Physiology of Infection Flashcards
Summarise the characteristics of the different types of pathogens (Viruses, bacteria, fungi and parasites)
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What are the components of bacteria?
Cytoplasmic membrane, cell wall, capsule, pili, flagella (movement), intracellular structures (nuceloid, ribosomes, inclusion granules, endospores)
What is the function of the cytoplasmic membrane in bacteria?
Made of proteins and PL, no sterols, synthesise and export cell wall components, respiration, secretion of EC enzymes and toxins, uptake of nutrients by active transport mechanisms
What is the function of the cell wall in bacteria?
Protective = rigid, so can withstand osmotic and mechanical stress, provides barrier against certain toxic chemical and biological agents; gives shape to bacterium; firm base for pilli, fimbriae and flagella; contains Ag which are used in virulence and host Ab production
What are the 2 types of cell wall?
Gram Positive -> 2 layers, THICK Ptg/Teichoic acids and inner plasma membrane; Negative -> 3 layers, THIN ptg layer and outer plasma membrane with LPS and outer membrane proteins
What is the function of the capsule of the bacteria?
Mucoid polysaccharide layer, consisting of polymerised D-glutamic acid; capsular Ag; anti-phagocytic activity and prevents attack by complement; adhesion; lab diagnostic tests; vaccines
What is the function of pilli in bacteria?
Sex pilli (transfer DNa by conjugation) and common pilli (attachment) -> adhesion and anti-phagocytic activity
What are the nutrients needed for bacterial growth?
C, N, H2O and source of energy; some need specific gases and growth factors; media developed to ID specific bacteria
What are the classifying features of bacteria?
Shape -> round, long curved, pairs, clusters, chains; Atmospheric requirement -> needs O2, tolerates O2, needs increased CO2 , O2 toxic; Spore production -> dangerous feature
Which tests can be used to identify bacteria?
Enzyme production -> Certain enzymes like urease, catalase, coagulase, oxidase; Toxin production -> protein synthesis inhibitors, haemolysins, superAgs; gram stain -> methyl violet and Lugol’s iodine followed by acetone then methyl red = GPB are violet/blue and GNB are red
How are Gram positive bacteria classified?
Based on shape the biochem test results and aggregation
How are GPB cocci further classified?
Staph differentiated based on coagulase test; strep by extent of haemolysis -> alpha further differentiated by optochin (toxic chemical), beta is Group A (S. pyogenes), B (S. agalacticae) and D (Enterococci); non-haemolytic = strep milleri group and anaerobic strep
Which infections are caused by staph?
S. aureus = Severe infections (skin/soft tissue, endocarditis and osteomyelitis); Coagulaste negative staph = skin commensals of low path potential, infest prosthetic material causing line/pacemaker infections and endocarditis
Which infections are caused by strep?
Group B = neonatal infection as 1/3 of women have group B strep in vagina; pneumonia by S. pneumoniae (found in airs on blood film and optochin sensitive)
What are the types of Gram positive rods?
Spores are dangerous feature -> 2 groups: Bacillus sp (B. cereus and B. anthracis) and Clostridium sp. (C. perfringens/tetani/botulinum/difficile)
What are endospores?
Contain bacterial DNA, cytoplasm, plasma membrane, ptg, very little water, dipicolinic acid and keratin-like coat; formed in response to adverse env conditions, resistant to chemical inactivation; function is protective, so survives drying, heat, dehydration and radiation
What is important about anthrax?
Chemical weapon -> prevented/treated by ciprofloxacin; causes Hide Porter’s disease, which is due to contaminated animal hides. Presents with black eschars, oedema and swelling
What are the different types of GNB?
NB: Listeria is a GPB.
What is present in the periplasm of GNB?
Hydrolytic enzymes, Antibiotic inactivator enzymes, oligosaccharides (osmotic pressure buffers)
What is Haemophilus influenza?
GN cocco-bacilli; found in nasal cavity and doesn’t cause much of a problem because most people are vaccinated against it
What is present in the outer membrane of GNB?
PL, LPS -> Toxic to humans, causes fever and endotoxic shock, consists of Lipid A (toxic part of endotoxin), core polysaccharide, O Ag
Which bacteria can be differentiated using oxidase test?
Oxidase -ve = Coliforms like E. coli, Klebsiella spp., Enterobacter spp., Seratia spp., Proteus spp. Oxidase +ve = Pseudomonas spp.
How do bacteria hang on?
