Infection Flashcards
What is an infection?
Invasion of a hosts tissues by microorganisms AND Disease caused by: Microbial multiplication Toxins Host response
Neisseria meningitis is very easy to kill using a single dose of penicillin. Despite this, patients with meningococcal meningitis experience severe symptoms and may die from the disease. Why?
The patient continues to have an infection caused by the inflammatory response triggered by neisseria meningitis which persists even after they have been killed.
How do people get infections?
HAEMATOGENOSUS
• Microbiota - commensals
HORIZONTAL TRANSMISSION
• Contact
◦ Common source - environment, food/water
◦ Direct contact
◦ Vectors
• Inhalation
◦ Droplets - droplets of moisture expelled from one individual to another through sneezing or coughing
◦ Aerosols - suspension of tiny particles or droplets in the air (can stay there for a long time before being inhaled)
• Ingestion
◦ Faecal-oral transmission
VERTICAL TRANSMISSION
• Mother to child, before (intra-uterine) or at birth
ANIMALS
• Zoonosis
How do microorganisms cause disease?
- Exposure
- Adherence - bacteria enters the skin through gaps and adheres to a surface/viruses bind to cell surface receptors on cells to enter and multiply
- Invasion
- Multiplication
- Dissemination- spread
Alongside:
VIRULENCE FACTORS - molecules produced by pathogenic organisms that add to their effectiveness.
• Exotoxins- released by bacteria
◦ Cytolytic
◦ AB toxins
◦ Superantigens
◦ Enzymes
• Endotoxins- structures of bacterial cell wall that stimulate an immune response as bacteria breakdown
HOST CELLULAR DAMAGE
• Direct
• Consequent to host immune response
What are the determinants for disease?
PATHOGEN DETERMINING FACTORS
• Virulence factors - some strains of bacteria/viruses are more likely to cause disease than others
• Inoculum size- in some diseases you need large concentrations of some microorganisms to cause disease/others only a small concentration is required
• Antimicrobial resistance- determines how easy it is to treat infections
PATIENT
• Whether patient is immune or not
• Site of infection
• Co-morbidities- factors in patient that increase vulnerability to infection
Describe how to identify whether a patient has an infection.
• History ◦ Symptoms ‣ Local, systemic ‣ Severity ‣ Duration ◦ Potential exposures ‣ Where have you been? What have you been doing? With who? • Examination ◦ Organ dysfunctions ◦ Lymph node enlargement • Investigations ◦ Specific ◦ Supportive
List some supportive investigations.
- Full blood count - neutrophils (bacterial infection), lymphocytes (viral infection)
- C-reactive protein (CRP)
- Blood chemistry-liver and kidney function tests
- Imaging - x-ray, ultrasound, MRI
- Histopathology- gram stains
List some specific investigations.
These investigations aim to identify the cause of infection.
Bacteriology • Specimen types ◦ swabs, fluids, tissues • MC&S ◦ Microscopy- bacterial cells (gram stain), patient cells (cerebralspinal fluid) ◦ Culture- 18 hours incubation ◦ Antibiotic susceptibility • Antigen detection- faster, use a tray with antigen put blood in it and see if any is bound using immunoassay • Nucleic acid detection
Virology
• Antigen detection (the virus)- same as above
• Antibody detection (the patient’s response)
• Detecting viral nucleic acid (DNA or RNA)
Why might white blood cell count be increased?
- infection
- inflammation
- leukaemia
- cancer
Why might white blood cell count be decreased?
- medication
- some autoimmune conditions
- viral or severe infections
- bone marrow failure
- enlarged spleen
- liver disease
- alcohol excess
- congenital marrow aplasia
Why might neutrophil count be increased?
They one of the first lines of defence against bacterial infection.
They can be increased in some bacterial infections and would support a diagnosis.
Does white blood cell count and neutrophil count increase with all infections?
No - can be suppressed in some infections
Do all viral infections increase lymphocyte count?
No - can be increased or decreased by viral infections
Why might lymphocyte count be increased?
Viral infection
Chronic lymphocytic leukaemia
What are prions?
Proteins that generate copies of themselves and can spread from person to person to cause disease
Withstand very high temperatures and pressures
What are the significance of prions in a clinical environment?
Infection control
If instruments are sterilised and used again, prions that cause disease can remain
What is the difference between mycoplasma, chlamydiae, rickettsiae and other bacteria?
Although we classify these as bacteria, they rely on host biochemistry for reproduction like viruses and are transferred from cell to cell.
Yeast, protozoa and human cells are all eukaryotic cells.
True or false?
True
Describe the structure of genetic material in a virus.
RNA or DNA
Single stranded or double stranded
What is the capsid of a virus?
Do all viruses have it?
A protein shell that protects the genetic material.
Comes in different shapes: helical, icosahedral
All viruses have a caspid
What is the lipid envelope on a virus?
Do all viruses have it?
Derived from host
Contains virus specific antigens
Not all viruses have a lipid envelope
What is the function of spikes on viruses?
Spikes on viruses are important for binding to cell surface receptors on host cells.
Binding to receptors is essential for adhesion and hence invasion and multiplication.
Are viruses with single stranded or double stranded DNA more susceptible to mutations? Why?
Single stranded DNA as there is no way for the DNA replicating mechanism to identify a defect.
Are +ve or -ve RNA strands read to make proteins in viruses?
-ve strands
+ve strands have to be converted to -ve strands
How can viruses be classified?
- Baltimore classification - DNA structure
DNA or RNA
Single stranded or double stranded - Enveloped or non-enveloped
Parvovirus is a single stranded, non-enveloped virus. It mainly affects children. It causes a temporary suppression of bone marrow which affects the production of red blood cells.
Some people do not notice the effects of this whereas others are significantly affected.
Who would be significantly affected and what determine whether someone is affected or not?
Some people are not affected by suppression of bone marrow because red blood cells have a fairly long life - 120 days, so the time period is sufficient for them to be asymptomatic.
People with red blood cell abnormalities who may not usually have any problems experience significant anaemia as their red blood cells have a shorter life so the drop in production of red blood cells affects them. Eg. Sickle cell trait
In general, is a virus more easy to kill if it is enveloped or non-enveloped?
Enveloped
Easily disrupted and deactivated with alcohol.
What determines which cell line a virus replicates in?
Many viruses have tissue trophisms - prefer specific cell lines in which to replicate in.
What are bacteriophages?
Viruses can invade bacteria forming bacteriophages and this can be a form of transmission of DNA from bacteria to bacteria.
What are plasmids?
Double stranded DNA molecules in bacteria.
They an replicate and can be transferred between different species of bacteria.
They can transfer genes for antibiotic resistance.
All strains of the same bacteria have the same capsule. True or false?
False
How does the capsule of bacteria act as a virulence factor?
Allows organism to enter blood stream
Protects against immunological responses in host
Describe how a gram stain is carried out.
- Heat-fix specimen to slide. Flood slide with crystal violet solution. Allow to act for 1 minute.
- Rinse the slide, then flood with iodine solution. Allow iodine to act for 1 minute. Before acetone decolorisation, all organisms appear purple.
- Rinse off excess the excess iodine. Decolourise with acetone for approximately 5 seconds.
- Wash slide immediately with water. Those that remain purple are gram positive. Those that are no longer visible are gram negative.
- Apply safranin counterstein for 30 seconds. Wash in water, blot and dry in air. Gram negative organisms are visualised as red.
After a gram stain, gram positive bacteria appear…
Purple
After a gram stain, gram negative bacteria appear…
Red
How can bacteria be classified by their shape?
