S Aureus? What dat?
What does Staph Aureus look like under under a microscope?
• A bundle of grapes (clustered cocci)
Staph Aureus can cause what on a medical implant?
• Biofilm
How do we treat patients with MRSA? (What medication?)
• Vancomycin - glycopeptide antibiotic
• Not as effective has side effects
• But also … VISA and VRSA exist ahhhhhh
Is Staph Aureus gram - or gram +? What color would this be on a gram stain?
• Staph Aureus is gram +, it will take up crystal violet stain appearing purple on the gram stain.
Mr. Beckett’s right foot wound culture came back as: “Gram stain shows gram positive cocci in clusters. Culture pending”. What does this tell us?
• We know it is a type of Staph
Can Staph Aureus be found on our bodies normally? If so, where??
Staph Aureus is a normal skin flora in 1/4 of the population. It colonizes the nostrils, armpits, and groin.
Name at least 1 toxin mediated disease caused by Staph Aureus
• Scalded skin syndrome - exfoliatin toxin - Painful patches with red skin or blisters - usually resolves in a few weeks
• Toxic shock syndrome - TSST-1 - Often due to leaving packing in for too long - Fever, rash, low BP, poor organ perfusion
• Staph food poisoning - Rapid onset because preformed - Mainly associated with vomiting
How did Staph Aureus become resistant to Penicillin? How did we counter??
• Now most strains of staph make beta lactamase which allow them disable beta lactam antibiotics by breaking down beta lactam ring.
• Next, clavulanic acid was used to bind and disable beta lactamases.
How would Staph Aureus appear on Hemolysis test? What is happening?
• Beta Hemolytic
• Complete Lysis of RBCs leaves a pale /clear area surrounding colony on blood agar
What pathology did Mr. Beckett have that was indicative of a Staph Aureus infection?
• Osteomyelitis
When grown on blood agar, what appearance does Staph Aureus have?
Yellow or golden appearance
Name at least 2 Inflammatory Diseases caused by Staph Aureus
• Pneumonia - patchy infiltrate on CXR - Secondary post viral super infection
• Skin infections - Localized: pimple, boil, carbuncle - Deeper: abscesses, cellulitis, impetigo
• Acute bacterial endocarditis - particularly in IV drug users
• Osteomyelitis (most common cause)
• Pyomyolitis in muscle
• Septic Arthritis in joints
• Septic thrombophlebitis - infected blood clot
What mutation does MRSA have that makes it resistant to lactam antibiotics?
• MRSA expresses gene mecA which has specialized PBP that are unaffected by beta lactam antibiotics
What is a catalase test looking for and how is it performed? How would Staph Aureus present if this test were done?
• Done to identify the organisms that produce the enzyme catalase which detoxifies H2O2 by breaking it down into water and O2.
• Add a drop of H2O2 to bacteria and look for bubbles.
• Bubbles would be present if Staph Aureus.
• Catalase is present in all Staph but not in Strep.
Mr. Beckett was prescribed Cephalexin 500 mg PO ID x 14 days at his initial hospital visit. If he had taken these antibiotics as prescribed would they have worked to manage his Staph Aureus infection?
• Yes, Cephalexin is a first gen cephalosporin which targets farm positive bacteria.
What is Staph Aureus main virulence factor AND how does it work?
• Staph Aureus Has a Protein A virulence factor, which is a component of its cell wall.
• Protein A can binds Fc region of immunoglobulins which prevents opsonization and phagocytosis.
Name at least 2 toxins released by Staph Aureus and either their mechanism of action or associated disease
• TSST-1 - Stimulates APCs to produce cytokines storm - Causes toxic shock syndrome
• Panton - valentine leukocidin toxin - Punches holes in leukocytes and kills them, they die through necrosis which causes inflammation ◦Can lead to tissue death (ex: necrotizing pneumonia)
• Hemolysin - Destroys Erythrocytes which releases their hemoglobin (including iron) - S Aurues uses this iron in its own metabolism
• Exfoliatin - causes SSSS or Ritters Disease
• Enterotoxin - Very stable in environment - can withstand high temperature - Causes food poisoning - vomiting and diarrhea - Can also cause toxic shock syndrome if gets into blood stream
Explain this meme:
Meropenem is active against methicillin susceptible S Aureus; however, has a has poor activity against MRSA.
What is a coagulase test looking for and how is it performed? How would Staph Aureus present if this test were done?
• Identifies whether an organisms produces the eco enzyme coagulase which causes the fibrin of blood plasma to clot.
• Place a drop of human plasma to bacteria and look for clumping.
Mr. Beckett was prescribed Cephalexin 500 mg PO ID x 14 days at his initial hospital visit. What type of medication is this? What is its mechanism of action?
• Cephalexin is a first gen cephalosporin. It belongs to a larger classification of antibiotics known as beta lactam antibiotics.
• Beta lactam antibiotics disrupt DD-transpeptidase which cross links amino polysaccharides that make up peptidoglycan cell wall. This ultimately causes death due to lack of ability to make a cell wall.