S. aureus Flashcards
Describe Staphylococci?
gram positive cocci that grow in clusters; non-motile
What are the 3 pathogenic species in humans?
S.aureus; s. epidermidis; s. saprophyticus
What type of infections does s. saprophyticus?
UTI
How can S.aureus be differentiated from S.epidermidis; S.saprophyticus?
S. aureus is coagulase positive whereas otehrs are negative; appaearnce- aureus is yellow vs others are white ; aureus is positive for protein A/clumping factor whereas otehrs are negative
Which pathogenic staph species use mannitol (sugar)?
S.aureus and S.saprophyticus
Why is S.aureus coagulase positive?
secretes coagulase which leads to localised blood clotting as cleaves prothrombin into thrombin which converts fibrinogen to fibrin
What is the main test for differentiating strep vs staph?
all staph are catalse positive whereas strep are negative
How does the catalase test work?
if bacteria produce catalase enzyme, are able to convert hydrogen peroxide to water and oxygen- resulting in lots of bubbles
What is the function of protein A?
binds Fc region of antibodies, inhibiting their normal function
What is the function of clumping factor?
causes clumping of blood plasma
What are the ways of differentitating staph. aureus from strep?
microscopy- clusters vs chain; culture: golden pigment of aureus; catalse test
What are the diseases the aureus causes by exotoxin release?
gastroenteritis; toxic shock syndrome; staph scalded skin syndrome
What is the exotoxin responsible for gastroenteritis?
enterotoxins
What exotoxin is responsible for toxic shock syndrome?
TSST-1
What exotoxin is responseible for staph scalded skin syndrome?
exfoliation toxin
What feature do endotoxins and TSST1 share?
both superantigens
What is a superantigen?
binds outside the antigen-binding groove reulsting in APC and T cell binding even wihtout peptide reuslting in hyperactivation of hte immune system
What is the target of exoliation toxin?
cleaves desmoglein-1: resulting in disintegration of a layer of skin
How is protein A linked to the cell wall?
covalently linked to peptidoglycan
What are the functions of peptidoglycan?
shape/structure; protection; platform to anchor surface proteins
How is peptidoglycan syntehsised?
by enzymes not the ribosome
What enzymes convert a monomer of peptiodglycan to a cross-linked peptidoglycan?
penicillin binding proteins
How many penicillin binding proteins does S.aureus have?
4
Which is the only PBP to have both transglycosylase and transpeptidase activity?
PBP2
what funtion do all PBPs have?
transpeptidase
How does the transpeptidase activity of PBPs work?
cleaves a peptide and binds itself (now tetrapeptide- monomer is pentapeptide) covalently then makes another peptide bond to join to another chain
Why does penicillin inhibit PBP action?
looks like a PBP substrate to PBP covalently binds and cleaves beta-lactam ring but cannot resolve itself
What gene encodes PBP2a?
mecA
What causes aureus resistance to penicillin?
beta lactamase
What causes aureus resistance to methicillin?
PBP2a which has low affinity for methicillin