Non-specific electrostatic interactions. Tethering via projections (fimbriae) or pili. Attachment via special receptors on human cell surfaces (e.g. fibronectin binding proteins). Internalisation into the epithelial cell
How do bacteria penetrate the epi/endothelial barriers?
Artificial penetration (breach of epithelium), entry into and through the cell, transit in between cell layers (S. aureus has exfoliative toxin that cleaves desmosomes leading to scalded skin syndrome
What is the innate immune response?
Recognition of pathogen components cause transcriptional changes that lead to cytokine production (TNFa and IL-1); NO is released during infection, leading to relaxation of blood vessels and inflammation
What are Toll-like Receptors?
Lipoproteins from bacteria detected by TLR2 (heterodimer with TLR1/6);
TLR4 detects endotoxins (LPS) for GNB infections;
TLR5 detects flagellin (some GNB) -> activation leads to signalling inside cell and transcriptional changes;
other PRRs are inside cells = NOD and other TLRs
What is the response to meningococcal infection?
Once PRRs activated they trigger release of inflammatory mediators and recruit innate immune cells. NB: many neutrophils in CSF, which is responsible for stiff neck in meningitis; complement is v. important in dealing with meningitis
How are GNB cleared from the body?
Ab opsonise the bacterium before phagocytosis; complements also opsonise; chemotaxis of neutrophils mediated by C5a. MAC (from complement activation) lyses some GNB; IgG (produced by vaccination)
How are GPB cleared from the body?
Don’t have LPS; complement can’t lyse GPB; must be phagocytosed -> IS triggered by ptg (TLR2 and NOD IC) and lipoteichoic acid. Bacterial DNA acts through TLR9. They are cleared via specific Ab opsonising, complement opsonising, chemotaxis of neurophils, clearance by phagocytosis then killing
Which pathogens trigger multiple PRRs and which PRRs do they trigger?
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What is gram positive Toxic shock?
S. aureus produces a superAg TSST-1;
superAg produce a more profound immune response ->
deceive the IS as they bind to MHC II outside of the Ag binding groove so around 20% of T cells see the sAg and think they are complementary to it, so proliferate ->
MASSIVE proliferation of T cells and production of cytokines, leading to TSS. Ab can neutralise the toxins
How does Strep pyogenes evade the immune system?
Anti-neutrophil strategies e.g. streptolysin Anti-opsonisation strategies e.g. complement binding Toxins that interfere with immune responses e.g. superantigens
How does Staph aureus evade the immune system?
Anti-neutrophil strategies e.g. PANTON VALENTINE LEUKOCIDIN Anti-opsonisation strategies e.g. Ig binding proteins Toxins that interfere with immune responses e.g. superantigens
How does N. meningitidis evade the immune system?
Anti-complement/Anti-MAC strategies e.g. capsule
What is Panton Valentine Leukocidin?
mecA gene confers resistance to beta-lactams; these strains found in hospitals but there are clones in the community that have acquired similar Abx resistance element and a phage that encodes PVL ->Toxin forms a heptamer (heptameric pore) which at low conc = makes human neutrophils apoptose, at high conc = makes human neutrophils lyse -> very nasty infections associated with abcesses that can’t be cleared by the pts
What is the function of innate immunity?
Prevents infection and can eliminate infection with/without interaction with acquired IR; early response to infection involving phagocytic cells; drives adaptive IR; no memory; recognises microbial conserved structures that aren’t found in host (PAMPs); no self reaction; targets products needed for microbial survival; germ line components
What are the components of innate immunity?
Physical barriers - skin, mucociliary escalator; cellular barrier (immune active); circulating effector leukocytes (monocytes/macrophages/neuts/NK cells); circulating proteins (complement, collectins and pentraxins (CRP), antimicrobial peptides (defensins)); commensal organisms (gut microbiome); cytokines; local enzymes
What are NK cells?
Specialised T cells, stim by IL-12 and IL-15; detect missing self and kill infected/malignant cells; secrete cytokines (IFN-gamma) which activate macrophages, activate receptors and signal via tyrosine kinase receptors (Syk and Zap70), inhibitory receptors have immunoreceptor tyrosine inhibitory motifs in cytoplasmic tail
What is phagocytosis and how does it work?
Cells -.> macrophages, monocytes and neutrophils, which have PRRs that recognise PAMPs; mannose receptors bind mannose and fucose receptors on microbial GP/lipids. Scavenger receptors = CD14 (no cytoplasmic tail; opsonisation by complement (C3!!) and Abs then ingestion into vesicles. Macrophages can disseminate mycobacterial infection
What are TLRs?