Cocci
Bacilli - rods
Spirillus
How can cocci be arranged?
Clusters
Chains
What is the difference between staphylococcus and streptococcus?
Staphylococcus = catalase positive
Streptococcus = catalase negative
What is the difference between gram positive and gram negative bacteria?
Gram positive bacteria have a peptidoglycan cell wall.
Gram negative bacteria have a lipopolysaccharide cell wall. Gram negative bacteria can also have some peptidoglycan in their cell wall.
What are obligate aerobic bacteria and obligate anaerobic bacteria
Obligate aerobes - require oxygen for survival
Obligate anaerobes - require oxygen-free environment for survival (unless able to form spores)
What is the function of pilli and fimbriae?
Adherence to host cells
What are virulence factors?
Properties that enable an organism to establish themselves within a host and cause disease.
How do enzymes such as collagenase act as a virulence factor?
Breaks down collagen - allows bacteria to spread between cells so it can invade cells
What are siderophores?
Siderophores are small high affinity iron chelating compounds that some bacteria produce enabling them to sequester iron.
What is an endotoxin?
Part of the bacterial structure
The term endotoxin and outer lipopolysaccharide membrane of gram negative bacteria are often used interchangeably.
What type of bacteria is often associated with sepsis?
Gram negative bacteria
Lipopolysaccharide is highly toxic and can trigger a major inflammatory response.
What is an exotoxin?
Proteins secreted by the bacteria that go out into the surrounding blood and tissue. They can damage tissues by a number of different mechanisms
Enterotoxin - exotoxin that acts on the bowel
Neurotoxin - exotoxin that acts on nerves
List some differences between prokaryotic and eukaryotic cells.
- Prokaryotes usually have a single circular chromosome although extra-chromosomal DNA may also be present (plasmids). Eukaryotes have multiple chromosomes
- Prokaryotes have no nuclear envelope or nucleoli. Eukaryotic DNA is membrane bound and nucleoli are present.
- Prokaryotes have no membrane bound organelles. Eukaryotes have membrane bound organelles.
- Prokaryotes have a cell wall, which may contain peptidoglycan. Eukaryotes do not (plants do but not made of peptidoglycan)
- Prokaryotes have a plasma membrane with no carbohydrates and few sterols. Eukaryotes have a plasma membrane with carbohydrates and sterols present.
- Prokaryotes have 70s ribosomes whereas eukaryotes have 80s ribosomes.
Why is it difficult to kill fungi without damaging our own cells?
Fungi are eukaryotic and have a very similar structure to human cells
What is the difference between yeasts and molds?
Give some examples of each.
Both are types of fungi
Yeasts are single-celled whereas molds are multicellular.
Yeast - Candida albicans, Cryptococcus neoformans, Pneumocystis jiroveci
Molds - Aspergillus species, Dermatophytes (ringworm, athlete’s foot)
What is a parasite?
A parasite is an organism that lives on or in a host and gets its food from or at the expense of its host
What are single celled parasites called?
Give some examples.
Protozoa
Plasmodium malariae
Plasmodium falciparum
Giardia lamblia
What are multi-cellular parasites called?
Give some examples.
Helminths or worms
Roundworms
Tapeworms
Flukes
Are males or females more susceptible to urinary tract infections and why?
Females because the distance between anus and urethra is shorter
What does calendar time mean in the context of infection?
Infections that occur at certain times of the year
What does incubation period mean?
Time between exposure to infection and appearance of first symptoms.
What is debridement?
Removal of damaged tissue or foreign objects from a wound
Why is PCR the best way to make a specific diagnosis of a viral infection?
It is possible to culture a virus but this must be done with live cells as this is the only way a virus can survive and replicate. Looking at a virus then requires an electron microscope. This is time consuming so PCR is much easier.
What is the human microbiome?
Total number and diversity of microbes found in and on the human body
What are the beneficial functions of normal flora?
- large number of harmless bacteria in the large intestine and mouth makes it unlikely that an invading pathogen could compete for nutrients and receptor sites
- some bacteria produce antimicrobial substances to which the producers are not susceptible
- bacterial colonisation of a newborn infant acts as a powerful stimulus for the development of the immune system
- bacteria of the gut provide important nutrients such as vitamin K (required for clotting), B vitamins and aid in the digestion/absorption of nutrients
How can our normal flora become harmful?
- Organisms are displaced from their normal site to an abnormal site
- Potential pathogens gain a competitive advantage due to diminished populations of harmless competitions
- Commonly ingested food substances converted into carcinogenic derivatives by bacteria in the colon.
- When individuals are immunocompromised, normal flora can overgrow and become pathogenic
Describe the structure of adenovirus.
Double stranded DNA
Non-enveloped
What is adenovirus the main causative organism for?
Respiratory tract disease
Can also cause:
- ocular infections
- GI infections
- UTIs
The site of the disease caused by an adenovirus infection is related to the mode of virus infection
What could injection of drugs below the skin rather than directly into a vein cause?
Infections caused by introduction of normal skin flora into subcutaneous tissue.
This includes:
Gram positive cocci:
Staphylocccus aureus
Staphylococcus epidermis
Streptococcus species
Gram positive bacilli:
Corynebacterium species
Propionibacterium acnes
Patient is unwell with increased cough and shortness of breath. He has developed conjunctivitis.
What investigation would be best to make a specific diagnosis?
Throat swab for viral PCR
How does adenovirus replicate?
- attaches to host cell receptor
- entry into cell by receptor mediated endocytosis
- viral genome transported to nucleus where transcription of viral genes, genome replication and assembly occurs
- reproductive cycle kills the host cell
In what tissue do adenoviruses replicate well in?
Epithelial cells - observed disease symptoms are related primarily to the killing of these cells
What is sepsis?
Sepsis is a life-threatening organ dysfunction due to an unregulated host response to infection.
What is septic shock?
Septic shock is persisting hypotension requiring treatment to maintain blood pressure despite fluid resuscitation
What is bactaraemia?
Presence of bacteria in the blood which persists as organisms are being constantly shed from an infected site
How do we recognise sepsis?
-Raised Early Warning Score (EWS). If 3 or higher, patient should be reviewed
If sepsis is suspected: Look for clinical features suggesting source: -pneumonia -UTI -meningitis
Check for red flags - if present then red flag sepsis
- high respiratory rate
- low blood pressure
- unresponsive
What happens if the patient has red flag sepsis?
Immediate action required
Inform senior doctor for review
Complete sepsis six bundle: (give 3, take 3)
- oxygen
- blood cultures
- IV antibiotics
- fluid challenge
- lactate
- measure urine output
What urgent investigations should be taken from a patient with red flag sepsis?
Each case is different, no set list but consider:
- FBC
- urea and electrolytes
- EDTA bottle for PCR
- blood sugar
- liver function tests
- C-reactive protein
- coagulation
- blood gases
- other microbiology samples (CSF, urine), glass test for meningitis
- imaging
What is the supportive treatment for sepsis?
What is the specific treatment for sepsis?
Supportive
- consider early referral to ITU
- sepsis 6
- regular monitoring and reassessment
Specific
-antibiotic treatment
What are the life threatening complications of sepsis?
- irreversible hypotension
- respiratory failure
- acute kidney injury
- raised intracranial pressure (in the case of meningitis)
- ischaemic necrosis of digits/hands/feet
How does sepsis result in cytokines (IL-1 and TNF) and nitric oxide being released locally into tissue fluid and systemically into the bloodstream?
