professional pathogens Flashcards
Define infection
when an organism enters the body, increases in number and damages the host in the process
Define colonisation
when an organism lies on us but doesn’t cause infection
Define a pathogen
an organism which can evade the immune defences of the normal host to cause infection
Define a commensal
an organism which lives on us but doesn’t cause infection
Define symbiosis
mutual benefit
Define parasite
unequal benefit, used colloquially for eukaryotic infections
What are the two types of pathogens and what do they do?
- PROFESSIONAL: cause disease in any susceptible host
- OPPORTUNISTIC: only cause disease in immunocompromised patients
Define virulence
fundamental properties of the organism which determine how it causes disease
Describe the appearance of staphylococcus aureus on agar?
golden colonies
Describe the appearance of Staphylococcus aureus down a microscope?
gram positive cocci in clusters which look like bunches of grapes
Which types of protein exotoxins can S. aureus secrete?
- cytotoxins
- exfoliative toxins
- enterotoxins (superantigens)
What are cytotoxins?
pore-forming toxins, which lyse host cells
What are exfoliative toxins?
proteases which target epidermal structural proteins
What are enterotoxins (superantigens)?
stimulate massive T cell activation, leading to immune evasion
Describe S. aureus coagulase?
- cell wall-bound enzyme
- stimulates clotting
- plays a role in immune evasion
What can the S. aureus coagulase be used for?
can be used as a test to distinguish S. aureus from other less virulent staphylococci
What is the role of the S. aureus capsule?
masks cell surface features from recognition by the immune system
Describe the S. aureus capsule
- polysaccharide capsule
- compared with other bacterial species, this one is thin (microcapsule) but helps avoid phagocytosis by neutrophils
What does the S. aureus cell wall contain to help it?
- adhesins
- adherence to nasal mucosa and deep body tissues
How exactly do adhesins on the S.aureus cell wall help?
- TISSUE ADHERENCE: for colonisation or deep infections
- IMMUNE EVASION: ‘cloaking’, e.g protein A
How does S. aureus use gene regulation to aid itself?
can survive and adapt to different conditions by sensing environmental cues and altering production of virulence factors needed to survive
Which gene cluster encodes the peptide quorum-sensing system in S. aureus ?
the accessory gene regulator (agr)
Which kind of skin infections can S. aureus cause?
- furunculosis
- impetigo
- staphylococcal abscess
How can S. aureus cause food poisoning?
by ingestion of staphylococcal enterotoxins in contaminated food
List the common S. aureus infections
- soft tissue infections
- surgical site infections
- vascular line-related infections
- bacteraemia
Where is s. aureus often found?
a normal commensal of anterior nares is found in 20-60% of healthy adults
What does protein A in the cell wall of S. aureus do?
it binds to the Fc portion of immunoglobins, holding them out like umbrellas
Why is it so dangerous when the lipopolysaccharide gets disseminated in the blood?
- there is a SYSTEMATIC activation of immune response
- can manifest as sepsis
What does Neisseria meningitidis like to adhere to?
- nasopharyngeal epithelium
- endothelial cells
- meninges
What are common streptococcus pneumoniae diseases?
- most common form of pneumonia (40%)
- upper respiratory tract infections
- bacteraemia
- meningitis
- endocarditis (infection of heart valves)
What does the IgA protease released by S. pneumoniae do?
breaks down secreted immunoglobulin A, preventing mucosal clearance
What does pneumococcal surface protein A (PspA) in S. pneumoniae do?
- binds to laminin receptor on brain endothelial cells triggering transcellular passage into the CSF
- neutralises lactoferrin’s bactericidal activity
How does the S. pneumoniae capsule help?
- hides ‘immunogenic’ cell wall
- specific anitbodies to capsule required
What type of adhesin does Neisseria meningitidis have?
type IV pilus
What does streptococcus pneumoniae look like under the microscopic?
gram positive diplococci
