Immunity and infection Flashcards
Classify microbes into 4 broad biological groups
microbes can be classified into: bacteria, viruses, fungi, parasites (Protozoa and helminths)
Describe and explain viruses
Viruses are 20-300nm, obligate intracellular organisms. Can have DNA or RNA (not both) core w/in protein capsid made up of capsomeres
Can have a helical, cubic or more complex arrangement
No cytoplasm
May have envelope derived from host cell
Has membrane bound proteins
Viruses replicate by a variety of mechanisms
Draw and label a simplified virus structure
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Describe some common viral infections in the UK and their origin
Upper resp tract infection: influenza, RSV, covid
Gastroenteritis: Norovirus and adenovirus
Rashes: varicella zoster virus (chickenpox), measles
Meningitis e.g. enterovirus
Hospital Acquired infection: Norovirus
Draw and label the basic structure of a bacterium
Describe gram positive bacterial cell walls, and how penicillin targets them
Gram-positive consists of a thick peptidog layer and teichoic acid.
Peptidoglycan cross linkages is the target site for penicillins.
Crystal violet in the stain binds to teichoic acid and resists decolouring to leave a purple colour
Describe gram negative bacterial walls
Gram-neg wall consists of an outer membrane. This is made of phospholipids, lipopolysaccs, proteins and protein channels called porins.
Between the outer membrane and cell membrane hay the periplasmic space. This stores antibiotic inactivating enzymes in some bacteria. Hay a thin peptidog layer in this space.
What other features do bacteria have?
Flagella/axial filaments for independent movement.
Some bacterial surfaces have pili or fimbriae, involved in conjugation and help attachment to different mucosal surfaces.
Outside the cell wall, a capsule and slime layer adhere to other membranes and prevent phagocytosis.They are also antigenic.
Describe how bacteria grow in certain conditions
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List some common bacterial infections and where they originate from
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Describe and explain fungi
Fungi are over 2μm, eukaryotic, w single or multiple nuclei. Has haploid or diploid DNA and a rigid chitin cell wall.
May be uni or multicellular
May be dimorphic (switch between yeast and mould).
Classified based on morphology and mode of reproduction: sexual, teleomorphasexual, anamorph
How else can fungi be classified?
It’s easier to classify fungi by the type of infection they cause:
Superficial mycoses of skin, nails, hair and mucous membranes e.g. ringworm or thrush
Subcutaneous mycoses e.g. mycetoma
Systemic mycoses e.g. Histoplasmosis.
Describe protozoa
Protozoa: Single cell. 5-300 μm. Single or multiple nuclei. Haploid DNA. Morphology varies throughout life cycle. May have flagella.
Eg Plasmodium sp. which causes malaria
Describe helminths, a type of parasite
Helminths: multicellular parasitic worms. Can be microscopic or visible to the naked eye.
Helminths can be divided into:
Cestodes: tapeworms
Trematodes: flatworms or flukes.
Nematodes: roundworms
How else are bacteria visualised apart from microscopy?
Suspended in liquid broth- when live, some bacteria are seen being motile, others seen spinning at the same spot (Brownian movement.)
Bacteria is seen more easily if stained by Gram
How can we classify bacteria by shape and arrangement?
Spherical bacteria= cocci, bacilli= rod shaped.
Spirochaetes= helical, Vibrios= comma shaped.
Lanceolate=shaped like a lancet (arched window).
Arranged in pairs, eg diplococci, in chains eg streptococci, or in clusters e.g staphylococci.
Describe explain a bacterial growth curve
Bacteria must adjust to the new environment before multiplying (lag).
Bacterial growth reaches full potential w exponential growth (log).
Depletion of nutrients slows growth and causes some death- (stationary)
When nutrient is fully exhausted, bacterial growth fully ceases (death phase)
What is microbiota?
The human microbiota consists of the 10-100 trillion symbiotic microbial cells harboured by each person.
What is the microbiome?
The microbiome is the collective genomes of the microbes (composed of bacteria, bacteriophage, fungi, protozoa and viruses) that live in/on the human body.
What is symbiosis?
Some microbes need a human/other animal to survive and multiply.
Symbiosis does not distinguish between relationships that are harmful or beneficial to either party
Describe the normal flora of the skin
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Describe the normal flora of the GI tract
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Describe the normal flora of the nose, Nasopharynx & upper resp. tract
Flora may vary at different sites like the teeth, gums, or throat
what is the difference between commensal microbes and pathogens?
Commensal microbes (normal flora) colonise the host. No harm is done to the host.
A pathogen is a microbe that can cause disease, often w small numbers, via natural routes
Usually the host forms an inflammatory response to a pathogen, but not to a commensal at a normally non-sterile site of the body.
What can some commensals do if the host is immunocompromised?
Sometimes some commensals act as pathogens. If the host is immunocompromised infection may occur (opportunistic infection).
In some patients w AIDS, lung infection is seen with Pneumocystic jiroveci (a low virulence fungus).
In healthy ppl exposed to Pneumocystis jiroveci the i.system prevents infection. Virulence is a term that describes the degree of pathogenicity.
Do all commensals behave the same in all body sites?
Some commensals at 1 body site may be pathogens at another body site e.g. Staph aureus in the nose is commensal but Staphylococcus aureus in a post-op wound causes infection.
E coli in GI tract is commensal but causes a UTI in urinary tract, so is a pathogen.
What are sterile sites?