- Gram negative bacteria has a lipopolysaccharide endotoxins. The bacteria dies and when it does this, the endotoxin is released.
- These endotoxins bind to endothelial cells of the circulatory system and activate macrophages.
- Macrophages secrete cytokines( IL-1, TNF) and endothelial cells release nitric oxide.
What are the local effects of the cytokines IL-1 and TNF that are released into tissue fluid in septic patients?
- Coagulation —> Prothrombin to be converted to thrombin —> brief vasoconstriction
- Inhibits fibrinolysis
- Promotes inflammatory response
- Recruits RE (reticulo-endothelial system)
What are the systemic effects of IL-1 and TNF released into the bloodstream in septic patients?
Fever as production of prostaglandins is stimulated
Acute respiratory disease syndrome —>lungs don’t work properly, destruction of respiratory epithelium —> Endothelial cells in blood vessels in the lungs are activated so neutrophils and macrophages are stuck here
What is the effect of nitric oxide being released into the bloodstream in septic patients?
Stimulates vasodilation —> low blood pressure
How does sepsis cause disseminated intravascular coagulation?
Cytokines IL-1 and TNF cause tissue factor to be released.
Tissue factor triggers the clotting cascade by the extrinsic pathway.
This causes:
Coagulation and inhibition of fibrinolysis
This results in:
• Widespread unregulated coagulation in all blood vessels resulting in microvascular thrombosis
• Compromised blood flow to tissues leading to ischaemia
• Leads to multiple organ damage
• Anti-coagulant system stimulated to a severe extent
• Platelet and clotting factors consumed- severe bleeding of various sites
What is the major cause of shock and multiorgan failure in sepsis?
Microvascular injury
Why can sepsis result in ischaemic necrosis of the hands, feet and digits?
Microvascular thrombosis blocks blood vessels
Low blood pressure
Body desperately diverts blood supply to heart, lungs, brain and kidneys so supply to distal limbs is compromised.
How is meningococcal meningitis diagnosed?
- glass test, non-blanching purpuric rash
- blood culture
- PCR of blood
- lumbar puncture if same - sample of CSF for microscopy, gram stain and PCR
Describe the structure of Neisseria meningitidis.
Gram negative Diplococcus - shaped like a kidney bean Encapsulated Pilli Oxidase positive
How is Neisseria meningitidis transmitted?
Inhalation of respiratory droplets from a carrier or patient in early stages of disease
Describe the virulence of Neisseria meningitidis.
Pilus - enhances attachment of the organism to nasopharyngeal mucosa
Capsule - prevents phagocytosis and promotes adherence
Lipopolysaccharide endotoxin - triggers inflammation
Where is the usual attachment site for Neisseria meningitidis?
Nasopharyngeal mucosa
Describe the pathogenesis of Neisseria meningitidis.
- Asymptomatic meningococcal pharyngitis
A. Colonises nasopharynx but nasopharyngeal mucosa normally serves as a barrier to bacteria - Bacteraemia
A. Susceptible individuals - penetrates nasopharyngeal mucosa and enters blood stream
B. Rapidly multiplies in bloodstream - Meningitis
A. Crosses blood-brain barrier and infects meninges
B. Acute inflammatory response - Sepsis:
A. Lipopolysaccharide endotoxins triggers release of cytokines which results an uncontrolled inflammatory response to infection
B. Hypotension that cannot be treated with fluid resuscitation alone
C. Multiple organ failure
What specific investigations are used to confirm diagnosis of Neisseria meningitis?
- blood culture
- lumbar puncture for isolation of CSF
- swab of nasopharyngeal region
INVESTIGATION OF THESE CULTURES BY….
—>MC&S
◦ Microscopy- diplococci often in associated with polymorphonuclear leucocytes, gram negative
◦ Glucose and protein estimation - In CSF, increased pressure, elevated protein and decreased glucose (consumption by bacteria), many neutrophils
—>antigen detection
What are the symptoms of meningitis?
Initially, • Joint symptoms • Purpuric rash that doesn't blanch (glass test) • Fever • Severe headache
Within several hours, • Rigid neck • Vomiting • Sensitivity to bright lights • Coma
Initially, • Joint symptoms • Purpuric rash that doesn't blanch (glass test) • Fever • Severe headache
Within several hours, • Rigid neck • Vomiting • Sensitivity to bright lights • Coma
How is bacterial meningitis treated?
Bacterial meningitis is a medical emergency so antibiotic treatment cannot await a definitive bacteriologic diagnosis.
Specific:
• Blood culture drawn
• Antibiotic therapy given (before lumbar puncture)
◦ effectively treated with high doses of penicillin or ampicillin (can pass blood-brain barrier) in large intravenous doses
◦ when etiology of infection is unclear- ceftriaxone recommended
Supportive:
• Corticosteroid given with first dose of antibiotic - to treat intense inflammatory reaction
Sepsis 6 bundle
Are there vaccines available for Neisseria meningitidis?
Yes
Meningococcal C conjugate vaccine
ACWY vaccines - originally for immunocompromised patients and travel protection
Serogroup B vaccine - B capsules poorly immunogenic and similar to neural tissue so was difficult to make. Current vaccine has 4 antigens. Given to babies at 2 3 and 12 months, and adults at increased risk.
How can bacterial meningitis be prevented?
- serogroup B vaccine
- antibiotic prophylaxis for close contacts
Which antibiotic is recommended to be used in the treatment of infection by Neisseria meningitidis?
Ceftriaxone - cephalosporin (inhibits cell wall synthesis)
Penetrates into CSF so is active against Neisseria meningitidis
What is infectious disease?
A pathogen succeeds in evading and/or overwhelming the host’s immune defences
What is the immune system?
Cells and organs that contribute to immune defences against infectious and non-infectious conditions (self vs non-self)
Innate immunity is not required for adaptive immunity to work.
True or false.
FALSE
State 4 differences between innate and adaptive immunity.
Innate immunity - immediate protection Fast (within seconds) Lack of specificity Lack of memory No change in intensity
Adaptive immunity - long lasting protection Slow (days) Specificity Immunologic memory Changes in intensity
What are the different types of innate barriers to infection?
What is their function?
- physical barriers
- physiological barriers
- chemical barriers
- biological barriers
- prevent entry and limit growth of pathogens
- prevent local infection from becoming systemic by entering the bloodstream
What are the different physical barriers to infection?
Skin
Mucous membranes - can sense microbes at surface of mucous membrane and initiate a local immune response via MALT
(Found in mouth, respiratory tract, GI tract, GU tract)
Bronchial cilia
What are the physiological barriers to infection?
These symptoms are also present in non-infectious conditions
• Diarrhoea ◦ Food poisoning • Vomiting ◦ Food poisoning ◦ Hepatitis ◦ Meningitis • Coughing ◦ Pneumonia • Sneezing ◦ Sinusitis
What is the pH of the skin, stomach and vagina?
Skin = 5.5 Stomach = 1-3 Vagina = 4.4
Low pH so kills microorganisms
What are the biological barriers of infection?
Normal flora in strategic locations:
- nasopharynx
- mouth/throat
- skin
- GI tract
- vagina
- under NO circumstances should they be found in blood/organs/tissues
Patient is taking anti-acids for a stomach ulcer. Which infection is he prone to develop?
Skin infection, UTI, food poisoning?