Give specificity to innate IS; recognise and discriminate specific microbial components. TLRs interact with various adapter molecules = CD14 and MyD88; activate proinflammatory cascade; act IC/EC
What are the types of cytokines?
Proinflammatory -> lead to release of many other cyto/chemokines = TNFa and IL-1; Regulatory -> activates adaptive immunity = IFNg and IL-12; Down-reg -> IL-10 and TGF-b; Chemoattractants -> Chemokines
What are cytokines?
Secretion is usually transcriptionally regulated and transient; para/endo/autocrine, systemic/local; bind to specific receptors; effector functions are multiple and cell specific, may act synergistically/antagonistically
What are the 4 main properties of cytokines?
Pleiotropism, synergism, antagonism and redundancy
What is innate immunity the balance of?
Destructive: Proinflammatory damage, hgih TNF, IL-1 ^^^ and MMPs; VS. Protective low TNF, IL-1 ^, IL-10 and TGF
What is TNFa?
Major source is mononuclear phagocytes; secreted early in infection; apex of cascade of proinflammatory cytokines; mediates LPS response; causes cachexia and septic shock; affects muscle function (depresses heart); drives met disturbance = decrease blood glucose and emerging interaction between met and immunity
What are TNF receptors?
2 main types that TNFa binds to -> p75 TNF and p55 TNF;
CD40 is also an important member of the TNFR family. CD40 member of TNF family;
some TNF receptors have DD which activates death signalling pathways involving caspases leads to cell death;
TNF can cause necrosis and apoptosis
What are NFkB transcription factors and how does the signalling pathway work?
Sits in cytoplasm as heterodimer of p65 and p50 bound to IkB; when athogen triggers IC signalling via receptors, IkB is phosphorylated, ubiquinated and degraded, allowing NFkB to pass from cytoplasm to nucleus, where it binds to binding sites which are nearly on all pro-inflammatory genes, with binding occurring in matter of minutes and is essential for some cyto/chemokines. NFkB and IkB have different forms; p65 alone is activating; p50 and p52 alone or with other molecule are inhibitory. IkBa is classical; IkBb binds to NFkB and keeps it in active state, so maintains NFkB as an active molecule
How are inflammatory genes regulated by transcriptional control?
TF binding sites near the genes, mRNA has regulatory sites (UA rich regions at 3’ end, providing stability) = more UA rich sequences means more proteins produced
How is IFNg important?
IFN -> IFNR -> phosphorylation of JAKs -> activation of STAT1 -> heterodimerisation to activate genes in nucleus. IFNg deficient pts are very susceptible to mycobacterial infections and salmonella -> can be caused due to deletions of IFNR, acquired AIAb.
Lesser clinical variants have IL-12 system mutations
What are chemokines?
They recruit leucocytes to areas of infection and inflammation;
important in angiogenesis. 2 groups (has 4 cysteine molecules but proximal 2 give name) = CC, usually recruit mononuclear cells (monocytes and lymphocytes = MCP-1 and RANTES) (2 adj cysteine residues) and CXC usually neutrophil attractants (IL-8 = neutrophil attractant with ELR leader sequence;
IP-10 = T cell attractant) (2 cysteine residues separated by another amino acid);
small family have just one cysteine residue = lymphotactin.
Can either be inducible or consitutively expressed
How are leucocytes recruited?
Roll along endothelial wall and if activated then binds to selectin molecules (loose binding) and then more tightly via integrins which transmigrate through the cells into the tissues
What is the role of IL-8 in sepsis?
Released by neutrophils and monocytes, present in animal models of sepsis, with raised IL-8 associated with poor prognosis; Ab protect in lung reperfusion injury
What is haplotaxis and how are chemokines involved in chemotaxis and adhesion?
Many chemokines attached to tissues but at different concentrations and the cells migrate down tissue-bound gradient; - anchored chemokines are potential adhesion molecules = chemokine bound to a cell or GAG and sits acting like adhesion molecule for cells; IL-8 upregs beta2 integrins. Chemokine receptor is GPCR (7-TMR)
How are chemokines involved in molecular mimicry?
Lot of chemokines mimicked by pathogens; HSV = ECRF3 which mimics IL8Rbeta; CMV US28 mimic ubiquitous chemokine receptor that bins MIP-1alphaR, RANTES and MCP-1
Why do viruses express chemokine receptors?
More chemokines = more spreading from one cell to next; expressing chemokine receptors on surface and acts as a sponge that soaks up chemokines and stops leukocytes being recruited to site of infection
What are the complement pathways and their effects?
