Principles of Infection Flashcards
(35 cards)
Recall the types of host-microbe interactions
- Commensalism- normal flora
- Mutualism?
- Parasitism
Summarise some opportunistic infections associated with HIV
Oral candidiasis
- Herpes zooster
- Pneumocytosis carnii
- Cytomegalovirus
How can be commensals be pathogen sometimes?
- At another site
- Due to immunosuppression
- By-passing defences
Give examples of commensals that turn into pathogens in different sites
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)
Recall sterile body sites- no commensal organism
- 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
What is a pathogen?
a microbe that can initiate infection, often with only small numbers, via natural routes, despite natural barriers and immune defences
Name a strict pathogen that will always cause disease
Bacillus anthracis (anthrax)
What are virulent microbes?
• Highly pathogenic microbes
What is virulence dependent on?
- Virulence – degree to which it causes disease
- virulent strains
- gene content alters phenotype
- host susceptibility
What is pathogenic potential?
potential to cause pathological disease
According to germ theory what does a microorganism have to do?
- Be present in every case of the infection
- Be cultured from cases in vitro
- Reproduce disease in an animal
- Be isolated from the infected animal
Describe the balance between pathogenic mechanisms and defensive mechanisms
Pathogenic mechanisms: adhesion, capsule, toxins
Defensive: neutral barriers, defensive cells, complement, immune response
Defences of tissue and blood
• usually involves tissue damage and controlled by feedback mechanisms
Natural non-specific barriers:
- physical conditions (dry, acidic), sloughing, microflora, lysozyme,
- toxic lipids, lactoferrin, lactoperoxidases, tight junctions, bile, mucin,
- cilliated epithelia, bile, cryptdins, phagocytes, intraepithelial lymphocytes
Natural adaptive barriers:
MALT( mucosa-associated lymphoid tissue), SALT(skin), GALT(gut), associated lymphoid tissue, secretory IgA
Types of infection
- Local surface infection; wound
- Invasive penetrate barriers – local spread
- Systemic via blood to other sites
- Effects at different site from colonisation - toxinsspecific disease, endotoxins SEPSIS
Recall local and systemic symptoms
Local symptoms (inflammation)
• Redness, swelling, warmth, pain
• Pus – pyogenic infection
Systemic symptoms
• Fever, rigors, chills, tachycardia(heart rhythm disorder), tachypnoea(rapid breathing)
Acute inflammation pathogen examples:
• Acute inflammatory response e.g. infection with Streptococcus pyogenes or Staphylococcus aureus
• Toxin mediated e.g. diphtheria (Corynebacterium diphtheriae)
tetanus (Clostridium tetani)
What is chronic infection?
- Slower onset or post-acute
- But may still have major local and systemic symptoms
- Chronic inflammatory response
- Results when host does not succumb immediately to infection, but cannot clear infection e.g. TB (Mycobacterium tuberculosis), Chronic osteomyelitis (Staphylococcus aureus)
What is asymptomatic 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
Example: No symptoms present e.g. Chlamydia trachomatis
(urethral infection in men, cervical infection in women)
50% males are asymptomatic
80% females are asymptomatic
e.g. herpesvirus shedding post-acute infection
What is reactivation of latent infection?
The virus never goes away it’s always in the body
- When you get older, the virus may be reactivated and this causes shingles or herpes Zoster
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
F-List:
- Fingers
- Fresh Air
- Fomites – (objects or materials which are likely to carry infection)
- Fluids (blood etc.)
- Faeces
- Flies
- Food
- Foetus
- Fornication
Examples of mucosal contact:
- genital tract – gonorrhea, chlamydia, HIV, HepB, syphilis
- Saliva – Herpes, CMV, EBV
- Skin – Staphylococci, VZV, HPV,
• fungal infections
