Infection Flashcards
Where do we have normal flora?
Skin, Vagina, Upper and Lower Bowel, Nasopharynx
What important organisms make up the flora of the nasopharynx?
- Streptococci
- Haemophilus
- Neisseria
- Mixed anaerobes
- Candida (fungus)
- Actinomyces
What important organisms make up the flora of the skin?
- Staphylococci
- Streptococci
- Corynebacteria
- Propiniobacteria
- Yeasts
What important organisms make up the flora of the upper and lower bowels?
• Upper Bowel - Enterobacteriaceae - Enterococci - Candida • Lower Bowel - Clostridium - Bifidobacteria - Bacteroides - Peptosteptococci
What important organisms make up the flora of the vagina?
- Lactobacilli
- Streptococci
- Corynebacteria
- Candida
- Actinomyces
- Mycoplasma hominis
What are the components of the bacterial cell envelope?
- cytoplasmic membrane
- cell wall
- capsule
- flagella
- pilli
What are the properties of the bacterial cytoplasmic membrane?
- Functions to synthesise and export components of the cell wall
- functions in respiration, secretion of extracellular enzymes and toxins, and uptake of nutrients by active transport mechanisms.
What are the properties/functions of the bacterial cell wall?
- protective against osmotic or mechanical stress
- barrier against certain toxic chemicals
- often the source of antigens
Describe the Gram+ and Gram- cell envelopes
- Gram+ (2 layers) contains a plasma membrane and a thick peptidoglycan layer. There can also be additional carbohydrates and proteins depending on the species of organism.
- Gram- (3 layers) contains a plasma membrane, a thin peptidoglycan layer and an outer membrane (which contains outer membrane phospholipids and LPS)
What are the properties/functions of the bacterial capsule?
- Contains capsular K antigens
- Protects against phagocytosis
- Helps with adhesion
- used in laboratory diagnostic tests
Describe the process of Gram staining
- Methyl violet (blue) + Lugol’s iodine is added. This is taken up by both Gram+ and Gram-
- Acetone is used to decolourise, by removing the stain from the outer membrane of the Gram-
- Methyl red (paler pink) is used to colourise the Gram- bacteria red.
Describe the classification of Gram+ cocci that form clusters
Clusters (catalase+):
- Staphylococcus Aureus (coagulase+)
- Staphylococcus epidermidis and saptrophyticus are coagulase-
Describe the classification of Gram+ cocci that form chains
B-haemolysis:
- Group A Strep = Streptococcus Pyogenes
- Group B Strep = Streptococcus agalacitae
- Group D = Enterococci
A-haemolysis:
- Streptococcus Pneumoniae (optochin resistant)
- Viridans Step
Describe the classification of Gram+ rods
Aerobes:
- Spores = Bacillus Athracis or Bacillus cereus
- Non spores: Listeria is motile in room temperature. Corynebacterium and Pallisaids
Anaerobes:
- Spores = Clostridium species such as botunilium, difficile and tentani.
- Branching = Acintomyces
What infections are caused by Staphylococcus Aureus?
- skin/soft tissue infections
- endocarditis
- osteomyelitis
- Toxic Shock Syndrome
What infections are caused by Streptococcus Epidermidis and Saprophyticus?
skin commensals of low pathogenic potential. However, are usually the cause of line, pacemaker and prosthetic infections.
What infections are caused by Staphylococcus Pyogenes?
- Causes quinsy (a painful puss-filled inflammation of the tonsils and the surrounding tissues)
- Causes cellulitis (inflammation of tissue bellow the skin)
- Toxic Shock Syndrome
What diseases are caused by Bacillus Anthracis?
- Cutaneous (Hide porter’s disease) - black eschar, oedema and swelling
- Gastrointestinal
- Inhalational
What are the components of the innate immune system?
- Physical barriers such as skin and mucociliary escalator
- Cellular barriers
- Circulating effector leucocytes such as monocytes/macrophages, neutrophils and NK cells
- Circulating components such as complement, collectins and pentraxins, and antimicrobial peptides
- Commensal organisms (microbiome)
- Cytokines
- Local enzymes
How are NK cells activated?