Some body sites don’t have a normal flora so are sterile. These inc: lower resp tract, blood, bone, joint and subcutaneous connective tissue, female upper genital tract, urinary tract, CNS, and other viscera
If any organism gains entry to a normally sterile site, an infection may occur.
But the Human Microbiome Project found some sterile organs do have a microbiota. Yet the organisms aren’t cultureable and is only detected by molecular methods.
When would a microbe behave as a pathogen?
Whether a microbe behaves as a pathogen depends on the properties of the microbe and the defensive properties of the host.
If the balance is in favour of the microbe, hay infection
If the balance is in favour of the host, no hay infection
Outline the steps required for establishment of Infection
- Survival of microbe and spread to the host
- Adherence of microbe to the host and entry into the host
- Multiplication
- Evasion of host defences
- Damage to the host
- Shedding of microbe and spread to the environment or to another host
How do microbes survive?
Some microbes only survive in the host environment. Others can survive in water, soil, air, dust etc.
Some organisms produce spores which survive extreme environments for many years e.g. Bacillus anthracis (the cause of anthrax). Others e.g. M.tb have a waxy coat.
Describe and explain airborne/droplet spread.
Airborne/droplet spread: usually occurs in resp tract infections e.g. TB, measles. When a person coughs etc, droplets are expelled into the surrounding air.
Droplets> 5 diameter microns behave ballistically, falling onto surfaces. Microbes in droplets can infect if they’re touched and then inoculated onto mucous membranes.
Droplets<5 microns are droplet nuclei/aerosols. They remain airborne and may travel long distances from the cough. The time that the microbes within these aerosols remain viable depends on humidity and temp.
Some viruses like measles spread easily by the aerosol route and have a high R0 (mean number of ppl infected by one infected individual).
Describe other ways in which microbes spread infection.
Through the skin, eg Plasmodium falciparum (malaria) after anopheles mosquito bite.
Fomites= contaminated objects that pass on infection.
Other epithelial surfaces e.g. oral or genital mucosa and STDs like gonorrhoea.
Microbes can spread via the GI tract in food and water. Faeco-oral route.eg Salmonella or enterohaemorrhagic E coli infection
What happens once microbes are internalised?
Once microbes are internalised, they multiply locally to produce a focus of infection.
During multiplication microbes produce enzymes which invade surrounding tissues and deeper sites. Some bacteria also produce toxins and tissue destroying enzymes.
Following multiplication, the infection may remain localised e.g. an abscess, or spread to contiguous tissues, or spread via the blood, eg septicaemia.
How does a microbe find a new host?
Sometimes a microbe needs to find a new host, which it does by:
Exiting via the resp tract (coughing, sneezing, talking), via the GI tract, epithelial surfaces like the genital tract, via blood-sucking insects, from hands or body fluids onto fomites.
Outline the chemical barriers
The chemical barriers are mostly innate Immunity.
Microbicidal substances e.g. lysozyme in tears, gastric acid and antibac substances in urine.
Acute phase proteins, interferons and complement inhibit microbes.
Phagocytic cells e.g. Mononuclear phagocytes-monocytes and macrophages, and NK cells.
Describe the symptoms of infection
Local symptoms (inflammation): redness, swelling, warmth, pain, loss of function
Pus – pyogenic infection
Systemic symptoms: Fever, rigors (shaking), chills, tachycardia, tachypnoea (fast breathing). These symptoms are from more serious infections
Some infectious agents cause asymptomatic infection
Describe acute versus chronic infection.
Acute–Rapid onset, brief period of symptoms and resolution within days
Chronic–Persistent infection lasting months or even years e.g. tb and HIV.
What kind of cells are in the innate vs adaptive immune system? Draw a venn diagram to illustrate this
Describe the cells in the innate immune response
Cell Mediated (Innate): Phagocytes, NK cells
Humoral (extracellular innate): Complement & Pentraxins (CRP), pattern receptors, Lysozyme, cytokines, binding proteins (Lactoferrin)
Most cells forming the innate system have granules which destroy nonself pathogens (granulocytes.)
What is the difference between innate and adaptive immune systems
The Innate Responds to new infections quickly. It acts on PRE-DETERMINED NON-SELF SIGNALS.
SOMETIMES the innate response can be primed for intense re-activation (INNATE MEMORY)
Adaptive responds to new infections slowly. It selects for SPECIFIC SIGNALS and generates MEMORY
Are some pathogens resistant to phagocytosis?
Salmonella dampens the inflammatory response, is resistant to ROS and RNS (reactive 02 + nitrogen species)
Mycobacterium prevents phagosome-lysosome fusion
Pathogens that cannot be killed by phagocytes persist dormantly in the phagosome (chronic infection). It’s there until the phagocyte dies, the pathogen is released and becomes active.
Describe chlamidya trachomitis
- Obligate intracellular bacteria- can only live inside a cell
- Gram negative
- It has 15 serotypes
- As well as causing sexual infections it is known for causing Trachoma (eye infections) in the developing world, and conjunctivitis in newbornbabies
- Can cause pelvic inflammatory disease, ectopic pregnancy and accounts for 30% of tubal factor infertility
What are risk factors for chlamydia?
How does chlamydia present? What are the symptoms upon examination?
Women; inflamed cervix- strawberry colour, discharge, abdominal pain, pain on moving the cervix- sign of PID
Men; urethral discharge, testicular tenderness
How is chlamydia treated?