Food poisoning because acidic environment prevents bacterial growth in the stomach and colonisation of the intestines
Patient is being treated for cellulitis in left leg. Which conditions are a probable cause for his medical problem?
eczema, dermatitis, insect bites
Cellulitis is infection of soft tissue under the skin. All of the above breach the integrity of the skin.
A patient is being treated for a UTI. She developed severe vaginal thrush. Which class of medicine is likely to be the cause?
Antibiotics can reduce normal flora allowing the colonisation by pathogenic microbes
Taking antibiotics can reduce normal flora in the gut. What condition does the predispose to?
Clostridium difficile associated disease
Why are people with cystic fibrosis more susceptible to infection?
Cystic fibrosis is caused by a defect in the CFTR gene which codes for a chloride ion channel.
Absence of this channel leads to less chloride and hence water exiting the cell and mucus becoming thick. This damaged cilia in the bronchi.
The cilia lose their ability to trap microorganisms in the mucus and sweep them to be excreted.
What does IgA do?
Where is it found?
Binds to microbe and prevents microbe from adhering to epithelia affecting its infectivity
Tears, salvia, mucous membranes
What does lysozyme do?
Where is it found?
Breaks the cell wall of the bacteria reducing its survival
Sebum, perspiration, urine
What do beta-defensins do?
Where are they found?
Highly toxic for gram positive and gram negative bacteria
Epithelium of mucous membranes
What normal flora is found in the nasopharynx?
Streptococcus pneumoniae
Neisseria meningitidis
Haemophilus species
What is the normal flora of the skin?
Staphylococcus aureus Staphylococcus epidermis Streptococcus pyogenes Candida albicans Clostridium perfringens
How can normal flora cause disease?
- normal flora is displaced from its normal location to a sterile location
- normal flora overgrows and becomes pathogenic when host becomes immuno-compromised
- normal flora is depleted by antibiotics
Poor dental hygiene can cause serious infections in high-risk patients. Who is at risk?
- asplenic patients
- damaged or prosthetic valves
- previous infective endocarditis
(Antibiotic prophylaxis given to these patients)
How can flora be displaced from its normal location to a sterile location?
- breaching of skin integrity
- faecal-oral route
- faecal-perineal-urethral route
- poor dental hygiene
Taking antibiotics can reduce normal flora in the gut. What condition does he predispose to?
Clostridium difficile associated disease
What is vaginal thrush caused by?
Depletion of normal flora of the vagina allowing colonisation by pathogenic microbes
When is the second line of defence in the innate response activated?
Integrity of one of the barriers that prevent entry and limit growth of pathogens is compromised eg. Break in skin
What are the second lines of defence in the innate immune response?
What is their overall function?
- phagocytes
- chemicals eg. Complement system, cytokines
- inflammation
These contain and clear the infection
What are the main cells of the innate immune cystem?
- monocytes which differentiate into macrophages
- neutrophils
- basophils
- mast cells
- eosinophils
- natural killer cells
- dendritic cells
What is the function of macrophages?
- phagocytosis
- antigen presentation
- produce cytokines/chemokines
What is the function of neutrophils?
How are they recruited to the site of infection?
-phagocytosis of pyogenic bacteria
Recruited by chemokines to the site of infection
Eosinophils are important in defence against…
Multi-cellular parasites (helminths/worms)
What do natural killer cells do?
Kill all abnormal host cells (virus infected or malignant)
What is the function of dendritic cells?
Antigen presentation to T cells
How are pathogens recognised by cells of the immune system?
-microbial structures - pathogen associated molecular patterns (PAMPs)
Can be carbohydrates, lipids, proteins, nucleic acids
Phagocytes have pathogen recognition receptors (PRRs) eg. Toll like receptors, that recognise PAMPs
-opsonisation of microbes - coating proteins called opsonins bind to microbial surfaces leading to enhanced attachment of phagocytes and clearance of microbes
What are toll like receptors?
A type of pathogen recognition receptor
Phagocytes that have most the toll like receptors are able to recognise gram positive and gram negative bacteria
Where are pathogen recognition receptors found?
- cell surface of phagocytes to identify bacteria
- inside the cell to identify viruses
What is the difference between pathogen recognition using PAMP-PRR and opsonisation?
When a PRR on a phagocyte binds to a PAMP on a pathogen, the phagocyte is activated to release cytokines and chemokines that are necessary to initiate an inflammatory response
When opsonin receptors bind to an opsonin on a pathogen, the phagocyte is simply activated to phagocytise the pathogen- no inflammatory response is activated
List some examples of opsonins.
Complement proteins = C3b , C4b
Antibodies = IgG , IgM
Acute phase proteins = C-reactive protein, mannose-binding lectin
What type of pathogen opsonins essential in the clearance of?
Encapsulated bacteria eg.
Neisseria meningitidis
Streptococcus pneumoniae
Haemophilus influenzae b
How does phagocytosis occur?
- Chemotaxis and adherence of microbe to phagocyte
- Ingestion of microbe by phagocyte
- Formation of a phagosome
- Fusion of the phagosome with a lysosome to form a phagolysosome
- Killing of ingested microbe by enzymes/oxidative burst
- Formation of residual body containing indigestible material
- Discharge of waste materials
What are the intracellular killing mechanisms of phagocytes?
Oxygen dependent pathway
Oxidative burst using hydrogen peroxide, hydroxyl radical, superoxide, nitric oxide
Oxygen independent pathway
Lysozyme - breaks down cell wall of bacteria
Lactoferrin/transferrin - deprives bacteria of essential nutrients
Cationic proteins
Proteolytic and hydrolytic enzymes
What is the complement system composed of?
20 serum proteins
Most important C1-C9
What are the two pathways of the complement system?
-Alternative pathway
Initiated by cell surface microbial constituents eg. Endotoxin
-MBL pathway
Initiated when MBL binds to mannose containing residues of proteins found on many microbes
What is mannose-binding lectin (MBL)?
It is an acute phase protein
Functions:
- opsonisation
- activates MBL pathway of complement system by binding to mannose containing residues of proteins found on many microbes
Why does the MBL pathway occur after the alternative pathway of the complement system?
The MBL pathway occurs later than the alternative pathway because MBL needs to be released by the liver. Chemokines released by macrophages stimulate the release of MBL from the liver.
The alternative pathway is activated by the microbial surface.
Therefore, this must occur first before the complement pathway is activated via the MBL pathway.
What do the complement pathways activate complement to do?
C3a and C5a = mediators for chemotaxis
C3b and C4b = opsonisation of pathogens
C5-C9 = killing of pathogens via membrane attack complex
Which chemokines are released by macrophages?
TNFa
IL-1
IL-6
What is the function of the chemokines TNFa, IL-1 and IL-6?
Liver - opsonins
CRP
MBL - complement activation
Bone marrow
Neutrophil mobilisation
Inflammatory actions
Vasodilation
Vascular permeability
Adhesion molecules —> attraction of neutrophils
Hypothalamus
Increased body temperature
Summarise the innate immune response.
- Innate barrier is breached: entrance and colonisation of the pathogens
- Complement, mast cells and macrophages activation (PRR)
Phagocytosis (opsonins)
Cytokine/chemokine production - Vascular changes - vasodilation + increased permeability
Chemoattraction - neutrophils, monocytes - Hypothalamus —> fever
Liver —> acute phase response - Local inflammation (redness, heat, swelling, pain)
Name a disease in which there is impaired neutrophil function.
Chronic granulomatous disease
NADP oxidase deficiency
You have been called to care for a 60-year-old patient in the intensive care unit who was admitted following a car accident.
