Principles of Infection Flashcards
Describe commensalism.
Commensalism is where the microbe (now called a commensal) derives its food and shelter from the host; normally it’ll do no harm to the host. The host may even benefit from the presence of the microbe (called mutualism).
Commensals are also sometimes referred to as the “normal flora” which may vary at different sites of the body.
How can commensals sometimes be pathogens?
In healthy individuals, the commensal microbes (normal flora) will do no harm, but, if the host’s defences are weakened (immunocompromised), an infection may occur. This is called an opportunistic infection.
An example would be a patient with leukaemia being infected with candida, which may be a commensal of the oral cavity in a normal healthy person. Another example would be that a person with HIV could be infected by an organism with low virulence. Virulence is a term used to describe the degree of pathogenicity.
How can commensals sometimes become pathogens?
- if found at another site
- due to immunosuppression
- by by-passing defences
Expand on the balance between the microbe and the host.
Whether a microbe behaves as a pathogen i.e. causes disease depends on the properties of the microbe and defensive properties of the host, this can be viewed as a balance between the microbial and host properties.
If the balance is in favour of the microbe (e.g. good adhesion, toxins, capsule etc.) then infection will occur.
If the balance is in favour of the host (e.g. natural barriers, defensive cells, immune response, complement) then no infection will result.
What are the three types of infection?
LOCAL: surface infection, wound
INVASIVE: penetrate barriers spread
SYSTEMIC: via blood to other sites
Describe the role of inflammation in responding to infection.
It functions to bring serum molecules and cells to the sight of infection.
- increases blood supply
- increases capillary permeability
- induced a migration of cells from blood to tissue (polymorphs, macrophages)
What is a pyogenic infection?
It is a pus-producing infection.
List some differences between acute and chronic infections.
ACUTE:
- rapid onset
- major local and systemic responses
- acute inflammatory response
- toxin-mediated
CHRONIC:
- slower onset or post-acute
- may still have major local and systemic symptoms
- chronic inflammatory response
- results when the host doesn’t succumb immediately to infection
What is an asymptomatic infection?
It’s an infection with a pathogenic microbe, but not showing symptoms when infected.
- inflammatory response is mild/ none at all
- damage to the host is mild/ none at all
- no symptoms present
What are some ways in which an infection can spread?
- Direct contact
- Indirect contact
- Airborne
- Blood products
A way to remember some is by the F-list:
- Fingers
- Fresh Air
- Fomites (touch)
- Fluids (blood, etc.)
- Flies
- Food
- Foetus
- Fornication
Describe microbe multiplication.
Once microbes are internalised, either by an active process as in many virus infections or introduced passively as most bacterial infections, they will multiply locally to produce a focus of infection. During the process of multiplication, many microbes produce various tissue-destroying enzymes which allow invasion of surrounding tissues as well as deeper sites.
Some bacteria produce toxins and tissue destroying enzymes, these toxins are capable of local or systemic actions. Following multiplication, the infection may remain localised, e.g. abscess or spread to contiguous tissues e.g. cellulitis or spread via the blood – septicaemia.
Expand on how microbes adhere and enter the host cell.
In order to gain a foothold in the host, a microbe needs to adhere to the host’s tissue.
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 (e.g. attaching to urethral epithelium).
How do microbes evade the host defences?
- antigenic variation
- capsules can stop contact with a phagocyte
- they can inhibit phagolysosome formation
- they can immunosuppress the host
Describe the functions of toxins in infection.
- they promote the spread or survival of bacteria
- they damage or destroy cells
- they interfere with cell metabolism
- they affect nerves
What are Koch’s Postulates?
They are four criteria that were established by Robert Koch to identify the causative agent of a particular disease.
A microorganism has to:
- 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