Natural Killer cells are specialised T-cells. They are stimulated by IL-12 and IL-15. They are activated by Syk and Zap70 protein kinases.
They kill infected and malignant cells particularly if lost normal MHC Class I molecule.
What is the function of NK cells?
- They kill infected and malignant cells particularly if lost normal MHC Class I molecule.
- secrete INF-y, which is an important activator of macrophages.
How do phagocytes recognise pathogens?
Phagocytosis is carried out by macrophages/monocytes and neutrophils. They detect pathogens through Pattern Recognition Receptors (PRR), which recognises molecular structures produced by microbial pathogens, called PAMPs (Pathogen Associated Molecular Patterns).
Phagocytes can also recognise and phagocytose opsonised pathogens, either by complement (especially C3) or antibodies. Phagocytosis usually involves ingestion into a vesicle that later becomes a phagolysosome.
What are the functions of cytokines?
- pro-inflammatory
- chemoattractant
- Regulatory (activates adaptive immune response)
- Down-regulatory
What are the important pro-inflammatory cytokines?
TNF-a
IL-1
What are the important chemoattractant cytokines?
IL-8
What are the important regulatory cytokines?
INF-y
IL-12
What are the important down-regulatory cytokines?
Il-10
TGF-b (Transforming Growth Factor beta)
What are the main receptor types of the innate immune system?
- PRRs
- Complement Receptors
- Fc Receptors
- NK Receptors
What are the main types of PRRs?
- Toll-like Receptors
- Nod-like Receptors
- Rig-like Receptors
- C-type Leptin-like Receptors
What do NLRs detect?
Bacterial cell wall peptidoglycan
What do RLRs detect?
Viral RNA
What do CLRs detect?
Mannose and fructose
Describe the TLR
Transmembrane receptors that consist of an extracellular recognition domain and an intracellular TIR (Toll-interleukin receptor) domain.
The TIR recruits MyD88 or TRIF adaptor proteins.
What TLRS recognise bacterial lipids?
TLR1, 2, 4 and 6
What TLRS recognise viral RNA?
TLR3, 7 and 8
What TLRS recognise bacterial DNA?
TLR9
What TLRS recognise bacterial and parasite proteins?
TLR5 and 10
What TLR detects LPS?
TLR4
What are the anatomical and physiological barriers?
- Skin
- Mucocilliary escalator
- Low pH environment
What are the phagocytic cells?
- Neutrophils
- Macrophages
- Dendritic cells
What are the states a macrophage can exist in?
- resting
- primed
- hyperactivation
Describe the role of the neutrophil
Engulf and breakdown pathogentic components via oxygen dependent and independent killing mechanisms.
Also release cytokines as well as other destructive chemicals.
Describe the resting macrophage
‘garbage collectors’ with few MHC Class II molecules
Describe the primed macrophage
Upregulation of MHC Class II and macrophages acting as APCs following detection of INF-y from Thelper and NK cells
Describe the hyperactivated macrophage
following direct PAMP detection, the rate of phagocytosis increases and the macrophage grows larger. They also produce IL-1, TNF-a, IL-6, IL-8, IL-12
What are the different principles of cytokine function?
- pleiotropism
- antagonism
- synergism
- redundancy
How does TNF-a cause a pro-inflammatory state in the cell?
TNF-a induces phosphorylation of IkB, activating NF-kB
What are the complement activation pathways?
- Classical pathway (antibody-antigen)
- Alternate pathway (foreign surface of a pathogen)
- Lectin pathway (pathogen-specific sugar patterns)
Describe the classical pathway of complement activation
Initiated by C1 complex binding to IgM and igG
Describe the lectin pathway of complement activation
Initiated by MBL and ficolin binding to carbohydrate complexes or PAMPs that are associated with MASP-1 and -2 proteases to activate the complement
Describe the alternative pathway
Spontaneously activated C3 binding to the surface of a pathogen
Describe the formation of the MAC after convergence of the pathways
C3-cleaving enzymes forms C3b, which produces C5-cleaving enzyme, forming C5b.
C3a and C5a act as inflammatory mediators. C3b and 5b remain on the pathogen forming MAC.
