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Bacteriology > Staphylococci > Flashcards

Staphylococci Flashcards

1
Q

What is the microscopic appearance of Staphylococci?

A

Gram-positive cocci in bunches

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2
Q

Is Staphylococcus catalase positive or negative?

A

Positive

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3
Q

Is Staph aureus coagulase positive or negative?

A

Positive

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4
Q

What are the coagulase-negative Staph?

A

S. epidermidis (Device-related infections)
S. saphrophyticus (UTI)
S. lugdenensis (Native-valve endocarditis) - more virulent

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5
Q

How can you differentiate Staph aureus from Staph epidermidis on culture?

A

Staph aureus appears gold, Epidermadis appears white

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6
Q

What is protein A?

A

Surface protein of Staph aureus with multiple Fc receptors, binds IgG and prevents neutrophils from phagocytosing the bacteria

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7
Q

What are MSCRAMM?

A

“Microbial surface component reacting with adherence matrix molecules”

Facilitates adherence of S. aureus to host tissues

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8
Q

What are virulence factors of Staph aureus?

A
  • Evasion of host immune system (capsule, Protein A)
  • Adherence to host tissues (MSCRAMM, slime layer)
  • Enzymatic destruction of host tissues (coagulase, hyaluronidase, catalase, fibrolysin, lipases, nucleases)
  • Toxin mediated destruction of host tissues (cytotoxins, cytolytic peptides)
  • Development of resistance to antibiotics (Penicillin, Methicillin aka Semi Synthetic penicillins, Vancomycin)
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9
Q

What toxins are always produced if the Staph strain expresses it?

A

Cytotoxins

Cytolytic peptides

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10
Q

What causes Staphylococcal scalded skin syndrome (SSSS)?

A

Exfoliative toxins

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11
Q

What causes Staphylococcal food poisoning?

A

Enterotoxins

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12
Q

What causes Staphylococcal toxic shock syndrome?

A

Toxic shock syndrome toxin-1 (TSST-1)

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13
Q

What is the mechanism of Penicillin resistance?

A

Penicillinase (beta-lactamase) hydrolyzes penicillin and other beta lactams

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14
Q

What is the mechanism of Methicillin resistance?

A

Acquisition of mecA gene encodes for an altered penicillin-binding protein (PBP2a)

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15
Q

What is the mechanism of Vancomycin resistance?

A

Acquisition of vanA gene alters vancomycin binding site, commonly found in VRE, and this gene can transfer to S. aureus via plasmid

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16
Q

What cutaneous pygoenic (pus-forming) infections are caused by S. aureus?

A
  • Impetigo: superficial pustule, thick yellow crust
  • Folliculitis: hair follicle
  • Furuncle: boils
  • Carbuncles: multiple infected hair follicles, deeper
  • Wound infections
17
Q

What systemic pyogenic infections are caused by S. aureus?

A

Pneumonia and/or empyema (abscess in lungs)
Osteomyelitis
Septic arthritis
Endocarditis (particularly with valve involvement)

18
Q

How does S. aures infection go from superficial cutaneous to systemic?

A

Bacteremia (in blood) from cutaneous infection or direct inoculation (e.g. catheter)

19
Q

What is the localized form of Staph Scalded Skin Syndrome (SSSS)?

A

Bullous impetigo

20
Q

Would you find S. aureus in a culture of the blisters from SSSS or bullous impetigo?

A

NO from SSSS - blisters are the results of toxins, so blisters are sterile (no organisms or inflammatory cells)
MAYBE from bullous impetigo - still caused by toxins, but localized so infection/colonization is in close proximity

21
Q

What are virulence factors of coagulase-neg Staph?

A

Slime layer - most important
Produce many of the same enzymes as S. aureus (catalase, hyaluronidase, penicillinase)
Resistance to antimicrobials
NOTE: don’t produce toxins like S. aureus

22
Q

How many POSITIVE blood cultures are needed to diagnose S. aureus vs. coagulase-neg Staph?

A

S. aureus - 1, very virulent so treat right away

Coagulase-neg Staph - multiple, could just be skin contaminant

23
Q

What is the first choice for empiric therapy (i.e. before susceptability results are known) of Staph?

A

Vancomycin - Don’t know resistance so this covers MRSA and other Gram pos

Could also use Daptomycin, but only for skin/soft tissue infections and not pneumonia (Daptomycin is inhibited by pulmonary surfactant)

24
Q

What would be the definitive therapy for MSSA (methicillin-susceptible) Staph that is causing a serious infection?

A

Anti-staphylococcal Penicillins (Nafcillin, cefazolin)

25
Q

What would be the definitive therapy for MRSA Staph that is causing a serious infection?

A

Vancomycin, Daptomycin (but not for pneumonia), Ceftaroline

26
Q

What would be the definitive therapy for MSSA Staph causing a less serious infection (outpatient treatment)?

A

Anti-staphylococcal Penicillins taken PO (Dicloxacillin, Cephalexin)

27
Q

What would be the definitive therapy for MRSA Staph causing a less serious infection (outpatient treatment)?

A

Clindamycin, TMP/SMX (doesn’t cover Strep), doxycycline (doesn’t cover Strep)
Linezolid (but $$)

Bacteriology (19 decks)

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