Staphylococci I and II Flashcards
What is the most common cause of nosocomial bacteremia?
S.aureus
What is the resistance determinant of vancomycin resistant staph aureus?
vanA
What are the three important staphylococcal species that we need to know? What does each infect?
- S. Aureus: community and hospital acquired infections
- S. epidermidis: intravascular catheter and foreign bodies
- S. Saprophyticus: urinary tract infections
What is the morphology of staphylococcus?
Gram + cocci in clusters
Is S. aureus motile?
No
Is S. aureus anaerobic or aerobic?
facultative
What happens if you plate S. Aureus on a blood agar plate?
Clearing of red cells around colonies, indicating hemolysis
Does S. aureus have catalase?
Yes. Because it is facultative aerobe.
If a species of staph is coagulase positive, what kind is it?
Staph aureus
If a species of staph is positive for nitrate reductase, what kind is it?
Either Staphylococcus aureus or epidermidis
If a strain of staphylococcus is DNAse positive, what kind is it?
Staphylococcus aureus. To do this, you would plate the bacteria on agar with methyl green (+) and DNA (-). Disintegration of the DNA will also show a clearing of the dye
How would you distinguish S aureus from group A strep?
S aureus would bubble under hydrogen peroxide because it has catalase. Group A strep would not.
How do you perform a coagulase test?
Add bacteria to rabbit plasma
What would indicate a positive nitrate reductase test?
Strip turns red
What color are S. epidermidis and S saprophyticus colonies?
White
What are two ways in which you could distinguish S epidermidis and S saprophyticus, after ruling out s. aureus?
S. epidermidis would be positive for the nitrate reduction test (would need to rule out S. aureus first). In contrast, S. saprophyticus is the only species that is positive for resistance to novobiocin.
How would you differentiate S aureus from S. epidermidis and S saprophyticus?
coagulase or DNAse test
How would you differentiate staphylococcus strains from streptococcus and enterococcus strains?
coagulase
How would you track an outbreak of S aureus?
pulse field electrophoresis
MLST
Also, phage typing, molecular typing, clonal complex typing
Staphylococcus aureus is a toxin mediated disease. What toxins does it release?
enterotoxins
toxic shock syndrome toxin
exfoliatins A and B “exfoliation syndrome”
Once the bug has adhered, s. aureus creates toxins to cause tissue damage:
- hyaluronidase
- staphylokinase
- P-V leukocidin
Note that even if you kill the bacteria, the toxins are heat stable
What factors does Staph aureus have to evade host defenses?
- complement inhibitors
- surface protein A
- capsule
What are the characteristics of the S. aureus cell wall that have allowed it to adhere to host cells?
Has fibronectin, fibrinogen, collagen, and platelet binding protein. These all help with adherence to the cellular matrix
What characteristics of the S. aureus cell wall have allowed it to evade host defenses?
teichoic acid makes surface less negatively charged. Protein A interferes with binding of IgG
What is the most common cause of food-borne diarrhea?
s. aureus contamination. Release of enterotoxins that are heat stable. However, diarrhea is usually self-limiting and nonbloody with an absence of white blood cells
What do the exfoliatin A/B toxins do?
Cleavage of desmoglein protein 1, which holds the epidermis to the dermis. Scalded skin syndrome. In TSS desquamation, the underlying epidermis is partially preserved.
how do you determine whether an S. aureus infection is invasive?
pus and abscess formation, either superficial or deep-seated
What are typical deep seated infections?
pneumonia, endocarditis, osteomyelitis
What are Janeway lesions? when would you see them?
emboli in the skin from S. aureus endocarditis. From bacteria adhered to neutrophil causing small vessel hemorrhage in distal limbs
What is a sequestrum?
Necrotic bone surrounded by a hyepremic region causing loss of mineralization
What are the toxin-mediated diseases caused by S aureus?
food poisoning, scalded skin syndrome, and toxic shock syndrome. Note that antibiotics are usually not helpful in these scenarios because it does not neutralize the toxin
How would you treat invasive S. aureus disease?
dicloxacillin, oxacillin, and vancomycin if MRSA is diagnosed
What happens if you plate S epidermidis on a blood agar palte?
little hemolysis
Is S. epidermidis positive for coagulase or catalase?
Coagulase negative.
Catalase positive.
If you take a blood culture from a normal pt and find S. epidermidis, does that mean that you should treat?
Not necessarily. S. epidermidis is a skin commensal and is often a sample contaminant
Describe the pathogenesis of S. epidermidis infections
Adherence to abiotic surfaces
Local bacterial proliferation
Formation of biofilm
Bacteria dislodge and then invade the blood stream
Why are biofilms so resistant to antibiotics?
These cells are not dividing, which is a prerequisite for many antibiotics (bacteriostatics, penicillins, etc.)
How would you treat an S. epidermidis infection?
Remove the prosthetic device
Treat with vancomycin
Is Staphylococcus saprophyticus catalase or coagulase positive? What is a diagnostic feature?
Catalase positive but coagulase negative. Resistant to novobiocin
What pathology does S. saprophyticus cause?
This is the second most common cause of UTIs aside from E. coli.
How do you diagnose S. saprophyticus?
polyuria and dysuria with positive urinary culture
How do you treat S. saprophyticus?
trimethoprim, bactrime, quinolones
Which species of staph have protein A? Cause hemolysis?
Only S aureus
What kinds of teichoic acids are found in S. aureus, epidermidis, and saprophyticus?
aureus: ribitol teichoic
epidermidis: glycerol teichoic
saprophyticus: different type entirely
What are the pyogenic diseases caused by S aureus?
- Skin infectiona (impetigo-honey colored crust erythema)
- septicemias from wound or IV
- endocarditis
- osteomyelitis
- wound infections
- lung abscess
What are the toxin mediated diseases caused by S aureus?
food poisoning (enterotoxin
toxic shock syndrome (TSST)
scalded skin syndrome (exfoliatin)
What are the important wall components of S aureus?
- Protein A
- Teichoic acid
- Capsule ( 11 serotypes)
- Peptidoglycan wall can act as an endotoxin
