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19 Cards in this Set
- Front
- Back
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Staphylococcus aureus: Microbiology
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Gram-positive cocci, arranged in grape-like clusters
From Greek staphyle – bunch of grapes Initially white colonies turn buff-golden (aureus) Most colonies β-hemolytic, facultative Ability to grow in 7.5% NaCl, mannitol salt agar (selective/indicator medium) |
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Staphylococcus aureus: Identification and subtyping
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Coagulase distinguishes S. aureus from other staphylococci
Most strains produce clumping factor -“bound coagulase” -Binds fibrinogen |
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Staphylococcus aureus: Epidemiology
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Any age
Risk factors: open wound Outbreaks: Hospital/healthcare associated - primarily MRSA, nasal carriers (30% of population). Community - MSSA (wrestlers), MRSA (drug addicts, children) New evidence indicates increase in CA-MRSA from 14% to 35% of cases |
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CA-MRSA infections, general
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1st or 2nd cause of soft tissue infections
2nd or 3rd cause of pneumonia 1st cause of osteomyelitis 1st cause of infectious arthritis (excluding STDs) |
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CA-MRSA Skin Infections
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Mimic Spider Bites
Fagan SP, Spider bites presenting with MRSA soft tissue infection require early aggressive treatment. Houston, TX ; 3/00 - 11/01 38 patients presented with infected “spider bites” 87% grew MRSA |
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CA-MRSA: differentiating from HA-MRSA, virulence factor
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Panton-Valentine leukocidin (PVL, pore forming cytotoxin)
-targets leukocytes |
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HA - MRSA infections, general
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1st cause of surgical site infections
2nd cause of pneumonia 2nd cause of bloodstream infections 10th cause of urinary tract infections |
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HA - MRSA: Pathogenesis
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Colonization mediated by number of surface proteins which bind host elements/foreign bodies
Trauma/foreign matter lower infectious dose Resistance to phagocytosis permits accumulation of toxins Resolution depends on ability to isolate the infection Granulation and fibrosis of boil In organs other than skin, spread and destruction prominent |
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S. aureus toxin mediated disease
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Food Poisoning:
Preformed enterotoxin resistant to boiling, action within hours of ingestion Exfoliative toxin: causes blisters when localized to site of infection, scalded skin syndrome when absorbed Staphylococcal toxic shock syndrome: Toxin(s) absorbed and circulated Not limited to TSST-1 |
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S. aureus clinical manifestations
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Furuncle and Carbuncle:
-Superficial infection of follicle -Infection resolves upon spontaneous drainage -Multiple boils yield a carbuncle Bullous impetigo: -Often secondary invader of GAS pustular impetigo -Exfoliative toxin strains Wide variety of deep tissue infections: -osteomyelitis (90% of acute cases in children) -pneumonia and deep tissue lesions highly destructive Scalded skin syndrome -exfoliative toxin Toxic shock syndrome |
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TSST-1 and Toxic Shock
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S. aureus
Superantigen: -non specific binding of TCR and MHC class II -hyperactivation of immune response, cytokine storm |
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S. aureus immunity
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Immunity poorly understood
Role of cellular and humoral components unclear Relapsing infections can occur over many years -Chronic furunculosis Current efforts directed at multicomponent vaccine -Fibronectin-binding protein, collagen-binding protein, fibrinogen-binding protein, capsular polysaccharide, alpha toxin mutant |
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S. aureus nasal carriage
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Nasal carriage of CA-MRSA is on the rise
-From 2001 – 2004, rates climbed from 0.8% to 9% for MRSA -36% carriage of MSSA -Similar studies showing increased carriage of PVL strains as well |
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S. aureus treatment
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80-90% of strains penicillin resistant
Penicillin sensitive – penicillin Pen resistant/Ox sensitive – oxacillin, nafcillin or cefazolin Methicillin resistant (MRSA) – vancomycin, synercid, linezolid, daptomycin Vancomycin resistant (VRSA) - synercid, linezolid, daptomycin Total resistance to antibiotics seems likely |
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CA-MRSA treatment
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10d - 3 weeks of clindamycin, minocycline; occasionally vancomycin or newer parenteral agents
Topical skin antiseptics Povidone iodine (10%) Chlorhexidine (4%) Phisohex soap Topical nasal agents Mupirocin, Bacitracin House cleaning Decolonize family members |
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New Approaches to Preventing S. aureus Infection: Vaccination
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Vaccination
polysaccharide-based vaccine known as StaphVAX Ineffective in a confirmatory phase 3 trial – halted human IgG preparation known as INH-A21 (Veronate) Ineffective at phase 3 An effective vaccine will have to be multicomponent, incorporating several surface proteins, toxoids, and surface polysaccharides |
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New Approaches to Preventing S. aureus Infection: Nasal decolonization
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Topical decolonization with mupirocin reduced the overall Staphylococcus aureus infection rate after surgery in Staphylococcus aureus nasal carriers
Topical decolonization with chlorhexidine gluconate resulted in a reduced overall nosocomial infection rate, but no effect was found on the Staphylococcus aureus infection rate |
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New Approaches to Preventing S. aureus Infection: anti-infective vascular catheters
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Minocycline/rifampin coated catheters significantly reduce adherence (P = 0.001)
M/R catheters significantly longer antimicrobial durability (28 days, P = 0.01) Downside: possible resistance to coating Anti-infective catheter flush solutions Minocycline/EDTA used as a flush solution in hemodialysis patients reduces the risk of CRB by up to 10-fold |
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Coagulase-negative Staphylococci
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S. epidermidis and other lesser recognized species
Commensals of skin, anterior nares, ear canals Commonly colonize implanted medical devices Viscous extracellular polysaccharide provides mechanical barrier to host defenses/antimicrobial agents – biofilm Most common skin contaminant in cultures |
