M1: How bacteria cause disease Flashcards
What are commensals
Bacteria colonised on a healthy host that do not cause disease
What are opportunistic pathogens?
Pathogens that arises caused by disturbance in the homeostasis between host and commensal
Define carriage
When a person asymptomatically carries a pathogenic microorganism which can be transmitted to other people
How does normal flora help prevent infection?
By competing with pathogens for colonisation sites
What is virulence
The quantitative ability of a pathogenic microorganism to cause disease
The virulence/pathogenicity of a microbe depends on what?
Depends on microbial and host factors
What microbial factors affect virulence? (4)
- Transmissibility
- Infectivity
- Invasiveness/toxicity
- Ability to evade host defences
What host factors affect virulence? (4)
- Age
- Genetic factors
- General and local host defences
- Immunodeficiency
What are 3 exogenous sources of infections?
- Other humans
- Animals
- Environment (water)
What is an endemic infection?
Always present in a population at a more or less constant level, although the level may be cyclic
What is an epidemic infection?
A higher than normal level in the population, usually much higher than the endemic level
What is an outbreak infection?
Localised increase in incidence of a disease
What is a pandemic infection?
An epidemic spreading between continents
How can congenital infections spread? (2)
- Via mother’s blood stream and by crossing the placenta
- Acquired shortly before or during delivery
What are fomites?
Inanimate object that can carry and transmit microorganism
How does skin to skin transfer normally occur?
Transfer usually occurs through abrasion in the epidermis
What is cellulitis?
How is it caused?
- A spreading infection of the soft tissues
- Streptococcus progenies getting into a small abrasion of skin
How is herpes simplex spread?
Transferred directly by contact with infected secretions (salivary spread)
What is respiratory spread dependant on? (3)
- The amount of infective discharge
- Size of aerosol droplets
- The resistance of the infectious agent to drying (or inactivation by UV)
How can blood borne transmission occur? (2)
- Use of blood products (dirty needles)
- Through insect host
How is congenital syphilis spread?
Spread can occur to the infant via birth canal
What are the most common types of nosocomial infections? (3)
- Urinary tract infections
- Wound/skin and soft tissue infections
- Respiratory tract infections
How can infection spread in a hospital setting be minimised? (3)
- Isolating patients
- Adhere to infection control
- Hand washing between patients
How easily the infection spreads depends on its? (6)
- Ability to survive
- Ability to find alternative host
- Shedding capacity
- Infectivity
- Virulence
- Ability to evade immune response
What is the degree of infectivity?
Ability to enter, multiply and survive in host
How do invading organisms colonise the body?
Invading organisms must attach themselves to host tissues to colonise the body
What does neisseria gonorrhoea adhere to?
Genital mucosa by fimbriae
How does the influenza virus attach to host cells?
By its hemagglutinin antigen
How do respiratory bacteria evade the immune response?
Respiratory bacteria secrete an IgA protease which degrades host immunoglobulin.
How does Streptococcus pyogenes evade the immune response?
Expresses protein A which binds to host immunoglobulin preventing opsonisation and complement activation
How does Streptococcus pneumoniae evade the immune response?
Has a polysaccharide capsule which inhibits uptake by polymorphs
How does Mycobacterium tuberculosis evade the immune response?
Mycobacterium tuberculosis can survive inside host macrophages
Exotoxins are excreted by?
Produced from organism
Gram positive bacteria
Endotoxin are excreted by?
Produced within organism
Gram negative bacteria
List some features of exotoxins (4)
- Highly toxic
- Act at specific target sites
- Neutralised by antitoxin.
- Often destroyed by heat
What is the mechanism of action for endotoxins?
Causes host cell to produce IL-1 and tumour necrosis factor causing fever and shock
How can IgM levels help diagnose an infection?
IgM is the first antibody to appear in response to initial exposure to antigen
