Introduction to microbial infection Flashcards
What are the four elements of Koch’s Postulates?
- bacteria must be present in every case of the disease
- bacteria must be isolated from the host with the disease and grown in pure culture
- the specific disease must be reproduced when a pure culture of the bacteria is inoculated into a healthy susceptible host
- bacteria must be recoverable from the experimentally infected host
What does the inmate immune system consist of? (4)
- normal microbiota
- physical barriers
- chemical barriers
- phagocytic cells
How do normal microbiota help protect against microbial infection?
- offer protection by competing with pathogens for colonisation sites
- produce antibiotic surfaces suppressing growth of competing organisma
- may produce toxic metabolic products to inhibit other microorganisms
- may alter pH
( - surpassed by antibiotics)
What are normal microbiota?
microorganisms that reside on the surface and deep layers of skin, in the saliva and oral mucosa, in the conjunctiva, and in the gastrointestinal tracts of every human being.
• These microbiota are not harmful to humans; some are even beneficial and most help maintain our health.
How do physical barriers help protect against microbial infection?
- skin - secretes sebum and fatty acids to inhibit growth (- microbes have evolved ways to penetrate)
- mucomuciliary clearance - particles settle on sticky mucus of respiratory epithelium - debris transported by cilia to oropharynx where it is swallowed
- flushing - urinary tract
- peristalsis - GI tract
How do chemical barriers help protect against microbial infection?
- mucus
- antimicrobial proteins - lysosome (tears, saliva), lactoferrin (breast milk, tears, saliva) - defensives
- gastric acid - pH 2
- plasma proteins - complement cascade, C-reactive protein(CRP), mannose-binding lectin (MBL), transferrin
How do phagoytotic cells help protect against microbial infection?
- process used by the human body to destroy dead or foreign cells - endocytose pathogens
What do phagocytes include?
- macrophages
- neutrophils
- monocytes
- dendritic cells
- mast cells
How does a infection occur?
a micro-organism causes ill-health in two ways
- invading host tissues
- exerting effects from mucosal surfaces
Commensal definition?
micro-organism which forms part of the normal host microbiota
Pathogen definition?
micro-organism capable of causing an infection
Pathogenicity definition?
the capacity to cause disease
Virulence definition?
measure of the capacity to cause disease
What are obligate pathogens?
almost always cause disease
eg. HIV
What are conditional pathogens?
may cause disease if certain conditions are met
eg. bacteroides fragilis, staphylococcus aureus
What are opportunistic pathogens?
usually only infects immunocompromised host
eg. pneumocystis jiroveci
What are the 5/6 steps of infection?
- recognition
- attachment and entry
- multiplication
- evasion of host defences
- shedding - expulsion and release of virus
(- damage - doesn’t always happen as not too much damage wants to be caused)
How can infection enter a host?
microbes with
- specific mechanisms for attachment and penetration of host’s body surfaces
- introduced into host by biting arthropods (spiders…)
- introduced into host via skin wounds or animal bites
- able to infect only when hosts defences are impaired
What is tissue tropism,
defines the cells and tissues of a host which support the growth of a particular microbe
- broad tissue tropism - infecting many cells and tissues
- or infect primarily a single tissue
What are the influencing factors of tissue tropism?
- presence of cell receptors
- transcription factors
- local temp
- physical barriers
- pH
List virulence factors?
- toxin secretion
- antibiotic resistance
- pilus formation
- capsule
- iron transport systems
- adhesion factors
- enzymes
What are the features of endotoxins?
- low toxicity
- part of cell wall of gram negative bacteria
- lipopolysacharide
- low specify
What are the features of exotoxins?
- highly toxic
- secreted from bacterial cells
- produced by gram negative and positive bacteria
- can be converted into toxoids for vaccine use - only toxin part causes disease
- eg. tetanus toxin, cholera toxin ..
What does transmission depend on?
- number of microorganisms shed
- number of microorganisms required to infect a fresh host
- microorganisms stability in environment
