Lecture 10 Flashcards
(21 cards)
True Pathogen (Obligate Pathogen)
Definition: A microorganism that can cause disease in healthy individuals with normal immune defences.
Examples:
Vibrio cholerae – causes cholera.
Chlamydia trachomatis – causes sexually transmitted infections (STIs).
Opportunistic Pathogen (‘Opportunist’)
Definition: An organism that only causes disease when the host’s immune defences are compromised (e.g. weakened immunity, breached skin/mucosa).
Note: Some are normally harmless commensals (organisms living in the body without causing harm) but become harmful under certain conditions.
Examples:
Staphylococcus aureus
HOW DO WE PROVE A MICROBE CAUSES DISEASE? Koch’s Postulates
Presence: The pathogen must be found in every case of the disease.
⚠️ Problem: Asymptomatic carriers may have the pathogen without disease.
Isolation: The pathogen must be isolated and grown in pure culture.
⚠️ Problem: Viruses can’t be grown without a host cell.
Reproduction: When introduced into a healthy susceptible host, the same disease should occur.
⚠️ Problem: Not all hosts will respond the same (host immunity differences).
Re-isolation: The pathogen should be recoverable from the experimentally infected host.
Infectious Dose and LD50 (Lethal Dose 50%)
Infectious Dose
The number of organisms required to cause infection.
Affects whether or not an infection takes hold.
LD50 (Lethal Dose 50%)
The number of organisms required to kill 50% of test subjects.
Lower LD50 = more virulent.
VIRULENCE FACTORS
Toxins
Disrupt host cell function or kill cells.
Corynebacterium diphtheriae – diphtheria toxin
Capsules
Protect bacteria from phagocytosis by immune cells.
Adhesins
Help bacteria stick to host tissues
COMMENSAL FLORA
Microorganisms that naturally live on/in the human body without causing disease (under normal conditions).
Bacteria, Protozoa, Fungi, Archaea, Viruses
Found on:
Large Intestine
Skin
Mouth
Nose
Throat
More anaerobes in colon
HARMFUL EFFECTS OF COMMENSAL FLORA
When they escape their normal site:
Peritonitis from Bacteroides and E. coli after appendix rupture
UTI (Urinary Tract Infection) – E. coli from gut to urinary tract
ALTERATIONS TO GUT FLORA (e.g. antibiotics)
Antibiotics kill off sensitive gut bacteria
Resistant species overgrow, e.g. Clostridioides difficile
C. difficile produces toxins, causing:
Diarrhoea
ALTERATIONS TO GUT FLORA (e.g. antibiotics) treatment
Stop antibiotic
Give:
Oral metronidazole
Or vancomycin
Recovery depends on restoring normal flora
Use probiotics or faecal transplant
OPPORTUNISTIC INFECTION – WHO IS VULNERABLE?
- Immunosuppressed:
Cancer
Transplant patients - Breached defences:
IV line
Catheter
Wounds - Foreign bodies:
Prosthetics
Splinters - General weakness:
Malnutrition
Illness
Old age
CLOSTRIDIOIDES DIFFICILE – INFECTION (oppurtunistic)
Features
Gram-positive rod
Spore-forming
Strict anaerobe
Symptoms
Frequent diarrhoea
Fever
Nausea, pain
Loss of appetite
At-risk groups:
Taking:
Broad-spectrum or multiple/long-term antibiotics
Proton pump inhibitors (reduce stomach acid)
Hospital stays
Age > 65
IBD, cancer, kidney issues
Weakened immunity
Digestive system surgery
CLOSTRIDIOIDES DIFFICILE – INFECTION (oppurtunistic) STEPS OF INFECTION
Contamination
Disruption of normal gut flora
Spore germination
Colonisation
Toxin production
Opportunistic Fungal Infection: Mucormycosis
Mucormycosis is a serious fungal infection caused by a group of molds called Mucorales.
commonly found in soil, decaying organic matter, and compost
These molds are opportunistic pathogens — they don’t usually cause disease in healthy people, but can cause severe and rapidly progressing infections in people with weakened immune systems.
Opportunistic Fungal Infection: Mucormycosis types
Rhinocerebral mucormycosis: Starts in the nose/sinuses and can spread to the brain — especially dangerous.
Pulmonary mucormycosis: Inhalation of spores into lungs — seen in transplant patients.
Cutaneous mucormycosis: Through broken skin (wounds or burns).
Gastrointestinal mucormycosis: Mostly in premature infants or immunocompromised.
Disseminated mucormycosis: Spread through the blood to multiple organs.
Opportunistic bacterial infection: Polar Leprosy
This describes the spectrum of disease caused by Mycobacterium leprae — the bacterium that causes leprosy
The form of disease depends on the host’s immune response, especially T-cell immunity.
Strong T-cell immunity = Tuberculoid Leprosy
Localised skin lesions and nerve damage. Low bacterial load. Less contagious.
Weak T-cell immunity = Lepromatous Leprosy
Widespread skin nodules, nerve damage, high bacterial load. Highly contagious.
Opportunistic infections: Latent pathogens
These are pathogens that can remain hidden in the body after the initial infection, and reactivate later, especially when immunity is low.
e.g. Mycobacterium tuberculosis (TB) — may stay dormant for years in lungs.
Splenic Function
The spleen plays a major role in immune defence, especially against bloodborne pathogens.
🛡️ Functions:
Produces antibodies (especially IgM)
Filters the blood — removes damaged/aged red blood cells and bacteria
Helps with clearance of encapsulated bacteria (e.g., Streptococcus pneumoniae)
Opportunistic infections: Post-splenectomy Sepsis (PSS)
After spleen removal (splenectomy), the person is at very high risk for rapid, overwhelming bacterial infection — called Overwhelming Post-Splenectomy Infection (OPSI).
🧨 Features:
Rapid onset — symptoms can develop within hours.
Very severe — leads to sepsis, organ failure, and death in up to 50–70% of cases if not treated early.
Opportunistic infections: Post-splenectomy Sepsis (PSS) - prevention
⚠️ Prevention is vital:
Educate the patient:
Carry a splenectomy alert card
Educate healthcare providers:
Vaccination:
Against encapsulated bacteria:
Streptococcus pneumoniae
Neisseria meningitidis
Prophylactic antibiotics:
Often long-term penicillin
Prevention of Infection
Immunisation — protect against key pathogens (e.g., HiB, pneumococcus).
Infection control — e.g., hand hygiene, PPE.
Engineering controls — physical systems to reduce exposure to pathogens.
Screening for latent infections — e.g., TB in immunosuppressed patients.
Prophylactic treatment — giving antibiotics, antivirals, or antifungals to prevent infection in high-risk individuals.
Engineering Controls in Infection Prevention: Aspergillus
🦠 Aspergillus prevention:
Air filtration: HEPA filters remove spores from air.
Barrier protection: During hospital renovations/construction, spores in dust can cause infection.
Protective isolation: HEPA-filtered rooms for stem cell transplant patients.
Respiratory protection: When patients must leave protective rooms, they wear masks.
