Principles of Infection Flashcards
What are commensals and pathogens?
Infection implies- harm is done to the host i.e. causes disease
A microbe that causes disease is a pathogen
Usually the host will manifest an inflammatory response to a pathogen, but not to a coloniser (commensal) at a normally non-sterile site
How can commensals sometimes be pathogens?
At another site
Due to immunosuppression
By-passing defences
Staphylococcus aureus in the nose (commensal)
Staphylococcus aureus in a post-operative wound infection (pathogen)
Escherichia coli in GI tract (commensal)
Escherichia coli in urinary tract causing UTI (pathogen)
Staphylococcus epidermidis on skin (commensal)
Staphylococcus epidermidis bloodstream infection following infection of an intravenous line (pathogen)
What are the normally sterile body sites?
Sites without a normal flora are sterile Lower respiratory tract Blood Bone, joint and subcutaneous connective tissue Female upper genital tract Urinary tract (not distal urethra) CNS including CSF and eye Other viscera e.g. liver, spleen, pancreas
How can pathogens vary?
A pathogen is a microbe that can initiate infection, often with only small numbers, via natural routes, despite natural barriers and immune defences
Some organisms are strict pathogens i.e. will always cause disease e.g. Bacillus anthracis (anthrax)
Some are conditional pathogens
Highly pathogenic microbes are said to be virulent
Virulence- degree to which it causes disease
Virulent strains
Gene content alters phenotype
Host susceptibility
What are Koch’s postulates?
Germ theory A microorganism has to: Be present in every case of the infection Be cultured from cases in vitro Reproduces disease in an animal Be isolated from the infected animal
What are some types of infection?
Local- surface infection; wound
Invasive- penetrate barriers- local spread
Systemic- via blood to other sites
Effects at a different site from colonisation-
Toxins—> specific diseases e.g. tetanus
Endotoxins (part of bacterial cell wall)—> sepsis
Immunopathology:
Inflammation causes tissue damage
Cross-reactive antigens e.g. Streptococci and rheumatic heart disease
Granuloma e.g. Tuberculosis
What is inflammation?
Response to tissue injury- functions to bring serum molecules and cells to site of infection
1. ↑Blood supply
2. ↑Capillary permeability
3. Migration of cells from blood to tissue (polymorphs, macrophages)
Ordered Process- regulated
Vasodilation, oedema, complement activation, mast cell degranulation, PMNs recruitment, clotting
What are the characteristics of an acute infection?
Rapid onset
Major local and systemic symptoms
Acute inflammatory response e.g. infection with streptococcus pyogenes or staphylococcus aureus
Toxin mediated e.g. diphtheria (Corynebacterium diphtheriae), tetanus (Clostridium tetani)
What are the characteristics of a chronic infection?
Slower onset or post-acute
But may still have major local and systemic symptoms
Chronic inflammatory response
Results when host does no succumb immediately to infection, but cannot clear infection e.g. TB (mycobacterium tuberculosis), Chronic osteomyelitis (Staphylococcus aureus)
What are the characteristics of a symptomatic infection?
Infection with a pathogenic microbe (not a commensal or part of the normal flora)
Inflammatory response is mild or none at all
Damage to the host is mild or not at all
No symptoms present e.g.:
Chlamydia trachomatis 50% males and 80% females are asymptomatic
E.g. herpesvirus shedding post acute infection
What are the stages of infection?
Acquisition from spread- 9Fs Colonisation- adherence Penetration and Spread- local or general Immune evasion Tissue damage Shedding and transmission Resolution Not all microbes need all stages
How does infection spread?
Direct contact:
Indirect contact
Airborne
Blood products
Human sources:
Bugs on fingers, toilet seat, surfaces etc.
Direct spread to other
Contamination of food
Contamination of drinking water by sewage
Contamination of vegetables by sewage
Breathing air-flu, TB
Mucosal contact:
Genital tract- gonorrhoea, chlamydia, HIV, HepB, syphilis
Saliva- Herpes, CMV, EBV
Skin- Staphylococci, VZV, HPV, fungal infections
How can a host damage its immune system?
Over-activity of immune defences
Endotoxin- all Gram-negative bacteria –> sepsis
Types of “hypersensitivity” e.g. Type 4- granuloma in TB
Cross-reaction of antibodies against streptococcal antigens with host antigens- myocardium, synovium, brain e.g. Streptococcus pyogenes in rheumatic fever
