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.
What does pneumolysin (S. pneumo) do?
Binds to cholesterol in host-cell membranes.
What are the clinical presentations of S. pneumo infections?
Meningitis Otitis media (in children) Pneumonia (rusty sputum) Sinusitis Sepsis in Sickle Cell
Where does the viridans group of Streptococci live?
Normal flora of the oropharynx.
How are viridans distinguished from S. pneumo?
Optochinin resistant.
What are dental caries caused by?
Strep mutans (mutilates the teeth)
What is subacute bacterial endocarditis caused by?
S. sanguinis (blood in heart)
How does S. sanguinis cause subacute bacterial endocarditis?
Makes extracellular dextrans, which bind to fibrin-platelet aggregates on previously damaged heart valves.
What sub-group of S. viridans causes brain of liver abscesses?
Strep intermedius group.
If a gram+ cocci is catalase negative, and beta-hemolytic, what is the next step in differentiating?
Bacitracin sensitive: Group A (s pyogenes)
Bacitracin resistant: Group B (s. agalactiae).
What is the significant of streptolysin O of S. pyogenes?
It is one of the enzymes that destroys RBCs and facilitates hemolysis; it is antigenic and can be detected on ASO titer.
What is the significance of the M protein of S. pyogenes?
70 types. Adherence factor on capsule, antigenic, induces antibodies which can lead to subsequent antibody-mediated disease.
What is pyrogenic toxin of S. pyogenes responsible for?
Pharyngitis, Skin infections (Cellulitis, Impetigo, Erysipelas),
What is exotoxin A of S. pyogenes responsible for?
TSST-1-like toxin creats toxic shock syndrome, scarlet fever, necrotizing fasciitis.
What are the immunologic-mediated complicates of S. pyogenes?
Rheumatic fever
Post-streptococcal glomerulonephritis
What is the disease presenting as rash with sandpaper-like texture, strawberry tongue, circumoral pallor, and desquamation?
Scarlet fever.
What are the sx of acute rheumatic fever?
Joint pain <3 carditis Nodules (subQ) Erythema marginatum Syndenham chorea
How does acute post-strep glomerulonephritis present?
Tea-colored urine weeks following skin/pharynx infection.
Is impetigo more likely to precede glomerulonephritis or rhuematic fever?
Glomerulonephritis.
Does S. pyogenes also have a bunch of tissue invading toxins?
Yes. Hyaluronidase, DNAase, anti-C5a peptidase, etc.
If a gram positive cocci is catalase negative, beta hemolytic, but not bacitracin sensitive, what is it
Group B strep - agalactiae.
Where does Group B strep colonize?
Vagina
What diseases does Group B Strep cause?
Neonatal meningitis, pneumonia, sepsis.
What factor does Group B strep create?
CAMP factor (not same as cylic AMP). Enlarges area of hemolysis around S. aureus
What other test is Group B positive on?
Hippurate test +.
When should pregnant women be screened for GBS?
35-37weeks. If+, intrapartum penicillin ppx.
What group do catalase negative, non-hemolytic gram+ cocci belong to? How to differentiate?
Group D strep. Both grow in bile.
Grows in 6.5% NaCl: Enterococcus CAN BE ALPHA HEMOLYTIC
Doesn’t grow in 6.5% NaCl: S. bovis, also called gallolyticus (or equinus)
Where are E. faecalis and E. faecium found?
Normal colonic flora.
What disease can enterococcus be associated with?
- subacute endocarditis (dextrans) following GI/GU procedures.
- biliary tract infections
- UTIs
Are enterococci susceptible to penicillin?
No. Treat w/ ampicillin.
Vancomycin-Resistant-Enterococcus = badness. (linezolid, dapto)
What is S. bovis associated with?
- bacteremia/subacute endocarditis
2. Colon cancer.