What are the functions of the complement system?
- host-defence against infection through MAC and opsonisation
- waste-disposal: clearance of immune complexes from tissue via RBCs and by the clearance of apoptotic cells via the classical pathway
- link between the innate and the adaptive immune response since complement receptors.
What are the functions of TNF-a?
- pro-inflammatory state through NF-kB
- mediates response to LPS
- causes cachexia and septic shock
- ## depresses muscle (and cardiac function)
How does IFN binding change the cell?
INF binds to a transmembrane INF receptor which heterodimerises. Through a series of series of phosphorylation events Tyk2 and Jak1 phosphorylate each other, as well as other proteins in the cytoplasm such as STAT proteins.
The STAT proteins enter the nucleus and bind to the promoter regions of ISGs. These promoter regions are called the Interferon Stimulated Response Elements.
What infections do IFN deficient people suffer from?
mycobacterial infection. They are also susceptible to salmonella.
What do people with late complement deficiency benefit from?
Milder invasive meningitis infections
What proteases does TB use to cause damage?
In tuberculosis MMPs drives tissue destruction. MMP1 is very destructive in causing GRANULOMAS and destruction.
What molecules produced by the innate immune system kills pathogens?
- reactive oxygen species
- reactive nitrogen species
- lysosomal enzymes
- Proteolytic enzymes
- Microbiocidal peptides
What are the portals of entry for infection?
- Skin
- Respiratory tract
- Faeco-oral transmission
- Urogenital tract
What are the ways bacteria can attach?
- Non specific electrophilic interactions
- Tethering via projections (fimbriae) or pili
- Attachment via special receptors on human cell surfaces
- Internalisation into epithelial cell
What are the different ways bacteria can penetrate?
- Artificial penetration e.g breech of epithelium via trauma
- Entry into and through cell e.g salmonella spp.
- Transit between cell layers e.g S. aureus
How does Staphylococcal Scalded Skin Syndrome develop in infants?
S. aureus produces an exofilative toxin, which cleaves desmosomes resulting in fluid filled blisters.
How do we usually clear gram- infections?
Most Gram- bacteria can be destroyed by complement.
What infections are people who are deficient in complement susceptible to?
Meningococcal infection
What Gram+ components activate the immune system?
Peptidoglycan and Lipoteichoic acid
What Gram- components activate the immune system?
LPS (endotoxin)
What does TLR2 detect?
Peptidoglycan and Lipoteichoic acid
What does TLR4 detect?
LPS
How does activation of TLRs elicit a response?
Through adaptor proteins MyD88 and TRIF –> activate NF-kB –> release of cytokines which act as chemoattractants (IL-8), IL-1, TNFa, and activate complement (important in Gram- infection). –>
Neutrophil activation, endothelial activation and vasorelaxation. (net effect)
Release of granules, antibacterial peptides, and metalloproteinases.
The endothelium releases NO which produces massive vasodilation
What does S. aureus produce to cause toxic shock syndrome?
Staphylococcus aureus produces TSST-1 (Toxic Shock Syndrome Toxin-1), staphlyococcal enterotoxin A, B and C.
How do S. aureus and S. pyogenes cause toxic shock syndrome?
Production of superantigens which cause massive TNF-a release. Leads to hypotension, shock, multi-organ failure and death.
Where do superantigens bind?
TCR and MHC Class II receptors on CD4 R-cells
What are the specific ways Streptococcus pyogenes evades the immune system?
- Anti-neutrophil strategies e.g SpyCEP, C5a peptidase, streptolysin
- Anti-opsonisation strategies e.g M protein, Ig binding proteins, Ig cleavage, complement binding
- Toxins which interfere with immune responses e.g superantigens.
What are the specific ways Staphylococcus aureus evades the immune system?
- Anti-neutrophil strategies e.g Panton Valentine Leukocidin
- Anti-opsonisation strategies e.g Ig binding proteins (protein A)
- Toxins which interfere with immune responses e.g superantigens
Describe the anti-neutrophilic action of Panton Valentine Leukocidin
PVL factor is encoded into the S. aureus chromosome and its genes lead to the formation of two toxins. LukS and LukF, which form heptametic pores on cell membranes of host defence cell, in particular neutrophils.