He is currently unconscious and has signs of cerebral oedema. The patient has a tube into his trachea, a central venous line and urinary catheter inserted. He was given high-dose of corticosteroids to reduce his cerebral oedema and the anti- convulsant phenytoin.
Why does he have an increased risk of developing a serious infection?
Urinary catheter provides entry route
What is an antimicrobial agent?
An agent that is active against a microbe. Includes • Antibacterial • Antifungal • Antiviral • Antiprotazoal
What is an antibiotic?
An agent derived from a creature and is active against a microbe
What is the difference between bactericidal and bacteriostatic antibiotics?
Bactericidal - kills bacteria directly
Bacteriostatic - prevents bacteria from reproducing while not necessarily killing them
What are the ideal features of antimicrobial drugs?
- selectively toxic- few adverse effects
- reach site of infection
- oral/IV formulation
- long half life - infrequent dosing
- no interference with other drugs
What are the different mechanisms of action of antibacterials?
- prevent cell wall synthesis
- prevent protein synthesis
- disrupt cell membrane function
- prevent nucleic acid synthesis
Which classes of antibacterials prevent cell wall synthesis?
Beta-lactams
Glycopeptides
Which classes of antibacterials prevent protein synthesis?
Tetracyclines
Aminoglycosides
Macrolides
Which class of antibacterials disrupts cell membrane function?
Polymixins
Which class of antibacterials inhibits nucleic acid synthesis?
Quinolones
Give some examples of beta-lactams
Penicillins
Cephalosporins
Carbapenems
Monobactams
How does penicillin work?
Beta-lactam - prevents cell wall synthesis
Penicillin binding protein is required to form cross-linkages between peptidoglycan side chains. These linkages prevent movement.
Penicillin binds to penicillin binding protein so that cross linkages cannot form between peptidoglycan molecules via the side chains.
How do co-amoxiclav and tazocin counteract resistance?
• Co-amoxiclav = Amoxicillin + clavulanate
Clavunalate is a beta lactamase inhibitor so this antibiotic is active in bacteria with a beta-lactamase enzyme.
• Tazocin = piperacillin + tazobactam
same as above
What type of bacteria is penicillin mainly active against?
Streptococci
What type of bacteria is amoxicillin mainly active against?
Streptococci and some gram-negatives
What kind of bacteria is flucloxacillin active against?
Staphylococci and streptococci
What are the main types of penicillins?
Penicillin
Amoxicillin
Flucloxacillin
Which cephalosporin has good activity in the CSF?
Ceftriaxone - used in the treatment of Neisseria meningitidis and sepsis
Which carbapenem is generally safe in a penicillin allergy?
Meropenem
How does vancomycin work?
Glycopeptide - prevents cell wall synthesis
Vancomycin binds to side chains on peptidoglycan molecules and prevents penicillin binding protein from binding.
Cross linkages cannot form between peptidoglycan molecules via the side chains.
Vancomycin is toxic so why is used?
- penicillin allergy/intolerance
- resistance to penicillin
What type of bacteria is vancomycin active against?
Most gram positive
Why can’t tetracyclines be used in children?
Stain bone and teeth yellow
What class of antibacterial is gentamicin?
Aminoglycoside - prevents protein synthesis
When is gentamicin used?
Aminoglycoside - inhibits protein synthesis
Profound activity against gram negatives. Good activity in blood and urine.
Generally preserved for severe gram negative sepsis
Which macrolide can be used as an alternative to penicillin for mild gram positive infections?
Erythromycin
Macrolide - prevent protein synthesis
What is the commonest type of quinolone?
Ciprofloxacin
How do quinolones work and are they broad spectrum?
Inhibits two enzymes in DNA replication, one of these is involved in supercoiling
Quinolones are broad spectrum
Use of quinolones is associated with a risk of…
C.difficile
How do trimethoprim and sulphonamides work?
Inhibit DNA synthesis by inhibiting folic acid synthesis
Which antibiotic is used in treatment of UTI’s?
Trimethoprim - inhibits folic acid synthesis and hence DNA synthesis
What is used to treat pneumocystic pneumonia?
Co-trimoxazole
Trimethoprim + suphamethoxazole
Inhibits folic acid synthesis and hence DNA synthesis
What are the different types of antibiotic resistance?
Intrinsic
Acquired
Adaptive
What is intrinsic antibiotic resistance?
Intrinsic - certain antibiotics do not work against certain bacteria
No target or access for the drug
Usually permanent
What is acquired antibiotic resistance?
Acquired - organism that was once susceptible is no longer susceptible
Acquires new genetic material or mutates
Usually permanent
What is adaptive antibiotic resistance?
Adaptive - a few organisms in the colony develop a potential to withstand the adverse circumstance
Organism responds to a stress eg. Sub-inhibitory level or antibiotic
Usually reversible - take away adverse circumstances, susceptible organisms will reappear
What are the mechanisms of antimicrobial resistance?
• Drug inactivating enzymes
◦ Eg. B-lactamases, aminoglycoside enzymes
• Altered target- Target enzyme has lowered affinity for antimicrobial
◦ Eg. Resistance to meticillin (MRSA), macrolides, trimethoprim
• Altered uptake- decreased permeability/ increased efflux
◦ Eg. Decreased permeability- B-lactams
◦ Eg. Increased efficacy - tetracyclines
What is the mechanism of antibiotic resistance to meticillin (MRSA)?
Altered target -
Target enzyme in bacteria has lowered affinity for the antimicrobial
What type of genetic material is found in bacteria?
- Chromosome - double stranded circular DNA
- Plasmids- non-chromosomal DNA
- Transposon- strands of non-chromosomal DNA
Transposons can be integrated into plasmids which can be integrated into chromosomes
What are transponons?
Strands of non-chromosomal DNA in bacteria
Describe how vertical gene transfer in bacteria can lead to antibiotic resistance.
◦ A spontaneous mutation occurred which enables the organism to be resistant. This may be in plasmid or in the chromosome.
◦ All susceptible organisms die and those with resistant genes survive.
◦ Organisms with resistant genes multiply and dominate the environment
◦ Genes for resistance passed on to all offspring.
Describe how horizontal gene transfer in bacteria can lead to antibiotic resistance.
◦ Conjugation
‣ Genetic material transferred from one organism to another
◦ Transduction
‣ Injected into bacterium by a virus forming a bacteriophage
◦ Transformation
‣ Free DNA can enter cell wall via pores
Why is minimum inhibitory concentration better than disc testing for measuring antibiotic activity?
• Disc testing
Grow bacteria on an agar plate.
Measure the circumference of space where the antibiotic has killed the bacteria.
• Minimum inhibitory concentration
Make solutions of different concentrations of antibiotic
See which solutions enable growth of the bacteria
Find minimum concentration that inhibits bacterial growth
MIC- gives quantitative value for antibiotic against bacteria to compare against different organisms and antibiotics. Gives a way of defining sensitive and resistant strains.
Which antifungal can be used to treat vaginal thrush?
Fluconazole - inhibits cell membrane synthesis
What are the two classes of antifungals?
Azoles - inhibit cell membrane synthesis
Polyenes - inhibit cell membrane function
Which antiviral drug can be used against Herpes simplex in the treatment of genital herpes and encephalitis?
Aciclovir- when phosphorylated inhibits viral DNA polymerase
Which antiviral drug can be used in the treatment of Varicella zoster in the treatment of chicken pox and shingles?
Aciclovir
What is antimicrobial stewardship?
