Staphylococcus aureus Flashcards
Describe Staphylococcus aureus.
S.aureus is a Gram-positive coccus which arranges itself in grape like clusters.
What are the diagnostic features of S.aureus.
> Extracellular enzyme, coagulase is produced which converts plasma fibrinogen into fibrin.
> Produces thermostable nuclease enzyme that breaks down DNA.
> produce surface bound antigen called clumping factor that also reacts with fibrinogen.
Describe the virulence factors of S.aureus.
Cell wall polymer
Peptidoglycan- inhibits inflammatory response.
Cell surface protein
Clumping factor- binds to fibrinogen.
Exoproteins.
Enterotoxins- induces vomiting and diarrhoea.
Coagulase- fibrinogen to fibrin.
Deoxyribonuclease- degrades DNA.
Describe S.aureus enterotoxin.
Enterotoxins- types A-E and G-I are commonly produced in 65% of strains. They withstand temperatures of up to 100 degrees for several minutes. When ingested as contaminated food, Staph food poisoning symptoms can occur within a few hours. (Nausea, vomiting and diarrhoea.)
Describe S.aureus TSST-1.
Toxic Shock Syndrome Toxin (TSST-1)
Toxic shock syndrome linked with TSST-1 or enterotoxin or both. Has an absence of a circulating antibody. Recognised as a superantigen. Cause multi-system clinical conditions
Describe S.aureus epidermolytic toxins.
Two kinds (types A & B) are commonly produced by strains belonging mainly to phage 2. They cause blistering diseases. Cause intraepidermal blisters at the granular cell layer. .
Describe the sources of infection of S.aureus.
Infected lesions
Healthy carriers
Animals.
Describe how Infected lesions may lead to a Staph infection.
Large number of staphylococci are disseminated in dried exudate and pus discharged from large infected wounds, burns and secondary infected lesions.
Direct contact is the most important node of spread but air-bourne dissemination may occur.
Describe how cross-infection can occur.
Hospitals- without the implementation of thorough hand washing regulation, there would be a lot more Staph infections.
Community- cross-infection of food by introducing an enterotoxin which could lead to food poisoning.
Describe how Healthy Carriers may lead to a Staph infection.
S.aureus grows harmlessly in the moist skin of nostrils in 30% of healthy individuals. Some carriers called shedders disseminate large numbers of Staphylococci.
Describe how animals May lead to a Staph infection.
Animals may disseminate Staph. Aureus.
Eg cows with mastitis create infected dairy produce which would lead to a Staph infection in humans.
Describe the Mode of infection of S.aureus.
May be exogenous (external source) or endogenous (from a carriage site or minor lesion in ones body).
Can remain alive on surfaces for several months when dried in pus, sputum, bed sheets or dust.
What is MRSA?
Methicillin resistant Staphylococcus aureus.
An endemic in many hospitals. Commonly colonise without symptoms but can cause severe infections particularly in vulnerable patients. MRSA can spread easily and cause severe infections involving the bloodstream, resp tract, ,bones and/or joints.
Control/prevention of MRSA involves early, reliable detection of MRSA in the lab. Patient isolation and good hygiene regulation with sensible use of antibiotics.
How can one diagnose S.aureus in a laboratory setting.
Specimen collection.
> Pus from abscesses, burns, wounds, etc.
Sputum from patients with pneumonia.
Faeces or vomit from patients with suspected food poisoning.
Blood from patients with suspected bloodstream infection.
Mid-stream urine form Px with suspected cystitis or pyelonephritis.
Anterior nasal or perineal swabs from suspected carriers.
Microscopy
> clusters cultured on blood agar.
Treatment of S.aureus.
Due to the ability of some strains to produce penicillinase, most penicillins are inactivated,. Some are stable to the enzyme such as, methicillin, oxacillin, cloxacillin and flucloxacillin. Cephalosporins and B-lactam inhibitors are also stable to the enzyme.
