Staphylococci Flashcards
What is the microscopic appearance of Staphylococci?
Gram-positive cocci in bunches
Is Staphylococcus catalase positive or negative?
Positive
Is Staph aureus coagulase positive or negative?
Positive
What are the coagulase-negative Staph?
S. epidermidis (Device-related infections)
S. saphrophyticus (UTI)
S. lugdenensis (Native-valve endocarditis) - more virulent
How can you differentiate Staph aureus from Staph epidermidis on culture?
Staph aureus appears gold, Epidermadis appears white
What is protein A?
Surface protein of Staph aureus with multiple Fc receptors, binds IgG and prevents neutrophils from phagocytosing the bacteria
What are MSCRAMM?
“Microbial surface component reacting with adherence matrix molecules”
Facilitates adherence of S. aureus to host tissues
What are virulence factors of Staph aureus?
- 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)
What toxins are always produced if the Staph strain expresses it?
Cytotoxins
Cytolytic peptides
What causes Staphylococcal scalded skin syndrome (SSSS)?
Exfoliative toxins
What causes Staphylococcal food poisoning?
Enterotoxins
What causes Staphylococcal toxic shock syndrome?
Toxic shock syndrome toxin-1 (TSST-1)
What is the mechanism of Penicillin resistance?
Penicillinase (beta-lactamase) hydrolyzes penicillin and other beta lactams
What is the mechanism of Methicillin resistance?
Acquisition of mecA gene encodes for an altered penicillin-binding protein (PBP2a)
What is the mechanism of Vancomycin resistance?
Acquisition of vanA gene alters vancomycin binding site, commonly found in VRE, and this gene can transfer to S. aureus via plasmid
What cutaneous pygoenic (pus-forming) infections are caused by S. aureus?
- Impetigo: superficial pustule, thick yellow crust
- Folliculitis: hair follicle
- Furuncle: boils
- Carbuncles: multiple infected hair follicles, deeper
- Wound infections
What systemic pyogenic infections are caused by S. aureus?
Pneumonia and/or empyema (abscess in lungs)
Osteomyelitis
Septic arthritis
Endocarditis (particularly with valve involvement)
How does S. aures infection go from superficial cutaneous to systemic?
Bacteremia (in blood) from cutaneous infection or direct inoculation (e.g. catheter)
What is the localized form of Staph Scalded Skin Syndrome (SSSS)?
Bullous impetigo
Would you find S. aureus in a culture of the blisters from SSSS or bullous impetigo?
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
What are virulence factors of coagulase-neg Staph?
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
How many POSITIVE blood cultures are needed to diagnose S. aureus vs. coagulase-neg Staph?
S. aureus - 1, very virulent so treat right away
Coagulase-neg Staph - multiple, could just be skin contaminant
What is the first choice for empiric therapy (i.e. before susceptability results are known) of Staph?
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)
What would be the definitive therapy for MSSA (methicillin-susceptible) Staph that is causing a serious infection?
Anti-staphylococcal Penicillins (Nafcillin, cefazolin)
What would be the definitive therapy for MRSA Staph that is causing a serious infection?
Vancomycin, Daptomycin (but not for pneumonia), Ceftaroline
What would be the definitive therapy for MSSA Staph causing a less serious infection (outpatient treatment)?
Anti-staphylococcal Penicillins taken PO (Dicloxacillin, Cephalexin)
What would be the definitive therapy for MRSA Staph causing a less serious infection (outpatient treatment)?
Clindamycin, TMP/SMX (doesn’t cover Strep), doxycycline (doesn’t cover Strep)
Linezolid (but $$)