- Appropriate use of antimicrobials
- Optimal clinical outcomes
- Minimise toxicity and other adverse events
- Reduce the costs of healthcare for infections
- Limit the selection for antimicrobial resistant strains
To prevent resistance, should patients complete every dose of antibiotics even after they feel better?
No because there is a greater chance of selecting for resistant bacteria with longer doses.
Do short courses of antibiotics cause resistance? If not, why are they disadvantageous over longer courses under some circumstances?
No - they are less likely to cause resistance
May not be long enough to kill all bacteria- need to find optimum dose.
Where are antigen presenting cells found?
Lymphatic tissue where antigen presenting cells can interact with B cells and T cells
• Skin (SALT)
• Mucous membranes MALT (GALT, NALT, BALT, GUALT)
• Lymphoid organs (lymph nodes, spleen)
• Blood circulation (plasmacytoid and myeloid DCs)
How do antigen presenting cells present antigens?
- Pathogen capture
A. Phagocytosis (whole microbe)
B. Macropinocytosis (soluble particles) - Process
A. Detect intracellular pathogens (virus) or extracellular pathogens (bacteria) using diverse pathogen sensors (PRRs- located inside and outside of the APC)
B. Process antigens using MHC molecules - Presentation
Which cells are antigen presenting cells and where are they found?
Dendritic cells
Location: lymph nodes, mucous membranes, blood
Langerhans cells
Location: Skin
Macrophages
Location: Various tissues
B cells
Location: Lymphoid tissues
Which antibiotic is used in the treatment of gram positive bacteria?
Vancomycin - glycopeptide
What do antigen presenting cells do?
For an antigen to be recognised by a T cell (the main cell of adaptive immunity), antigen presentation by antigen presenting cells is essential.
Why can’t asplenic patients mount a full response to blood-borne pathogens?
- site of phagocytosis of opsonised pathogens
- encapsulated bacteria are phagocytised here
Which type of T cells do dendritic cells and Langerhans’ cells present antigens to?
Naive T cells - T cells that have not yet encountered an antigen
Which cells do macrophages present antigens to?
Effector T cells - T cells that have encountered an antigen before
Which cells do B cells present antigens to?
Effector T cells - T cells that have encountered an antigen before
Naive T cells - T cells that have not yet encountered an antigen
What are naive T cells?
T cells that have not yet encountered an antigen
What are effector T cells?
T cells that have encountered an antigen before
Which MHC molecule is present on the surface of antigen presenting cells (dendritic cells, Langerhans’ cells, macrophages, B cells)?
MHC class II
What is the difference between Class I and Class II MHC molecules?
Class I molecules • found on all nucleated cells • present peptides from INTRACELLULAR microbes • Recognised by CD8+ T cells • Genes they are coded for by are found on p and q arm of chromosome 6 ◦ HLA A ◦ HLA B ◦ HLA C
Class II molecules
• found on APC’s (dendritic cells, macrophages, B cells)
• present peptides from EXTRACELLULAR microbes
• Recognised by CD4+ T cells
• Genes they are coded for are found on the q arm of chromosome 6
◦ HLA DR
◦ HLA DQ
◦ HLA DP
Which class of MHC molecule is found in all nucleated cells?
Class I
Which class of MHC molecule is found in antigen presenting cells?
Class I
Class II
What does MHC Class I do?
Presents peptides from intracellular microbes
Recognised by CD8+ T cells
What does MHC Class II do?
Present peptides from extracellular microbes
Recognised by CD4+ T cells
Where are the genes that code for MHC Class I molecules found and what are they?
P and q arm of chromosome 6
HLA A
HLA B
HLA C
Where are the genes that code for MHC Class II found and what are they?
Q arm of chromosome 6
HLA DR
HLA DQ
HLA DP
Why is it advantageous to have a diverse class of MHC molecules?
More microorganisms can be processed and presented by antigen presenting cells with a greater variety of MHC molecules.
What determines the diversity of MHC molecules that an individual has?
• Co-dominant expression - Both parental genes are expressed
◦ Increased number of different MHC molecules
• Polymorphic genes - Different alleles among different individuals
◦ Each individual can present different antigens/microbes
Describe the structure of MHC Class I and explain how this relates to its function.
There are 2 subunits- a large subunit (spans the membrane) and a small subunit.
• Peptide binding cleft = variable region with highly polymorphic residues so many different types of peptides can bind and be presented by the same MHC molecule (broad specificity)
◦ Binds to peptides between 9 and 10 amino acids in length
Describe the structure of MHC Class II and explain how this relates to its function.
There are 2 subunits equal in length and both transmembrane
• Peptide binding cleft = variable region with highly polymorphic residues so various different types of peptides can bind and be presented by the same MHC molecule (broad specificity)
◦ Binds to larger peptides
◦ Peptide binding cleft is between the 2 subunits
A patient has contracted a flu virus. He has a terrible headache, fever and is coughing a lot.
The viral proteins on the airway epithelial cells are presented by the MHC molecules called HLA-DP, HLA-DQ, HLA-DR.
True OR False?
False- viruses are only presented by MHC class I, these are HLA-A, HLA-B, HLA-C
A patient has contracted a flu virus. He has a terrible headache, fever and is coughing a lot.
The viral proteins on the airway epithelial cells are presented by the MHC molecules called HLA-A, HLA-B, HLA-C.
His dendritic cells will also express the viral peptides
True OR False?
True- dendritic cells have MHC I and MHC II because they are nucleated and APC’s
It is the nature of the pathogen that determines the way it is processed.
True or false?
False
It is not the nature of the pathogen that determines the way it is processed. Instead, it is the route of trafficking into the cell.
Describe antigen presentation of all nucleated cells via the endogenous pathway.
- The virus/tumour or self antigen (intracellular antigen) enters the cell.
- This is targeted for degradation by the cell. Ubiquitin is added to the antigen and signals for their degradation via proteasomes. Proteasomes are protein complexes that degrade unneeded or damaged proteins by proteolysis.
- Antigenic peptides are produced by the the proteasomes which is transported into the ER via a specific transporter.
- There is a dynamic system inside the ER where MHC class I is constantly produced. When there is a match between MHC class I and the viral antigenic peptide/self antigen, the MHC class I binds and moves to the cell surface.
- The viral antigenic peptide/self antigen is presented alongside MHC class I at the cell surface.
- This activates CD8+ T cells.
Both self and non-self peptides are presented to T cells.
True or false.
True
What are proteasomes?
Protein complexes that degrade unneeded or damaged proteins by proteolysis
What is ubiquitin?
It is added to antigens and signals for their degradation via proteasomes
Where is MHC class I produced?
There is a dynamic system in the ER where MHC class I is constantly produced
Where do MHC Class I molecules bind to antigens?
In the ER -
The MHC Class I and antigen complex then moves to the cell surface
Describe how antigens are presented by antigen presenting cells via the exogenous pathway.
- The exogenous antigen is engulfed by the cell by endocytosis.
- The vesicle fuses with lysosomes and the proteins are digested forming a phagosome
- The phagosome fuses with a vesicle containing the MHC class II molecule. when there is a match between MHC class II and the exogenous antigen, the MHC class II binds to it and moves to the cell surface.
- The exogenous antigen is presented alongside MHC class II at the cell surface.
- This activates CD4+ T cells.
Are all peptides from the same microbe presented by the same MHC molecule?
No- peptides from the same microbe may be presented by different MHC molecules
What does susceptibility to infections depend upon in relation to MHC molecules?
