidal
- Not well absorbed in gut so uses IV
What are ways which pathogens can resist aminoglycosides? (4)
- efflux
- Modified outer membrane leading to reduced entry
- ribosomal mutation leading to reduced binding
- enzymatic modification leading to reduced entry
Which microbe produces chloramphenicol and how does it work? (5)
- Produced by streptomyces spp
- Targets translation by binding to 50S subunit and blocking action of peptidyl transferase, preventing peptide bond formation
- Bacteriostatic
- Has high affinity for bacterial peptidyl transferase (compared to the mammalian enzyme)
- Can be used to treat Salmonella Typhi but could result in toxic affects on bone marrow which could lead to aplastic anaemia.
What are ways which pathogens can resist chloramphenicol?
- Acquisition of a resistance gene encoding inactivation mediated by chloramphenicol acetyl transferase which adds acetyl group to the drug rendering it unable to bind to target.
What are 3 ways of inhibiting nucleic acid synthesis?
- Inhibit synthesis of nucleotide precursor
- Inhibit DNA replication
- Inhibit RNA polymerase
What substance can inhibit synthesis of nucleotide precursor and how does it do that? (3)
- Sulfoamides a synthetic sulfa drug that is an analogue of PABA.
- It competes for active site of dihydropteroate synthetase preventing PABA being converted into the next molecule in the pathway and so inhibits folic acid production.
- bacteriostatic
List the steps (4) in synthesis of nucleotide precursors.
- PABA -> dihydrofolic acid -> folic acid -> purines and pyrimidines
What substance can inhibit DNA replication and how does it do that? (4)
- Quinolones is a synthetic substance which interacts with bacterial DNA gyrase blocking the DNA coling mechanism during replication.
- Broad spectrum activity
- Can be bacteriostatic (Nalidixic acid) or bactericidal (fluoroquinolones).
- Possibly toxic affecting cartilage growth
What substance inhibits cytoplasmic membrane function and how does it do that? (3)
- Polymyxins produced by gram +, is a cyclic peptide that act as a cationic detergent disrupting the PM.
- Active against gram negative outer membrane
- Bactericidal
Which plasmid encodes multiple resistance genes? (3)
- R100
- Affect uptake or inactivate drug
- Self trasmissible via conjugation
Where do mobile resistance genes come from?
- Spontaneous mutation in a gene with similar function (random, low frequency events)
- From microbe that make the antibiotic naturally
