How Bacteria Causes Disease and How Infections Spread Flashcards
What are commensals
These are bacteria that are colonised in a healthy host and do not cause disease
What are opportunistic pathogens
Pathogens that tend to cause disease when the opportunity arises caused by disturbance in the homeostasis between host and commensal
Give examples of opportunities for infection that opportunistic bacteria might take advantage of
- In immunocompromised hosts
- When they get to sites where they don’t normally live
- Antibiotics and other therapies
- Dietary imbalances
Give examples of pathogenic bacteria that can be spread due to asymptomatic carriage
- Staphylococcus Aureus in the nose
- Streptococcus Pneumoniae in the respiratory tract
- Salmonella typhi in the gut or the gall bladder
How can the normal flora help prevent infection
The bacteria competes with pathogens for colonisation and invasion by undesirable, exogenous organisms
What is Virulence
This is the quantitative ability of a pathogenic microorganism to cause disease
What microbial factors influence virulence
- Transmissibility
- Infectivity
- Invasiveness/Toxicity
- Ability to evade host defences
What host factors influence virulence
- Age
- Genetic factors
- General host defences and local host defences
- Immunodeficiency
What are some exogenous sources of infection
- Person to person
- Animal pathogens may spread to humans - zoonosis
- Human can be infected by organisms in the environment
Define an endemic infection
Always present in a population at a more or less constant level though the level may be cyclic
Define an epidemic infection
Higher than normal level in the population usually much higher than the endemic level and usually short term
Define an outbreak infection
Localised increase in incidence of a disease e.g. e.coli food poisoning
Define a pandemic infection
An epidemic spreading between continents
What is horizontal and vertical infection transmission
Horizontal - Transmission or organism between individuals
Vertical - Transmission from mother to offspring in utero/around birth
How can vertical spread of congenital infections occur and give examples of infections that can be spread by each pathway
- From mother’s bloodstream and by crossing the placenta e.g. syphilis, rubella, toxoplasmosis
- May be acquired shortly before or during delivery e.g. Herpes simplex, hepatitis B virus
Name some modes of transmission for infections
- Respiratory spread
- Faecal-oral spread
- Via the skin or mucous membranes
- Blood
- Fomites - inanimate object that can carry and transmit microorganisms
- Zoonoses
- Sexual transmission
What is cellulitis and what is it often caused by
A spreading infection of the soft tissues, most commonly caused by streptococcus pyogenes, getting into a small abrasion of the skin
What is erythema and oedema
Erythema - reddening
Oedema - swelling
What are some infections that can spread by faecal-oral spread
Salmonella spp.
Shigella spp.
Hepatitis A
How can blood borne transmission occur
- Needles
- Direct transmission
- Insects like mosquitos and fleas
What is a Nosocomial infection
Hospital Acquired infection
What are the most common types of nosocomial infection
- Urinary tract infections
- Wound/skin and soft tissue infections
- respiratory tract infections
How can spread of infection in hospitals be minimised
- Isolating patients with known resistant organisms if at all possible
- Sensible adherence to sensible infection control policies - isolating patients in side rooms if required
- Handwashing between patients
What affects how easily an infection can spread
- Ability to survive
- Ability to find alternative host
- Shedding capacity
- Infectivity
- Virulence
- Ability to evade immune response
What respiratory infections can be spread by shedding
- Influenza
- Measles
- M. tuberculosis
What skin-contact infections can be spread by shedding
- Herpes Simplex
- Staphylococcus aureus
What facial-oral infections can be spread by shedding
- Salmonella typhimurium
- Hepatitis A
What is infectivity
This is the ability of an infection to enter, multiply and survive in host
What factors do infections need to be able to resist outside the host
- Drying
- UV
- Water
- Soil
What are some ways that invading organisms can attach themselves to host tissues to colonise the body
- Neisseria gonorrhoea adheres to the genital mucosa by fimbriae
- Giardia laamblia attaches to the jejunal mucosa by a special sucking disc
- Influenza virus attaches to host cells by its hemagglutinin antigen
How do respiratory bacteria overcome host immune defences
They secrete an IgA protease which degrades host immunoglobulin
How does streptococcus pyogenes
Expresses protein A which binds host immunoglobulin preventing opsonisation and complement activation
How can organisms avoid destruction by host phagocytes
- Streptococcus pneumonia has a polysaccharide capsule which inhibits uptake by polymorphs
- Some organisms e.g. mycobacterium tuberculosis can survive inside host macrophages
What kinds of enzymes can pathogens express to damage the host
Coagulase (s.aureus) Proteases Lipases Nucleases Glycosidases
What kind of bacteria release exotoxins
Gram positive bacteria
What kind of bacteria release endotoxins
Gram negative bacteria
Name some properties of exotoxins
- Highly toxic, acts at specific target sites
- Neutralised by antitoxin
- Often destroyed by heat
Name some types exotoxin
Streptococccal erythrogenic toxin - can lead to scarlet fever
S. Aureus enterotoxin = diarrhoea
Vibrio cholera enterotoxin = stimulates adenyl cyclase = watery diarrhoea
Describe the morphology of vibrio cholerae
- Curved bacterium rod-shaped
- Gram-negative
- Motile
How do endotoxins damage the host
They cause the host cells to produce IL-1 and TNF causing fever and shock
Where are endotoxins found
Found int he outer later of gram negative bacteria cell walls
What steps should you go through in the diagnosis process
- History
- Clinical examination
- Laboratory investigations
- Others as needed - x rays an dat
What general questions should you ask when asking for patient history
- Travel
- Contact with animals and infected people
- Food
- Drugs
- Symptoms and duration
What ways can pathogens be detected for certain
Culture - gold standard
- Microscopy
- Immunofluorescence
- Colorimetric
- Nucleic acid extraction/pCR amplification/ sequencing tech.
What do you need to look for in a general serological diagnosis
at least 4x rise in antibody titre in acute infection or presence of IgM
Why do you look for IgM in general serological diagnosis
It is the first antibody to appear in response to initial exposure to antigen
When is serology particularly useful in diagnosis
In viral infections or when it is hard to grow the presumptive bacteria
