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Microbiology > Antibiotic susceptibility testing > Flashcards

Antibiotic susceptibility testing Flashcards

1
Q

What are some examples of enzymatic cleavage/inactivation forms of bacterial resistance?

A
  1. Beta lactams and aminoglycoside antibiotics

2. Beta lactamases and aminoglycoside modifying enzymes

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2
Q

What are some examples of bacterial resistance due to altered receptors/binding proteins?

A
  1. altered penicillin binding proteins
  2. Strep pneumo resistance to Penicillin
  3. ** MRSA resistance to Methicillin**
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3
Q

What are some examples of bacterial resistance due to altered permeability?

A

Pseudomonas resistance to aminoglycosides

(Gram neg bacilli alter influx and efflux pumps stopping passage through porins)

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4
Q

What is an example of bacterial resistance due to a metabolic block?

A

Enterococcus resistance to TMP/SXT

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5
Q

What organization sets the standards for antibiotic susc testing?

A

CLSI: Clincial laboratory standards institute

  • publish new standards yearly
  • decide which are appropriate abx to test
  • Set QC standards and proper procedures
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6
Q

What are necessary steps for preparation of an organism for testing?

A
  • Organism must be in LOG phase
  • Must use a pure culture (1 organism)
  • Amount of organism is set to 0.5 MacFarland Standard (=turbidity of 10^8 bacteria CFU/mL)/ or can use spectrophotometer
  • Incubate at 35C for 16-24hrs
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7
Q

What are some cons to Kirby Bauer testing?

A

-Is qualitative NOT quantitative (reports whether sensitive or resistant, does NOT give MIC)
-Is manual and susceptible to human error (interpretation size and set up)
-If ions too high–> Aminoglycosides are falsely resistant;
If ions too low–> Aminoglycosides are falsely susc

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8
Q

How do you measure a double zone in Kirby Bauer testing?

A

Measure the inside or more resistant zone.

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9
Q

How do you arrive at MIC for Kirby Bauer testing?

A

Must use regression analysis to calculate MIC value related to KB zone size

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10
Q

What are pros/cons of E-tests?

A 
Pros: 
-Flexible, can use any strip
-Quantitative, gives MIC outright
-Can use for anaerobes, yeast and bacteria
Cons: 
-Higher cost, $3 per strip
-Still have that damn human error!
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11
Q

How is broth dilution performed?

A

Organism further diluted to 5x the 0.5 MacFarland standard (10^5 org)

  • inoculated into microtiter trays with growth medium and known 2 fold dilutions of antibiotic.
  • *Must have 4 or more dilutions to be true MIC?
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12
Q

What is the difference between MIC and MBC?

A

MIC: minimum INHIBITORY concentration= minimal amount of antibiotic needed to stop growth or an organism, doesn’t mean it’s not still around
MBC: minimum bactericidal concentration= minimum amount of abx to KILL organism

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13
Q

How is MIC vs MBC determined?

A

MIC: dilution at which org is no longer growing in well
MBC: plate every dilution at and after to MIC to see if microorganism grows on the plate; no growth= MBC

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14
Q

What is tolerance?

A

When an organism is inhibited by antibiotics but cannot be killed, fortells resistance. MBC/MIC ratio will be <32

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15
Q

How are bacteria tested for beta-lactamase?

A

With a filter paper disk that has a chromogenic cephalosporin (Nitrocefin).
Interpretation: Red color= breakdown in beta lactam ring POSITIVE for beta-lactamase
Negative= retains yellow color

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16
Q

What antibiotics are bacteria resistant to if the beta lactamase test is positive?

A

Ampicillin, Penicillin, Cephalosporin

17
Q

Which are two key organisms that may produce beta lactamase? What abx does a positive test imply resistance to?

A
  1. Haemophilus influenzae–> 20 -40% produce beta lactamase, if so resistant to Ampicillin
  2. Bacteriodes fragilis group–> primary mech of resistance is beta lactamase production, resistant to Penicillin
18
Q

How is methicillin resistance tested for S. aureus?

