Introduction to infection Flashcards
what is an infection?
Invasion of a host tissue by micro-organisms (where they should not be)
disease caused by
- Microbial multiplication
- Toxins
- Host response
how do people get infected
From a source directly or indirectly
- from person to person
- fromt environment
- from a vector
- from animals
microbiota
“ecological communities of commensal, symbiotic and pathogenic microorganisms” found in and on all multicellular organisms studied to date from plants to animals.
where are microbiota usually found
on the skin or mucosal surfaces
*usually harmless and even beneficial*
when can commensal microbiota be harmgul
when they are transfered to others sites e.g. Neisseria meningococcal meningitis
Neisseria meningococcal meningitis
- Found in around 1 in 10 people
- Will very rarely cause disease
- Cause disease when they go to non-commensal places-Cystitis
how do people get ifnected
- microbiota
- physical contacgt
- e.g. sexually tranmitted
- airborne spread
- e.g. chickenpox
- vectors
- e.g. mosquito for malaria
- through the environment
- ingestion of contaminated food or water
- e.g. food borne diarrhoea
- cholera
- ingestion of contaminated food or water
- inhalation of air contaminated
- contact with contaminated usrfaces inc medical devices
modes of transmission can be….
horizontal or vertical
horizontal mode of tranmission via
- Contact
- Inhalation
- Ingestion
contact can be
direct
indirect
vector
inhalation
airborne transmission- coughing
- droplets
- aerosols
ingestion
faecal-oral transmission
vertical transmission
Mother to child, before or at birth e.g. HIV
how can micro-organisms be spread in the body?
contiguous spread
haematogenous spread
contiguous spread
- infection spreading locally
e. g. a spot that spreads across the skin
Haematogenous spread
- blood borne
Tissue troponism
Viral infections e.g. enterovirus’ invade via the gut
virulence factors
Virulence factors are molecules produced by bacteria, viruses, fungi, and protozoa that add to their effectiveness and enable them to achieve the following:
- colonization of a niche in the host (this includes attachment to cells)
- immunoevasion, evasion of the host’s immune response[1]
- immunosuppression, inhibition of the host’s immune response
- entry into and exit out of cells (if the pathogen is an intracellular one)
- obtain nutrition from the host
types of virulence factors
exotoxins
endotoxins
exotoxins
Exotoxins are actively secreted by some bacteria
◦Cytolytic
◦AB toxins
◦Superantigens
◦Enzymes
endotoxins
Endotoxin is a component (lipopolysaccharide (LPS)) of the cell wall of gram-negative bacteria. It is the lipid A part of this LPS which is toxic. Lipid A is an endotoxin. Endotoxins trigger intense inflammation.
*recongised by cellular defences*
micro-organisms can also cause disease due to
host response
- Sometimes direct- will quickly eradicate local infection
- Can sometimes cause disease e.g. sepsis and meningitis
Disease determinants: Pathogen
Virulence factors
Inoculum size
Antimicrobials
Disease determinants: Patient
- Site of infection
- Co-morbidities
- As you get older more likely to have chronic condition
- E.g. those with diabetes more likely to get infections
questions we need to ask when we think someone is infected?
- Is there an infection?
- Where is the infection?
- What is the cause of infection?
- What is the best treatment?
taking a history of someone with a potential infection
Symptoms
- focal, systemic ?
- severity
- duration
Potential exposures
examination of someone with a potential infection
organ dysfunction tests
investigations of someone with a suspected infection are…
specific (blood culture) or supportive (CRP)
supportive investigations
- Full blood count- neutrophils, lymphocytes
- CRP
- Blood chemistry- liver and kidney function tests
- Imaging- X-ray, US, MRI
- Histopathology
specific types of bacteriology: M, C &S
microscopy
culture
antibiotic susceptibility
Virology
- Antigen section (virus)
- Antibody detection (the patients response)
- Detecting viral nucleic acid (DNA or RNA)