List the causes of variability in the body’s response to bacteria
- general health and nutritional states
- previous infection (immunological memory)
- a lack of immune deficiency: can be genetic (SCID), acquired (HIV) or induced (steroids)
- polymorphisms
Why are commensal microbiota useful?
- essential for tissue integrity
- essential for immune system
- innate immune system includes commensal bacteria (to outcompete pathogens)
What evidence do we have that commensal microbiota are essential for tissue integrity and immune system function?
- Bacteria colonising specific locations have a ‘site-specific fingeprint’ - similar characteristics
- Bacteria increase the variability of response
- Mice studies show that deleting commensal bacteria leads to a developmentally defective immune response
- Microbial composition is upset in COPD, which leads to abberant response to allergens.
What are the different time-scales in the resolution of viral infection?
- Acute viral infection and clearance (e.g by the Flu)
- Acute viral infection and clearance, with re-infection (e.g by RSV) - unknown mechanism of how the memory is lost.
- Acute viral infection and chronic infection (e.g by HIV)
- Slow chronic infection (e,g by CMV)
What two factors determine weather a disease spreads?
- Basic reproductive rate
2. Proportion of infections (being infectious) that occur before becoming symptomatic
What are the mechanisms in place to prevent infection at mucosal epithelia?
- Mucus: produced by goblet cells, traps dust and pathogens, which are ingested and destroyed by –>
- Proteolytic enzymes: in the stomach (pepsin) and small intestine (trypsin, chymotrypsin, and pancreatic proteases) are cytotoxic to pathogens.
- Antimicrobial molecules are produced to specifically target pathogens. Such as lactoferrin, which binds to iron, and inhibits bacterial growth, preventing them from accessing them. Lysozyme cleaves the cell wall of Gram+ bacteria. Defensins are peptides that disrupt the cell membranes of bacteria and fungi causing lysis.
- Commensal organisms compete with pathogenic bacteria for space and nutrients. Not only in the GIT but also respiratory and genitourinary tract. In Clostridium difficile infections, these tend to occur when large doses of antibiotics kill GIT commensals, allowing C. diff to proliferate. Foecal transplants may be used to re-introduce commensal organisms
Describe antigen presentation in nucleated cells
Proteins made in the cell are degraded by proteosomes into peptides. These go through a TAP channel. then endoplasmic reticulum. Peptides get associated with MHC I at ER
Describe antigen presentation in APC cells
Exogenous bacterial material endocytosed and moved into lysosomes. The acidic pH cleaves invariant peptide on MHC-II, so bacterial material can bind with MHC-II instead.
What are the three signal stages in the priming of T-cells?
Stage 1: MHC-TCR
Stage 2: CD80/86-CD28 (on CD4 cell)
Stage 3: various cytokine signalling by APC cell
What are the viral passive immune evasion mechanisms?
- Antigenic drift and shift leading to epitope escape
- Anatomical seclusion/hiding
- Infection of immune cells
What is the difference between antigenic sift and drift?
Antigenic drift is due to a high mutation rate in surface proteins.
Antigenic shift is the acquisition of whole new genes.
What are the viral active immune evasion mechanisms?
With the innate immune system:
- meddling with cytokine/chemokine network
- avoiding type 1 IFN
- alteration of cell cycle/ inhibition of apoptosis
- blockade of complement
Can inhibit NK cells
With the adaptive immune system:
- Interfering with MHC
- Inhibition of antibody
How does HIV evade the immune system?
- surface proteins are coated in carbohydrates
- RNA genome undergoes rapid change
Compare how you would look for disease risk genes vs disease causing genes
- GWAS (genome wide association studies) are cheap and widely used to identify SNPs, which only have a modest impact in susceptibility
- Next Generation Sequencing is able to identify rare variants of genes which have a much greater impact on the resulting phenotype.
What is the human model of genes that predispose to infectious disease?
- Mendelian defects leads to infections that take place in early childhood as a child encounters new pathogens.