Alternative (microbe activated), classical (Ab activated), lectin (lectin activated - MBL) -> early effects: inflammation (C3a), opsonisation and phagocytosis (C3b); late effects:inflammation (C5a), lysis of microbes (MAC)
How are MMPs involved in TB?
MMPs involved in leucocyte recruitment to sites of infection;
drive tissue destruction in TB (matrix degrading phenotype, required for spread of infection (pulm cavitation creating immune-privileged site).
acts in networks - granulomas in TB, MMPs as potential therapeutic targets -> TB infection in monocyte leads to: MMP-1 = Type 1,2,3 collagen (critical for tissue destruction) and MMP-9 = Type 4 collagen.
Shift to matrix degradation in TB;
Cavitation key feature in TB which results from upreg of protease activity leading to lung EC matrix destruction
What are the mechanisms of avoiding innate immunity?
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How does the innate immunity kill microbes?
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What are the 4 different types of resolution of infections?
RSV immuno memory wanes over time, so reinfection can occur by same pathogen.
For HIV there is acute phase but then virus suppressed by body but not totally cleared and viral load builds up slowly over time.
CMV = no acute phase, gradual build up of amount of virus in body over time
Why do some diseases spread?
2 main variables -> Basic reproduction number (how many people does one virus infect) an proportion of infections that occur PRIOR to symptoms (symptomatic before being infectious then quarantine subjects to prevent spread)
What are the 4 main routes of infection and the type of organisms that use each route?
NB: Resp infections are the most common type; GIT is also common site of infection
How does the immune response work?
Pathogens invade the epithelium and triggers the innate IS (is specific but recognises a smaller family of molecules than adaptive IS;
innate IR leads to Ag release which leads to bridging between the 2 arms of the immune response;
Ag are taken up by APC and shown to T cells on MHC
What are the 3 phases of the immune response?
The immediate phase is characterised by release of preformed molecules
How is the MHC class I loaded?
Virally infected cells produce protein ad some of these proteins will be misfolded - misfolded proteins tagged by ubiquitin and driven into the proteosome which processes the protein and produces smaller peptides;
peptides get pumped through TAP and loaded onto MHC I which translocates to the cell membrane
How is MHC class II loaded?
MHC II presents external proteins, with 2 IC bodies involved = MHC containing lysosome and phagolysosome containing phagocytosed proteins.
External proteins are taken up into phagolysosome and get chewed up into smaller peptides.
2 IC bodies merge and peptide loaded onto MHC class II by HLA-DM;
loaded MHC II then moves to cell membrane
Summarise the Ag presentation pathways to CD8 and CD4 cells
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What are CD4+ T helper cells?
CD4 cells amplify the IR by recruiting and activating other cell types by producing cytokines
What is the active immune invasion?
Bacterial/viral proteins can block various steps within the innate and acquired IR
How does HIV evade the immune response?
They target CD4+ T cells;
HIV also infect via APC;
HIV particles have coat proteins that are covered by carb to evade recognition;
epitopes in gp41 are masked (important in viral entry);
RNA genome undergoes rapid change both CTL and Ab are evaded - high mutation rate;
vaccination increases CD4 T cell infiltrate to the infected area, therefore increasing target cells for the virus
Summarise the factors that affect infection outcome after exposure to the microbe
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How does the pt progress to sepsis and severe sepsis?
- Endotoxin binds to LBP in circulation, with E-LBP complex binding to CD14 receptor on macrophages/monocytes,
- sending a message to the genome of the cell so it reacts in a particular way - CD14 doesn’t have transmembrane domain,
- so can’t directly talk to inside of cell, so binds to TLR4,
- which talks to the genome leading to transcriptional changes in the cell.
- Neutrophils are large cells which can get stuck and end up in alveoli, releasing damaging cytokines, causing leaky vascular endothelium so oedema occurs, also unreg release of NO happens causing vasodilation and drop in BP
What is a SOFA score?
The method of scoring sepsis in ICU, with qSOFA being carried out in GP/non-hospital settings -> if infection is suspected and SOFA is more than 2 OR there’s an increase of SOFA by 2.
What infections are caused by group A strep?
Invasive: affecting all gender/ages, with rapid onset; associated with all strains of S. pyogenes.
Most strains 5+ superAg genes; usually bacteraemic; 20-60% mortality
Tx: ITU, debridement, Abx, antitoxin Abx IVIg
What is spretococcal toxic shock?
Toxic shock = subset of invasive group A strep infections (10%), with 45% mortality rate; necrotising fasciitis associated with 7x increased risk
