S1: Principles of Infection I Flashcards
Name some microbes
Virues Bacteria Fungi Protozoa Helminthes Ectoparasites
What are saprophytes?
Harmless microbes that live on inanimate material and derive their nutrition from environmental sources
What are symbionts?
Microbes that require a human/animal host to survive and multiple –> It means ‘living together’
Note it does not distinguish between harmful and beneficial relationships
What is commensalism?
It is a relationship in which the symbiont benefits from the relationship (e.g. shelter, food) but the host is neither helped or harmed
What is mutualism?
Where both the host and symbiont benefit from each other
What are commensals sometimes called?
Normal flora
Difference between colonisation and infection
- Commensals (normal flora) are said to colonise the host, no harm is done to the host
- Infection implies harm to the host. Usually the host will manifest an inflammatory response to a pathogen but not a colonizer at a normally non-sterile part of the body.
Give examples of situations where commensals can act a pathogen
- If hosts defences are weakened (immunocompromised) infection may occur –> This is called opportunistic infection
e. g. HIV patient infected by an organism with low virulence - Also some organisms are commensals at one site but pathogens at another
e. g. staphylococcus aureas is commensal in the nose but in a wound is a pathogen - If an organism gains entry to a normally sterile site
What is a pathogen?
A microbe that causes disease (initiates infection) with only small number via natural routes
What does virulence mean?
It is a term used to describe the degree of pathogenicity
Name some body sites that are normally sterile and do not have normal flora
- Lower Respiratory Tract (LRT)
- Blood
- Bone, joint and subcutaneous connective tissue
- Female upper genital tract
- Urinary tract
- CNS including CSF and eye
- Other viscera e.g. liver, spleen, pancreas
What is a strict pathogen?
Organisms that always cause disease
Highly pathogenic microbes are said to be virulent
What are the 2 main factors that determine if infection with result
- The properties of the microbe
- adhesins
- toxins
- capsule - Defensive mechanisms of the host
- natural barriers
- defensive cells
- complement
- immune response
If balance is in favour of microbe, then infection will occur
If balance is in favour of the host, then infection will result
What are the 6 steps that need to be completed before infection is established?
1) Survival of microbe and spread to host
2) Adherence of microbe to host and entry into host
3) Multiplication
4) Evasion of host’s defences
5) Damage to the host
6. Shedding of the microbe and spread to environment or another host
Explain step 1a of establishing an infection: survival of the microbe
Microbes need to be able to survive in the enviroment they live in
- Physical environment: some produce spores that survive heat and dryness, others have a waxy coat
- Competition with others: There may also be competition with other microbes so they release bacteriocins to kill these off
Explain step 1b of establishing an infection: routes of spread
Airborne/droplets - Aerosol
GI Tract/Faecal-Oral Route
Skin
Through inanimate objects (called fomites)
Other epithelial surfaces
Explain step 2 of establishing an infection: Adherence and entry into host
This is done by various mechanisms including attachment to tissue via fimbriae (pili), adhesins and other molecules present on the surface of some of these microbes
Explain step 3 of establishing an infection: Multiplication
During the process of multiplication, many microbes produce tissue destroying enzymes which allow invasion of surrounding tissues as well as deeper sites.
The toxins are capable of local or systemic actions
Explain step 4 of establishing an infection: Evasion of host’s defences
The hosts defence mechanisms soon mobilises its resources to contain the infection and microbes use a variety of methods to evade the hosts response. As mentioned earlier a successful pathogen will tip the balance in their favour in comparison to hosts and evade the hosts defences.
Explain step 5 of establishing an infection: Damage to host
This may result from actions of the microbe or sometimes from the host’s own immune response
Explain step 6 of establishing an infection: Shedding infection
In order to perpetuate the infection, the microbe must find a new host.
Describe our natural barriers of infection
- Major natural physical barriers are the skin and mucous membrane. Skin covers the largest surface of the host and is impermeable to most infectious agents. When the continuity of intact skin is broken e.g. by cuts, lacerations, injection and animal and insect bites, microbes may be implanted into those sites from the environment. An infection may then be established.
The chemical barriers are generally the constituents of Innate Immunity
Name the characteristics of innate immunity
a) Immunity is non-specific i.e. are active against a variety of infectious agents.
b) Non-specific immunity can not be enhanced by repeat exposure.
c) Microbicidal substances e.g. lysozyme in tears, gastric acid and antibacterial substances in urine.
d) Acute phase proteins, interferons and complement are chemical substances and are all associated with mild to moderate inhibitory effect against various microbes.
e) Phagocytic cells e.g. Mononuclear phagocytes-monocytes and macrophages, and natural killer (NK) cells.
List some local and systemic symptoms of infection
Local symptoms (inflammation): Redness, swelling, warmth, pain, loss of function, pus (=pyogenic infection)
Systemic symptoms = Fever, rigors, chills, tachycardia, tachypnoea
What is acute infection?
Infection established and disease progresses rapidly with major local and systemic manifestations. E.g. pyogenic skin lesion or toxin disease.
What is chronic infection?
Usually slower onset, but may still have major local and systemic symptoms, a chronic inflammatory response results when the host does not succumb immediately to the infection but cannot clear it e.g. TB
What is asymptomatic infection?
Infection with a pathogenic microbe and the inflammatory response is mild or none at all and damage to the host is mild or none at all. No symptoms present.
