week 3 - staphylococci Flashcards
What is the shape of staphylococcus aureus?
Gram positive (purple) cocci arranged in irregular grape like clusters
Non sporing/motile
Resistant to dry salty conditions
What do the colonies of staph aureus look like on blood agar?
circular, smooth, 2-3mm, grey/wjite/pale yellow/golden.
Facultative anaerobe
Catalase positive
Where is staph aureus and staph epidermis found?
Staph aureus - only 30% of ppl are colonised with it, can be found in the nose, hand, pharynx etc
Staph epidermis - 100% of people are colonised with it as it is a commensal of the skin
What is the difference between staph aureus and staph epidermis?
Use the coagulase test =
Coagulase positive - staph aureus
Coagulase negative - 40 species like staph epidermis
What are the results of a catalase test for gram positive cocci?
Catalase positive, observation - bubbles are formed
How can you use mannitol salt agar to confirm the presence of staph aureus?
Grow the bacteria on mannitol salt agar
Staph aureus (coagulase positive) will ferment the mannitol so colonies appear yellow
Coagulase negative staph does not ferment mannitol so colonies are pink
How can you use DNAse agar to confirm the presence of staph aureus?
Grow the bacteria on DNAse agar
Staph aureus (coagulase positive) will have yellow colonies
Coagulase negative staph won’t have any colour colonies
How can you use the staph latex agglutination test to confirm the presence of staph aureus?
Staph aureus (coagulase positive) has a positive result as the clumping factor is detected
Coagulase negative staph has a negative result asa there is no clumping factor detected
What are the virulence factors found on staph aureus?
Toxins
Secreted enzymes
Cell surface proteins
Describe staph aureus cell surface proteins
Protein A - Inhibits phagocytosis, interferes with humeral response
Clumping factor - he=ides cell from immune components
List staph aureus toxins
Cytolytic toxins
Leukocidin toxin
Superantigens
They disrupt the membrane
Describe staph aureus enzymes
Hyaluronidase - tissue invasion through ECM
Collagenase, elastase - Tissue destruction
What are the different type of staph aureus infections?
Superficial Infections - boils, sties, impetigo
Serious infections - endocarditis, septicaemia etc
Toxin mediated - food intoxication, TSS, scalded skin syndrome
What is impetigo and how is it caused by staph aureus?
Staph aureus enters through a minor skin abrasion.
Is a localised infection
Forms small, honey coloured crusty lesions and very infectious
How does staph aureus cause furuncles/abscesses
A furuncle is a boil that is tender, swollen and pus filled on the skin and tissues.
It is an infection around a hair follicle where staph aureus cells multiply and spread producing an inflammatory response
Treat via drainage and antibiotics
How is food intoxication caused by staph aureus?
Staph aureus lies in contaminated foods and produces enterotoxins causing diarrhoea, vomiting, nausea
What is toxic shock syndrome and how is it caused by staph aureus?
How is scalded skin syndrome caused by staph aureus?
Skin infection
Caused by exfoliative toxins A, B and D
Causes the skin cells to become unstuck, blisters start to form and epidermis starts to peel.
Describe staphylococcal endocarditis
Infection of the endothelial lining of the heart.
Manifests as clots (vegetations) on heart valves
How are cardiac vegetations (clots) formed?
- endothelial layer surface becomes damaged
- Platelets/fibrin deposited
- Bacteria attaches
- Vegetation (clot) increases in size
- Continuous bacteraemia (continuous presence of bacteria in the bloodstream)
- Possible embolisation
How are blood stream infections diagnosed?
Via a BacT/alert 3D blood culture:
Microorganisms are allowed to multiply in the media, generating CO2
As CO2 increases, sensor in the bottle turns from grey to yellow
Reflected light is measured and positive sample is detected.
What happens once a positive sample is obtained?
- Gram stain using a venting needle
- Culture
- Direct sensitivity test using blood to inoculate the plate
What antibiotics are used to treat S. aureus?
Most s. aureus are resistant to penicillin
Severe infections caused by methicillin SENSITIVE S. aureus treated via flucloxacillin, erythromycin, vancomycin
Severe infections caused by methicillin RESISTANT S. aureus treated via vancomycin
What are the virulence factors found on staph epidermis?
- polysaccharide slime layer
- biofilm associated cells are tolerant to antibiotic treatment