The more diverse MHC molecules an individual has, the less susceptible they are to an infection
In a healthy individual, antigen presenting cells present both self and non-self peptides. Why do their normal tissues not get damaged?
T cells do not react with self antigens on antigen presenting cells. They only react with foreign antigens.
Why do people who are infected with HIV have different responses to the virus?
Slow progressors have MHC molecules that present key peptides that are essential for the survival of the virus eg. P24 (unmutated). Therefore, there is an effective T cell response.
• High CD4+ count
• High CD8+ cytotoxic T lymphocyte count
• Viral load is controlled
Rapid progressors have MHC molecules that present peptides that are not essential for the survival of the virus and can be mutated. Therefore, there is poor recognition of virally infected cells and a poor T cell response.
• Low CD4+ count
• Low CD8+ cytotoxic T lymphocyte count
• Viral load is not controlled
A 27 year old male
• History of injection drug use
• Pneumonia (Pneumocystis jirovecii) and CNS toxoplasmosis
• His viral load was 200,000 copies/ml
• CD4+ T cell count <200 cell/μl (mm3).
Treatment: Highly Active Anti-Retroviral Therapy (HAART)
What disease is this patient likely to have and what is he at risk of?
IV drug use—> blood borne infection, HIV?
CD4+ T cell count is low—> HIV destroys CD4+ T cells. CD4+ T cells very important in adaptive immune response.
Pneumocystis jirovecii and high viral load—> He won’t die from the virus but because his CD4+ T cell count is low, he is immunosuppressed and can die from an opportunistic infection
What is the importance of MHC molecules in transplant rejection?
GRAFT-VERSUS HOST REACTION (GVH)
MHC molecules are one of the major causes for transplant rejection.
HLA molecules mismatch between donor and recipient (allograft)
Why does it matter if HIV attacks CD4+ T cells when CD8+ T cells can be activated by the endogenous pathway by all nucleated cells?
CD8+ T cells require activation by CD4+ T helper cells to replicate. Therefore, APC’s can process intracellular microbes by endogenous and exogenous pathways using MHC class II and MHC class I
Where do T cells mature?
Thymus
What is the antigen receptor on T cells called?
TCR
What determines which TCR a lymphocyte has?
Random process which depends on gene rearrangement
What determines the function of T effector cells?
The nature of cytokines they produce
Describe the structure of TCR.
Has a variable region and a constant region.
Does not work in isolation- works as part of a complex with other receptors
CD4+ on T cells that recognise MHC Class II
CD8+ on T cells that recognise MHC Class I
How are naive T cells activated?
A complementary peptide binding to the complementary TCR forming a complex.
They are only fully activated when costimulatory signals (due too the release of cytokines) are produced by the binding of CD80/86 presented by mature APC’s with CD28 on the T cell.
What happens to antigen presenting cells when they present a peptide?
They mature and present a different class of proteins (CD80/86) that enhances the attachment between APC’s and T cells as it is recognised by CD28 on T cells.
Describe the T cell response against intracellular microbes.
Antigen-presenting cells express antigenic peptides via MHC Class I and MHC Class II.
MHC Class I antigen TCR complex and costimulation activates naive T cells to become CD8+ T cells.
MHC Class II antigen TCR complex and costimulation activates naive T cells to become T helper 1 CD4+ T cells.
CD8+ T cells are stimulated by T Helper 1 CD4+ T cells to divide and develop into cytotoxic T lymphocytes.
Cytotoxic T lymphocytes bind to MHC Class I antigen complexes on infected cells and kill them by secretion of
• Perforins - creates holes on the virally infected cell membrane
• Granzymes - initiates an apoptotic process in the virally infected cell
B cells are activated by cytokines released by T Helper 1 CD4+ T cells to create antibody IgG for opsonisation of microbes.
Macrophages are activated by cytokines released by T helper CD4+ T cells to phagocytise opsonised microbes.
Describe the T cell response against extracellular microbes.
Antigen presenting cells express antigenic peptides via MHC Class II
MHC Class II antigen TCR complex and constimulatory signals activates naive T cells to become CD4+ T helper 2 cells and CD4+ T helper 17 cells.
Eosinophils are activated by cytokines released by CD4+ T helper 2 cells.
Eosinophils kill parasites.
B cells are activated by cytokines released by CD4+ T helper 2 cells.
B cells produce antibodies for opsonisation and phagocytosis.
Mast cells are activated by cytokines released by CD4+ T helper 2 cells.
Mast cells cause local inflammation in response to allergies when activated by IgE
Neutrophils are activated by CD4+ T helper 17 cells.
Neutrophils phagocytise the pathogen.
Describe how the ratio of IgG and IgM changes with primary and secondary response to an antigen.
• Primary response - first encounter High concentration of IgM Low concentration of IgG, lower than IgM concentration Ratio of IgM: IgG is high Total antibody count lower
• Secondary response
Same concentration of IgM as primary response
Higher concentration of IgG than in primary response, exceed IgM concentration
Ratio of IgM: IgG is low
Total antibody count higher
What is the difference to the primary and secondary antibody response to an antigen?
It is faster
It is stronger as there is more antibodies produced
It lasts for a longer duration
The antibodies have a higher affinity for the antigen
There is an isotype switch from IgM to IgG as these are better for opsonisation
What are the functions of IgG?
Opsonisation
Complement activation - (classical pathway is activated when it binds to an antigen-antibody complex)
Neonatal immunity
Toxin/virus neutralisation
What are the functions of IgE
Immunity against helminths
Mast cell degranulation (allergies)
What is the function of IgA
In breast milk
Mucosal immunity
What is the function of IgM?
Complement activation
Which factors from adaptive immunity are used against a virus?
CD8+ —> cytotoxic T cells
CD4+ T1H —> macrophages / IgG
Cell - dependent immunity
One of the nurses in charge of a ward is 2 months pregnant. How would you check whether she has had chicken pox before?
VzF IgM: IgG ratio
It would be high in a primary response
It would be low in a secondary response
Which factors from adaptive immunity are used against helminths?
CD4+ TH2 —> eosinophils/ B cells/ mast cells
CD4+ TH17 —> neutrophils
Humoral immunity
What are healthcare infections?
Infections arising as a consequence of providing healthcare. They are a major cause of harm in patients receiving healthcare and limit the potential benefits of healthcare innovations.
What are hospital acquired infections?
Infections arising as a consequence of a patient being admitted to hospital. In hospital patients, it is neither present nor incubating at time of admission. This means onset is at least 48 hours after admission. This includes infections in hospital visitors and healthcare visitors.
List some viruses that cause healthcare infections.
Blood borne viruses (Hep B, Hep C, HIV)
Norovirus
Influenza
Chicken pox
List some bacteria that cause healthcare infections.
Staphylococcus aureus including MRSA
Gram negative and tend to be resistant to many antibiotics: Clostridium difficile Escheria coli Klebsiella pneumoniae Myobacterium tuberculosis
What patient factors can increase the risk of them developing a hospital acquired infection?
Obesity/malnourished
Particularly before elective surgery
Diabetes
Blood sugars to be kept stable particularly before elective surgey. High blood sugar levels can weaken the patient’s immune system defences and can cause peripheral vascular damage
Cancer
Immunosuppressed
Smoker
More prone to lung infections eg. Pneumonia
Increased risk of surgical wound infection as less oxygen is transported to tissues
Surgical patient
Emergency admission
Already unwell and immunosuppressed
What are the 4 Ps of infection prevention and control?
