Lecture 10 - Opportunistic infection Flashcards
Define:
True pathogen
Commensal
Opportunistic pathogen
Pathogenicity
Virulence
True pathogen (Obligate) - An organism that can cause infection in individuals with normal host defences
Commensal - An organism that is found normally on those parts of the body that are exposed to, or communicates with, the external environment
Opportunistic pathogen (Opportunistic) - An organism that can cause infection in individuals with abnormal host defences. Commensals may be opportunistic pathogens.
Pathogenicity - is the capacity of the organism to cause disease
Virulence - is the severity or harmfulness of a disease ability to cause damage to a host
What are kochs postulates?
Most pathogens aren’t ever pathogenic however many are potentially pathogenic. Very few microbes are always pathogenic.
Koch’s Postulates
* The pathogen must be present in every case of the disease (asymptomatic carriers)
* The pathogen must be isolated from the diseased host and grown in pure culture (viruses)
* The specific disease must (should) be reproduced when a pure culture of the pathogen is inoculated into a healthy susceptible host
* The pathogen must be recoverable from the experimentally infected host
Limitations of Koch’s postulates include viruses, host effects (make difficult to grow in culture), polymicrobial infections (more than one microorganism, infectious doses (not al pathogens infect in the same numbers).
What is the difference between virulence and infectivity.
Virulence and infectivity are not the same. E.g. the plague has a high virulence and infectivity whereas HIV has a high virulence and low infectivity.
Virulence factors
* Toxins
* Capsules
* Adhesins
Knowledge of these virulence factors allows exploitation allowing vaccines and antitoxins to be produced
E.g. Tetanus - antitoxin, Toxoid immunisation
Infectivity of infectious agents can be compared by using the LD50 (infectious dose) however the host effect can interfere
What is evidence for a pathogen being clinically significant.
- Isolated in abundance
- Isolated in pure culture
- Isolated on more than one occasion
- Isolated from deep tissues
- Evidence of local inflammation
- Evidence of immune response to pathogen
Fits with clinical picture
What is ment by the commensal microflora.
More bacterial cells are associated with the human body than the human ones.
The commensal flora includes: bacteria, protozoa, fungi, archaea and viruses
They are found in the skin and the mucosa
The composition of the normal flora varies from individual to individual
* Some bacterial species are carried only transiently - passing through
* Most however are fairly permanent
It is difficult to alter the composition of the normal flora of the gut in a healthy individual.
What are the bodys defences.
- Intact integument
- Surface antibody
- Lysozyme and fatty acids
- Cilia
- Normal flora
Immune system
What can cause changes in the normal flora.
Changes in normal flora
Changes in hormonal physiology and development
* Female genital tract and lactobacilli
When antibiotics select for a ‘resistant flora’
* Candida overgrowth
New organisms may be acquired
* Neonate from maternal genital tract during birth
* Gram-negative colonisation of the gut and urinary tract in hospitalised patients
* Cross infection with C. difficile, MRSA, VRE…
What are some fo the harmful effects fo the gut bacteria.
- Escape of normal flora to abnormal site - appendicitis
- Cholecystitis (gall bladder swelling) and cholangitis (inflammation of the bile duct system)
- UTI
Alterations to the gut microbiota can be harmful. They can be caused by
* Antibiotic use - leads to sensitive gut flora killed
* Overgrowth with resistant flora inc. Clostridioides difficile which leads to C. difficile toxin
Prevention of this is to stop prescribing antibiotics, give oral metronidazole or vancomycin. Recovery requires re-establishment of normal flora (probiotics and faecal transplant)
What may allow an opportunistic pathogen to cause infection.
Where the occurrence or severity of an infection is determined by patient, not microbial factors.
* Immunosuppression
* Breaching defences - Urinary catheter, IV access
* Foreign body - prosthesis, splinter
Debility - malnutrition, Ill health, old age
How does a clostridioides difficile infection occour.
Clostridioides difficile
* Gram-positive, rod, spore forming, strictly anaerobic bacterium
* Symptoms - Diarrhoea, fever, loss of appetite, feeling sick, pain
* Vulnerability -
○ Taking broad spectrum antibiotics
○ Proton pump inhibitors (reduce the amount of stomach acid)
○ Have had stay in healthcare setting
○ Underlying conditions
○ Surgery of digestive system
* Virulence factors
○ Adhesins - cwp family of proteins (cell wall protein), fibronectin-binding proteins, collagen binding proteins, lipoprotein, heat shock proteins.
○ Invasins - proteases, flagella
What is a murcomycosis infection
Mucormycosis is any fungal infection caused by fungi in the order Mucorales. The disease is often characterised by hyphae growing in and around blood vessels and can be potentially life-threatening in diabetic or severely immunocompromised.
What causes infection in immunosuppressed patients.
Causes of infection in immunosuppressed patients
* Shift from infections due to Gram-positive to Gram-negative pathogens
* Increased importance of fungi
Increasing incidence of resistant pathogens
What are latent pathogens.
Where the patient is unaware they are infected until they are immunosuppressed - e.g. HIV
what is the role of the spleen
- Main antibody production site
- Efficient clearance of bacteria
- Filter
Sepsis is more likely to occur post splenectomy. To prevent this rapid intervention and immunisation is very important
