Staphylococcus Flashcards
is S. Aureus coagulase + or -?
S. Aureus is coagulase +
This test differentiates S. Aureus from other staphylococci
Where is Staphylococcus aureus found?
in moist skin folds, mucosal surfaces or the nasopharynx
- increased in diabetes mellitus, IV drug users, and where a foreign body is present
Describe the pathogenesis of S. Aureus.
- gets in - portal of entry
- attaches to the cells
- defeats/evades the immune system
- cuases damage to host cells
- gets out and spreads further
What is the most common portal of entry of S. Aureus
- Ingestion
- Penetration of the skin
How does S. Aureus attach to the cells?
- surface proteins - attach to epithelial and endothelial proteins (laminin and fibronectin of the extracellular matrix)
- Capsule - inhibits chemotaxis, phagocytosis and facilitates adherence ot foreign bodies
- fibrin/fibrinogen binding protein - attachment to blood clots and traumatized tissue
- matrix binding proteins - fibronectin, fibrinogen and collagen binding where adhesion promotes collagement attachment found in strains that cuase septic arthritis
how does S. Aureus defeat/evade the immune system?
inhibition of phagocytosis with capsule
and
production of extracellular substances that promote invasion
- invasins
- enzymes
what is the effect of coagulase in S. Aureus?
coagulase is an extracellular protein which binds to prothrombn
*remember that S. aureus is coagulase +*
clots then protect the bacteria from phagocytosis and other host defences
what is the effect of S. Aureus’ Protein A ?
it is a cell wall surface protein that binds IgG molecules in the inverted orientation by their Fc Region and therefore prevents phagocytosis
What is the role of Alpha toxin in S. Aureus?
it binds to platelets and monocytes cuasing small pores - osmotic lysis
What is the effect of leukocidin in S. Aureus?
acts on polymorphonuclear leukocytes2% of all S. Aureus isolates express it
nearly 90% of isolates from severe dermonecrotic lesions are +
what is the effect of staphylokinase in S. Aureus?
plasminogen activator that lysses fibrin and dissolves fibrin clots and helps bacterial spread
what is the effect of hyaluronidase in S. Aureus?
helps the spread by breaking down hyaluronic acid in connetive tissues
how does S. Aureus damage the host cells?
- direct damage (peptidoglycan wall - provides osmotic stabiity and stimulates the release of cytokines)
- enzymes
- toxins
- Super antigens -
- exfoliative toxins
- cytotoxins
how do you classify staphyloccocal infections?
- skin and soft tissue ex) styes, folliculitis, mastitis, boils
- systemic - invasive ex) bacterial stream infection, endocarditis, bone/joint infections
- systemic - toxic mediated ex) food poisoning/gastroenteritis, scalded skin syndrome, toxic shock syndrome
What is Toxic Shock Syndrome?
Historically associated with high absorbency tampons - leads to pyrexia hypotension due to dilitation due to cytokines , rash with subsequent desquamation, other organ involvement (renal failure, CNS etc)
Due to
- TSS toxin 1 acts as a super antigen cytokine release
What is MRSA?
methicillin resistant S. Aureus
- results from production of an altered penicillin bidning protein - confers resistance to most beta-lactam antibiotics
a number of treatment options remain
Wha tis scalded Skin Syndrome?
S. Aureus has toxins that are exfoliative - meaning they split the intracellular bridges in the skin layer
classically described in young children - very contagious
huge risk of secondary skin infection
How should we manage patients with MRSA?
- history/clinical exam
- investigationas depend on the infection - ex) abscess need pus swab, BSI need blood cultures, Food poisoning need piece of food
- Antibiotic treatment - no treatment for mild infections, but 7-10 days for skin/soft tissue infections and RTI
- Fluclozacillin if susceptible
- Vancomycin if MRSA suspected
where are coagulase negative staphylocci found?
they are natural inhabitants of human skin and mucosa - harmless while on skin
much less virulent than S. aureus
what are S. Epidermidis infections most often associated with?
with prosthetic device infections ex) artificial joint infections
what does S. Saprophyticus cause?
most commonly UTIs
What is the treatment for a UTI?
generally infection = S. Saprophyticus = 3 days treatment for lower UTI with Trimethoprim
generally we treat coagulase + S.cocci with what?
How about coagulase - ?
coagulase + = flucloxacillin or vancomycin b/c risk of MRSA
coagulase - = Trimethoprim for general UTIs