- Polygenic predisposition sees infection in later life.
Give three single in-born errors of immunity and the specific infectious diseases they predispose to.
- complement deficiency = meningococcal infection
- MyD88/IRAK-4 = invasive pneumococcal infection
- TLR3 = herpes simplex encephalitis
What does invasive pneumococcal disease cause?
Pneumonia, Septicaemia and Meningitis
What increases your risk in developing invasive pneumococcal disease?
- MyD88/IRAK-4 deficiency
- not having a spleen
- B cell deficiency
- Antibody deficiencies
- Certain complement diseases
Why are IRAK-4 and MyD88 deficiencies clumped together?
IRAK-4 and MyD88 deficiencies are clumped together as they have the same phenotype. This is because they are protein kinase systems that depend on each-other. All types of deficiencies have no expression or the protein, or non-functional protein.
What do MyD88/IRAK-4 deficient patients present with?
Invasive bacterial infection (tends to be pneumococcal):
- deep inner organ abscesses, arthritis or osteomyelitis, sepsis and meningitis.
What is the timeline of infection caused by Herpes Simplex Virus?
Herpes Simplex Virus is an enveloped virus with dsDNA and causes lytic or latent infection. First time we encounter it causes lytic infection. It will then cause a more latent infection in the terminal ganglion. When it reactivates, you see a cold sore in the exact same spot where the primary infection occurred.
What are the symptoms of HSV lytic infection?
(90% asymptomatic). The rest:
- Labialis
- Gingivitis
- Gingibostomatitis
- Whitlow
- Keratitis
How can TLR3 deficiency be tested?
The phenotype is tested by sprinkling dsDNA on the skin of patients; deficient patients do not produce type 1 or type 3 INFs nor IL-6.
What virulence factors does S aureus have?
- fibrontectin-binding proteins
- capsule
- haemolysin and leukocidin
- superantigens
Describe the classification of Aerobic Gram- rods
Simple Growth Requirements:
- Lactose fermenters: E.coli; Klebsiella
- Non-lactose fermenters: salmonella, shigella and proteus
- Oxidase +ve: pseudomonas requires O2, vibrio is curved
Complex Growth Requirements:
- Bordatella
- Haemophillus
- Brucella
- Legionella
What bacteria stain poorly by gram stain?
- mycobacteria (because acid-fast)
- mycoplasma (lack cell walls)
- spiral bacteria
- intracellular
Define the components of the Sepsis continuum
SIRS (Systemic Immune Response Syndrome) is defined as two of the following:
- Temperature of >38 or 90bpm
- Resp rate >20/min
- WBC count >12000/mm, or 10% neutrophil
SEPSIS is SIRS comfirmed with infectious process. Severe sepsis involves organ failure, septic shock is sepsis with hypotension.
What are the components of LPS?
- Lipid A
- Core Region
- O-antigen polysaccharide
How does Lipid A trigger an immune reponse?
Part of LPS which is detected by TLR4
Lipid A directly triggers complement, coagulation, fibrinolytic and kinin pathways
What is the role of the vascular endothelium in immunity?
- Interaction with leukocytes
- Release of cytokines and inflammatory mediators
- Release of mediators of vasodilation and vasoconstriction
- Functional effects of the coagulation system
What are the effects of sepsis on the cardiovascular system?
- Hypovolaemia
- Hypotension
- Shock
Why can there be renal failure in sepsis?
- hypovolaemia
- hypotension
- intrinsic vasoconstriction
- acute tubular necrosis
Outline the Sepsis 6
- Administer high-flow oxygen
- Take blood cultures and consider source
- Administer IV antibiotics
- Give IV fluids
- Check haemoglobin and serial lactates
- Commence hourly urine output measurements
Outline the surviving sepsis campaign
Within 3 hours:
1) measure lactate
2) obtain blood cultures
3) administer broad spectrum antibiotics
4) administer 30ml/kg of crystalloid for hypotension
Within 6h:
5) Apply vasopressors (for hypotension that does not respond to fluid)
6) In event of shock (persistent hypo) measure central venous pressure and oxygen saturation
7) Remeasure lactate if initial lactate was elevated