A
  1. Staph. aureus is tested against OXACILLIN because it is more stable than methicillin and methicillin is no longer commercially available.
    * * If resistant to Ox then IS RESISTANT TO ALL BETA LACTAMS AND ALL CEPHALOSPORINS ARE REPORTED AS RESISTANT**
19
Q

What is the mech of resistance for MRSA**??

A

mecA gene encodes penicillin binding proteins

20
Q

What are two screening methods used for MRSA?

A

1) 6mcg/ml of Oxacillin and 4% NaCl (enhances detection of resistance) plated to Mueller Hinton agar
2) Cefoxitin screens (Cefoxitin is a better inducer of the mecA gene and therefore more sensitive at detecting resistance by production of PBPs)

21
Q

What is the theory behind the D test?

A

MRSA that is susceptible to Clindamycin has the ability to attain inducible resistance if certain enzymes are present. The D-test uses Erythromycin resistance to determine if the MRSA has occult resistance to Clindamycin

22
Q

How do you interpret a D test?

A

A blunted edge between the Erythromycin and Clindamycin test means Clinda resistance can be induced by erythromycin.

23
Q

Enterococci are naturally resistant to???

A
  1. Cephalosporins
  2. Clindamycin
  3. Tmp/SXT
24
Q

What two drugs have a synergistic effect against enterococci?

A

Ampicillin/ Gentamycin (beta lactam/aminoglycoside)

25
Q

How do you screen for synergy? Why do it?

A

put organism in a tube with 500 mcg/ml of gentamycin, if no growth then it is susceptible
**If growth present is not amenable to Gentamycin and Gent can cause nephrotoxicity and ototoxicity

26
Q

How do enterococci acquire resistance to Vancomycin?

A

It is plasmid mediated

  • vanA: Enterococcus faecium
  • vanB: Enterococcus faecalis
27
Q

How do you test for VRE?

A

can use Kirby Bauer, E test or automated systems

Drugs of choice-Linezolid and Synercid*

28
Q

Which organisms are capable of producing extended spectrum beta-lactamases? How is the resistance acquired?

A

E. coli, Klebsiella and Proteus

-plasmid mediated (Tem 1 beta lactamase plasmid)

29
Q

What is the significance of testing positive for extended spectrum beta-lactamase production?

A

You must report all CEPHALOSPORINS and PENICILLINS as RESISTANT
-typically still susceptible to carbapenems (Imipenem), Cephamycins (Cefotetan) and beta lacam inhibitors (Tazobactam or Clavulinic acid)

30
Q

What is the significance of Klebsiella pneumonia Carbapenemase (KPC)?

A

Not only resistant to Cephalosporins or Penicillins, but also Carbapenems.
- Test with Ertapenem, Imipenem, Carbapenem

31
Q

What is two step testing for Strep pneumo?

A
  1. Test S. pneumo for Penicillin susceptibility against Oxacillin (more sensitive), if susceptible, then susc to Penicillin; IF RESISTANT
  2. Confirm with MIC test against Penicillin (E-test, broth dilution, etc)
32
Q

What is the significance of the MIC values when testing S. pneumo for Penicillin susc?

A

Different breakpoints for type of infection:
CSF: Resistant if >= 2mcg/ml
Resp/blood: Resistant if >= 8 mcg/ml

33
Q

What drugs can you use if S. pneumo is resistant to Penicillin

A

Cefotaxime, Vancomycin or a quinolone

34
Q

What strain of C. difficile is hypervirulent?

A

NAP1 strain; more relapse, more serious infection, community acquired

35
Q

What toxins are responsible for disease in C. difficile?

A

Toxin A=enterotoxin (fluid)
Toxin B= cytotoxin
Test of choice previously EIA, now PCR***

36
Q

When amplifying in PCR, what does it mean when your negative control is positive? What must you do?

A

Means there is contamination in the system, you must rerun the ALL patients from the patients ORIGINAL sample

37
Q

Why should each patient sample have an internal control in addition to the positive and negative control used for the assay?

A

The internal control will suggest issues with the sample and not the system if negative such as inhibitors to amplification (Vagisil, zinc)

38
Q

When is the term “indifferent” used as a result in PCR amplification?

A

When you cannot tell if the results are positive or negative (I’m assuming this is after re-running the sample?)

Microbiology (5 decks)

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