Patient
- general and specific patient risk factors for infections
- interactions with patients/HCW/visitors
Pathogen
- virulence factors
- ecological interactions (antibiotics/other bacteria)
Practice
- general + specific activities of healthcare workers
- policies and implementation
Place
-healthcare environment
What is the most common mode of transmission for healthcare infections?
Most healthcare infections are caused by the patients own flora moving somewhere on themselves where it is now pathogenic.
Eg.
E.coli moving from the large bowel where it is normal flora, moving to the urinary tract where it can cause kidney infection or cystitis (UTI)
Staphylococcus aureus moving from the mouth/nose where it is normal flora, moving into a wound where it can cause infections of skin and soft tissue and septicaemia
How can we reduce patients from getting hospital acquired infections due to their own flora moving to a site where it is now pathogenic?
General
- optimise patient’s condition (smoking, nutrition, diabetes)
- antimicrobial prophylaxis
- skin preparation
- hand hygiene
Specific
- MRSA screens
- disinfectant body wash
How can we reduce the risk of infections being transmitted from patient to patient?
Physical barriers
- isolation of infected patients in sideroom/ positive pressure room
- protection of susceptible patients in sideroom/ positive pressure room
How can we reduce the risk of healthcare workers transmitting infections to patients?
Healthy (disease free + vaccinated)
Good practice
- good clinical techniques
- hand hygiene
- personal protective equipment (things to protect healthcare worker from being contaminated by the patient e.g. Plastic aprons, rubber gloves, face masks, respiratory protection)
How can we reduce the risk of the environment being a mode of transmission of infection in hospitals?
- appropriate kitchen and ward food facilities
- good food hygiene practice
- theatres
- positive/negative pressure rooms for immunosuppressed/infected patients
How can healthcare workers identify patients who are at high risk of infection transmission?
Identify
- abroad
- blood borne infections
- colonised
- diarrhoea/vomiting
- funny looking rash
Isolate
Investigate
Inform
Initiate
What promotes antimicrobial resistance?
All exposure of bacteria to antimicrobials - whether appropriate or not
What are the consequences of antimicrobial resistance?
- Treatment failure
- Prophylaxis failure - cannot perform surgery without a high risk of infection
- Economic costs
What does MDR (multi-drug resistant) mean?
Non-susceptibility to at least one agent in three or more antimicrobial categories.
What does XDR (extensively-drug resistant) mean?
Non-susceptibility to at least one agent in all but two or fewer antimicrobial categories
What does PDR (pan-drug resistant) mean?
Non-susceptibility to all agents in all antimicrobial categories.
What are the three types of antimicrobial stewardship interventions?
- persuasive
- restrictive
- structural
Describe the structure and virulence of Clostridium difficile.
Structure:
Bacilli
Gram positive
Virulence: Anaerobic Forms endospores Toxin A - excessive fluid secretion Toxin B - disruption of cell cytoskeleton
How is Clostridium difficile transmitted?
C.difficile is a minor component of the flora of the large intestine
After its introduction to a site, the environment becomes permanently contaminated with spores and new residents are easily colonised
Faecal-oral route
Describe the pathogenesis of Clostridium difficile.
• C.difficile is a minor component of the normal flora of the large intestine
• When antimicrobial treatment suppresses more predominant species in this community, C.difficile proliferates. Virtually all antimicrobial drugs have been reported as predisposing to diseases caused by C.difficile.
◦ Most commonly, clindamycin, ampicillin and cephalosporins
• Pathogenic strains produce two toxins
◦ Toxin A = enterotoxin that causes excessive fluid secretion and stimulates an inflammatory response
◦ Toxin B = disrupts protein synthesis and causes disorganisation of the cytoskeleton
• Forms endospores
What are endospores?
An endospore is a dormant, tough, non-reproductive structure produced by a small number of bacteria.
These are formed when favoured nutrients are exhausted.
Endospores can survive environmental assaults that would normally kill the bacterium.
What diseases are caused by C.diff?
- antibiotic associated diarrhoea
- pseudomembranous colitis (PMC)
Which specific investigation would you request if you suspected an individual of having a C.diff infection?
Stool sample
MC&S
Antigen detection
How is C.diff treated?
Discontinuance of predisposing drug
Fluid replacement
Antibiotic:
fidaxomicin (UHL)
metranidazole (anti-bacterial and anti-protozoal agent, used to treat anaerobic bacteria)
Reconstitution of normal colonic flora
Describe the structure and physiology of Staphylococcus aureus.
Cocci Gram positive Catalase positive Coagulase positive Facultative anaerobe
How is Staphylococcus aureus transmitted?
Staphylococcus aureus is frequently carried by healthy individuals on the skin and mucous membranes. Carriers serve as a source of infection to themselves and others.
• Direct contact
• Contamination of surfaces
• Contamination of food
What are the risk factors for Staphylococcus aureus transmission?
A significant host compromise is required for S. aureus infection
• Break in the skin
• Insertion of a foreign body eg. Wounds, surgical infections, central venous catheters
• Obstructed hair follicle
• Compromised immune system
Describe the virulence of Staphylococcus aureus.
Capsule - prevents phagocytosis by preventing attachment of antibodies/complement
Protein A - binds to IgG exerting an anti-opsonin and anti-phagocytic effect
Cytolytic exotoxins - attack cell membranes (including erythrocytes-haemolysin) resulting in osmotic lysis
Leukocidin - pore-forming toxin loses polymorophonuclear leukocytes
Coagulase - converts fibrinogen to fibrin causing clot formation
Superantigen exotoxins - stimulate enhanced T cell response which can cause toxic shock syndrome by increased release of cytokines
Which skin and soft tissue infections can be caused by Staphylococcus aureus?
Furuncles, carbuncles - boils/ infected hair follicles infected with bacteria
Wound infection
Cellulitis - bacterial skin infection involving the dermis and subcutaneous fat
Impetigo - bacterial skin infection involving the superficial skin
What are the signs and symptoms of toxic shock syndrome caused by Staphylococcus aureus?
Fever
Hypotension
Multisystem organ dysfunction
Erythematous rash with desquamation occurring
How do Staphylococcus aureus infections cause the formation of abscesses?
The development of an abcess is a complex process that involves both bacterial and host factors. S. aureus infections cause a rapid and extensive influx of white blood cells eg. Neutrophils. S. aureus lyses neutrophils (leukocidin) and red blood cells (cytolytic exotoxins and haemolysin) that have entered the infection area. The lysed neutrophils pour out large numbers of lysozymal enzymes, which damage surrounding tissue.
What is pus?
Dead or dying white blood cells and cellular debris and serous fluid and fibrin and bacteria
What diseases are caused by toxins released by Staphylococcus aureus?
- toxic shock syndrome
- scalded skin syndrome
- food poisoning (gastroenteritis)
What is the recommended antibiotic for skin and soft tissue infections caused by Staphylococcus aureus?
IV vancomycin
What infection control measures are important for MRSA?
• Active screening of high risk patients and exposed healthcare workers for carriage
• Strict implementation of transmission based precautions
◦ Hand washing- remains the cornerstone for effective control in hospitals. MRSA spreads from patient to patient via hands of healthcare providers so this is possibly the most important measure to reduce the spread of MRSA in a healthcare setting
◦ Isolation
• Treatment of carriage using topical applications of mupirocin nasal cream (a topical antibiotic) and washing with disinfectant agent, such as stellisept or chlorhexidine
How is norovirus spread?
• Mainly faecal-oral • Respiratory Can be spread by: • Direct person-person contact • Contact with contaminated surfaces • Ingestion of contaminated food
