Gram positive cocci Flashcards
What is the first step in differentiating gram positive cocci?
Catalase +: Staph
Catalase -: Strep
If a gram+ cocci is catalase +, what is the next step in differentiating?
Coag+: Staph aureus
Coag-: S. epidermis, S. saprophyticus
If a gram+ cocci is catalase + and coag -, what is the next step in differentiating?
Novobiocin sensitive: S. epidermis
Novobiocin resistant: S. saprophyticus
What are the main protective factors of Staph Aureus
Coagulase: allows fibrin clot formation around organism
Protein A: binds Fc-IgG, preventing opsonization and phagocytosis.
Also: hemolysins, leukocidins, penicillinase
What are the tissue-destroying proteins of Staph Aureus?
- hyaluronidase (breaks down connective tissue)
- Staphylkinase (lyses formed clots)
- lipase (breaks down fats)
By what three mechanisms does Staph Aureus cause toxin-mediated disease?
- Exfoliatin (scalded skin syndrome)
- Enterotoxin (food poisoning)
- Toxin-Shock-Syndrome-Toxin 1 (TSST): toxic shock syndrome
What does staph aureus exfoliatin toxin cause?
Staph scalded skin syndrome.
Exfoliative toxin A and B can diffuse far from local infection site, cause cleavage of middle epidermis, with fine sheets of skin peeling to reveal moist red skin beneath.
How does staph aureus enterotoxin work?
Pre-formed heat stable toxin, causes rapid-onset (2-6hrs) food poisoning with non-bloody diarrhea and vomiting < 24 hrs.
How does staph aureus TSST1 toxin work?
Superantigen that binds to MHC II and t-cell receptor, resulting in polyclonal t-cell activation by stimulation of TNF and IL-1.
What is the presentation of toxic shock syndrome? What is it associated with?
fever, vomiting, rash, desquamation, shock, end-organ failure.
Associated with prolonged tampon use/nasal packing.
Where does staph aureus cause disease by direct invasion?
Pretty much everywhere:
Skin infections, organ abscesses, meningitis, pneumonia, endocarditis, septic arthritis, osteomyelitis, bacteremia/sepsis, etc…
What differentiates MRSA from MSSA?
Methicillin-resistant staph aureus has altered penicillin binding protein, and community acquired MRSA has a mean Panton-Valentine Leukocidin that forms abscesses.
Where is S. epidermis found?
Normal skin flora.
What is the virulence factor of S. epidermis?
Forms adherent biofilms.
What is a biofilm again?
extracellular polysaccaride network that forms scaffold around organism. Very difficult for abx to penetrate.
What infections are associated with S. epidermis?
Infects prosthetic devices - hip implant, heart valve and IV catheters.
What is S. saprophyticus associated with?
Second most common cause of UTI in young women. (first is E coli).
If a gram+ cocci is catalase negative, what is the next step in differentiation?
Hemolysis:
Alpha (partial): S. Pneumo or S. Viridans
Beta (complete): Group A (s pyogenes) or Group B (galactic)
Gamma (not): Group D (enterococcus or not)
If a gram + cocci is catalase negative and alpha hemolytic, what is the next step in differentiation:
S. pneumo: optochinin sensitive, +capsule, +bile lyses.
S. viridans: optochinin resistant, - capsule, - bile lyses
What is significant about strep pneumo’s capsule? What shape are strep pneumo?
Shape: lancet shaped. Capsule: Highly varied. 83 serotypes. Vaccinations contain 13/23 most common. No virulence without capsule.