Infectious disease 1/2 Flashcards
(14 cards)
Briefly explain Koch’s postulates
> Came up with a system to show a particular microbe can cause a specific disease:
- He took a sample from an infected animal (e.g. sheep)
- Isolated bacteria using microscopes and rudimentary forms of agar from sheep that were infected but not from healthy sheep
- Grew bacteria in pure culture
- Put the culture into a rabbit
- Observed the rabbits – if they got sick showing the same symptoms then they had the same disease
- Then they isolated bacteria from rabbits and tested on another animal etc. (Forming Koch’s postulates)
Rules:
- The microorganism must be present in all sick organisms, and not present in healthy organisms
- The microorganism must be able to be isolated from the sick organism and grown in pure culture
- After growth in pure culture, the microorganism should cause the disease when introduced into a healthy individual
- The same microorganism must be isolated from the newly infected organism
Koch’s postulates: exception?
- Some bacteria may not cause disease In everyone although it is present in the individual
- Viruses and many bacteria that don’t grow in agar
List and briefly describe the general principles of microbial pathogenesis
Principles of Microbial Pathogenesis:
- Causative agent
- Host defences
- Microbial entry
- Spread/dissemination within the host
- Tropism within the host
- Pathogenesis (how disease is caused by the pathogen)
- Pathology (what disease is caused)
- Transmission and epidemiology
Name the different types of infectious agents and explain the principles of their classification
- Parasites
- Unicellular (protozoa)/multicellular (helminths)
- classified into:
> Protozoa
- 1-50um, single-celled eukaryotes
> Arthropods (ectoparasite)
- 1-5mm, remain outside the host (skin)
> Helminths
- 3-10+ mm
- Parasitic worms (multicellular organisms)
- complex life cycles - Fungi
- normally superficial infections - hair, skin, fingernails
- systemic infections usually only in immunocompromised host
- eukaryotes
- single or multicellular forms
- 2-200um
- can grow as: slender filamentous hyphae, rounded yeast cells - Viruses
- o Obligate intracellular parasites
o Require host biosynthetic and replicative apparatus for proliferation
o Composed of nucleus, capsid, lipid membrane (some)
o Size (20-300nm)
o Classification of viruses
Genome (RNA or DNA)
Shape of capsid
* E.g. icosahedral or helical
Lipid envelope
Mode of replication
Tropism
Pathology - Bacteria
- single-celled
- prokaryotes
- can make own nucleic acids and proteins
- can survive and replicate inside and outside cell
- oxygen requirements (aerobic/anaerobic)
- cell wall structure (Gram staining)
- shape (rod, coccus, spiral, etc.)
- surface structure (pilli, flagella, capsules) - Prions
o Transmissible protein
o Misfolded form of normal cellular prion protein
Briefly explain the potential causes of newly emerging infectious diseases
New & emerging infectious diseases
- Changes in human demographics and behaviour
- Changes in the environment
- Drug resistance
- Could not culture the pathogen until recently
o Helicobacter pylori
- The disease is genuinely new to humans (zoonosis or otherwise)
o Borrelia burgdorferi (Lyme disease)
o New viral strains (e.g. influenza, coronavirus)
- Infection more common due to immunosuppression
o AIDS, therapy for transplants and cancer (e.g. fungal infections)
- Introduction into a new area
o West Nile virus in the US, Zika virus in Central and South America
- Emerging viruses – Influenza
o New seasonal influenza strains – antigenic drift
Arising from mutation of existing subtypes
o New potentially pandemic influenza strains – antigenic shift
A completely new subtype (e.g. H5N1) introduced into the human population – antigenically very distinct virus so few have immunity
Can arise in a number of ways
* Zoonosis (moves from animals to humans)
o Infection with an animal influenza virus that has mutated to infect humans
o Genetic reassortment – mixing of two different viral strains in compatible host cells (e.g. pigs) to make a new human subtype
* Re-emergence of historical strains
Describe the various means the body uses to defend against microbial invasion
Skin
- Microbial growth is inhibited by
o Dense keratinised outer layer
o Low pH
o Fatty acids
o Exception: normal microbiota
- Most microorganisms penetrate through breaks in the skin
o Wounds, burns. Needle sticks, intravenous catheters, insect/animal bite
Gastrointestinal tract
- Microbial growth (other than normal microbiota) and entry inhibited by
o Acidic gastric secretions
o Viscous mucous lining
o Lytic pancreatic enzymes and bile detergents
o Defensins (anti-microbial peptides)
o Peristalsis – mechanical
o Normal microbiota – biological
- Infection via the GIT occurs when:
o Local defences are weakened
Antibiotics can alter normal microbiota
Immunocompromised host
o Organisms develop ways to overcome these defences
Adhesion, proliferation, invasion
Respiratory tract
- A large number of microorganisms are inhaled daily
o Large particles: removed by the mucociliary elevator
o Smaller particles: phagocytosed by alveolar macrophages
- Some microorganisms can overcome these defences
o Some bacteria impair ciliary activity
o Influenza binds to the surface of epithelial cells
o M. tuberculosis is engulfed by macrophages but prevents killing by the phagolysosome
- Defences can be weakened by:
o Smoking
o Cystic fibrosis or other chronic disease
Describe ways microorganisms spread within and between hosts
Explain the general principles of how microorganisms cause disease
Discuss, with examples, how viruses and bacteria cause disease
Define DALY
DALY: Disability-adjusted life year
o So it encompasses morbidity (condition of suffering from a disease) and mortality of diseases
Morbidity is the state of being unhealthy, whereas mortality is the rate at which people die from a specific cause.
DALYs for a disease or health condition are the sum of the years of life lost to due to premature mortality (YLLs) and the years lived with a disability (YLDs) due to prevalent cases of the disease or health condition in a population
Differentiate between communicable and non-communicable diseases
Communicable vs non-communicable diseases
o Communicable: infectious diseases spread from person to person, animal to person or person to animal
o Non-communicable: chronic diseases that aren’t contagious, linked to lifestyle factors, genetics and environmental conditions
Define pathogen
Pathogen:
o Infectious agents that cause disease and damage tissue (I e. cause pathology)
Pathogenic microorganisms or pathogens
o Pathos = disease or suffering
o + genic = producing
Pathogen = disease-producer
o Infection = success
Persistence or multiplication of a pathogen on or within the host
Describe normal microbiota
(Also cslled Commensals, normal flora or resident flora)
- Microorganisms that live on or within the body in non-sterile areas
o Skin
o Mucous membranes
o Bowel/rectum
- Not subject to inflammatory or immune attacks as long as skin and mucosa are intact
- Balance of homeostasis to prevent infection/disease
o Antibiotic use can change normal microbiota profile
o Immunocompromise can lead to opportunistic infections by normal microbiota
- Influences development of healthy immune system
Miasma theory
Diseases such as cholera and the Black Death were caused by vapours from contaminated water, foul air, and poor hygienic conditions
- “Spontaneous generation”: formation of living organisms occurs without descent from similar organisms
Germ theory
Microorganisms dont spontaneously generate
- Nutrient broth is boiled to kill microbial life
- broth in experiment 1 exposed to air and broth in experiment 2 protected by swan neck of flask
- broth in experiment 1becomes cloudy and broth in experiment 2 remains clear
