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37 Cards in this Set
- Front
- Back
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Characterize Staph aureus
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Gram + (blue gram stain)
cocci clustered |
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MoA of MSSA?
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more beta lactamase production -> breaks down more pcn (penicillin) which is a beta lactam
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DOC (drug of choice) for MSSA?
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methicillin, cefazolin, others
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MoA of MRSA?
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mecA gene -> changes in PBP (pcn binding protein)
specifically: MSSA's mecA (in SCCmec) -> PBP2a -> MRSA |
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DOC for MRSA?
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vancomycin
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MoA for VISA?
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Alteration in cell wall (thicker)
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DOC for VISA?
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none yet?
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MoA for VRSA?
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Transfer of vanA gene from Enterococcus
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DOC for VRSA?
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none yet
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What is MRSA resistant too?
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Penicillins and cephalosporins, both of which are B-lactam ABs
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Give examples of penicillins.
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-Methicillin
-Dicloxacillin -Nafcillin -Oxacillin -Amoxicillin -Ampicillin -Flucloxacillin (there are more) |
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Give examples of cephalosporins.
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Any drug that starts with "Cef"
eg Cefalexin |
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Which MRSA is resistant only to B-lactams and which is multidrug resistant?
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CA-MRSA: B-lactam resist
HA-MRSA: Multidrug resist |
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What do the B-lactams attach to on the bacteria?
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PBP (penicillin binding proteins).
Attachment of B-lactams prevents cell wall synthesis. |
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How did MSSA become MRSA?
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When the (variant) mecA gene of MSSA started producing PBP2a, B-lactams could no longer attach to the new PBP2a
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Where is the mecA gene located?
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In the SCCmec (Staphylococcal cassette chromosome mec)
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How does the size of the cassette matter?
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The larger the cassette, the more AB resistance genes it contains.
eg as seen in HA-MRSA |
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What are the SCCmec types for HA-MRSA and CA-MRSA?
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HA-MRSA: types 1,2,3(main)
CA-MRSA: type 4 |
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Which SCCmec types contain the most resistance genes?
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Types 2 and 3 (both HA-MRSA)
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Which SCCmec type is associated with the exotoxin gene PVL?
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Type 4 (CA-MRSA)
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What is the age and infection site seen in CA-MRSA vs HA-MRSA?
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CA-MRSA: younger, skin/soft tissue infection
HA-MRSA: older, bloodstream/respiratory(pneumonia) infection |
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Why aren't erythromycin and clindamycin very effective against S. aureus?
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They both bind to the 50s ribosomal subunit of the bacterial ribosome, but aureus has an efflux pump, and can sometimes alter its ribosome to prevent binding (only if it has the erm or ermB gene)
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You suspect aureus, how do you know if clindamycin can be used to treat it?
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Do a D test.
If the test is negative you can use clind, if test is positive you CANNOT use clinda. |
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A positive D test induces which gene within aureus?
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The D test uses clindamycin and erythromycin, which induce the erm and ermB gene to alter aureus's ribosome to increase resistance to these drugs.
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What's the most effective drug against both CA-MRSA and HA-MRSA?
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Vancomycin..
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What drugs are good against CA-MRSA?
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Vancomycin (if severe)
Clindamycin (if D --), or bactrim (trimethoprim + sulfamethoxazole), or linezolid. |
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What can you use to treat MSSA intravenously?
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Cefazolin (1st gen cephalosporin), clindamycin, nafcillin.
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What can you use to treat MSSA orally?
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Cephalexin (1st gen cephalo), clindamycin, dicloxacillin.
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Why has it taken so long for aureus to become (somewhat) vancomycin resistant?
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Because vanco has two MoA: polymerase and transpeptidase inhibition in cell wall synthesis.
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VRE is seen more with which specific microorganism?
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E. Faecium is the most vancomycin resistant enterococcus
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What gene encodes vancomycin resistance in Enterococcus?
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VanA gene (in US)
VanB in europe VanC everywhere else Note: resistance is plasmid mediated |
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How does vanA in enterococci confer resistance to vancomycin?
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Vanco attaches to ~~D-ala-D-ala in cell wall peptidoglycan, but vanA codes for ~~D-ala-D-lac, which vanco cant attach to, and therefore cannot inhibit cell wall synthesis.
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What are the risk factors for VISA?
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-Previous MRSA (always)
-Previous vanco use (always) -renal failure (sometimes) |
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When does VISA cause death?
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When it gets in the blood (bacteremia).
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What was the cause of VISA?
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Upregulation of cell wall synthesis caused (intermediate) vancomycin resistance.
Thicker cell wall, vanco cant get in. MIC of vanco increased (more needed to kill VISA) |
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VISA treatment?
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Multiple ABs, anything that the particular VISA is sens too, eg:
Trimethoprim/Sulfamethoxazole + tetracycline (or doxycycline), linezolid, quinupristin/dalfopristin |
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What resistance genes are in VRSA?
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mecA and vanA.
mecA is in MRSA, vanA came from enterococcus transfer to MRSA. |
