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MICBI - General microbiology > L06 Antibiotics susceptibility testing > Flashcards

L06 Antibiotics susceptibility testing Flashcards

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1
Q

Describe how the disc diffusion method is done.

A
  1. Place a blotting paper disc impregnated with a drug onto plates with organism to be tested.
  2. The degrees of susceptibility depends on the diameter of the zone of inhibition around the discs.
  3. There bigger the zone of inhibition, the more susceptible the organism is to that Abx.
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2
Q

Name 3 methods to determine the minimal inhibitory concentration (MIC) of an antibiotic.

A
  1. Broth dilution method (liquid)
  2. Agar dilution method
  3. E-test
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3
Q

What is E-test?

A
  • to obtain MIC of an organism
    1. a plastic strip immobilised with pre-defined and a concentration gradient of an antibiotic agent
    2. Standadized inoculum of organism is applied to the agar plate.
    3. Strip is placed on the surface and incubated overnight
    4. MIC is read at the organism zone edge that intersects the strip.
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4
Q

What is MBC?

It can only be determined when the MIC is tested by which method?

A

Minimum bactericidal concentration (MBC)
The minimum concentration of antibiotic which kills an organism.

It can only be determined when the MIC is tested by the broth dilution method.

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5
Q

Certain microorganisms produce an enzyme called beta-lactamase. What effect does it have?

A

Beta-lactamase hydrolyses the beta-lactam ring of penicillin to produce an inactive compound, penicilloic acid.

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6
Q

List examples of microorganisms that produces beta-lactamase.

A
  1. Staph aureus
  2. Hemophilus influenzae
  3. Neisseria gonorrhoeae
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7
Q

How do we detect beta-lactamase producing organisms?

A

Beta-lactamase defection paper strip.

- impregnated with benzylpenicillin and a pH indicator > lowering of pH and a change in color (to red) of indicator.

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8
Q

Give 2 examples of antibiotics in which pre and post drug levels are determined. Why?

A
  1. Aminoglycosides
  2. Vancomycin
  • narrow therapeutic window
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9
Q

Trimethoprim + Sulphamethoxazole shows ____________ in disc test. Therefore, they are combined as septrin, useful in what organisms?

A

Synergism;

E.coli, ESBL (extended spectrum beta-lactamases) organisms, PCP (pneumocystis pneumonia by jiroveciii)

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10
Q

Nitrofurantoin = Furadantin
and
Nalidixic acid (quinolone)
shows _______ effect on disc test.

A

Antagonistic
1+1 <2

(oD shape on disc)

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11
Q

Why is synergism important in the treatment of endocarditis?

A

Because vegetation in IE, antibiotics have to penetrate the vegetation to kill the bacteria

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12
Q

An 83 year old female with uncomplicated UTI. Which Abx of choice should be used?

MSU culture
- Large numbers of WBCs seen >100,000 cell/ml

Route culture:
>100,000 CRU/ml of E.coli

Sensitive to:

  1. Amoxycillin/clavulanate
  2. Cefuroxime (oral)
  3. Amikacin
  4. Nitrofurantoin

Resistant to

  • Ampicillin
  • Ciprofloxacin
  • Cotrimoxazole
  • Gentamicin
A
  • Augmentin should be used for E.coli UTI

Not nitrofurantoin

  • C/I in renal failure, only concentrates urine
  • 83/F may be poor RFT

Not Amikacin (aminoglycans)

  • do not use it as monotherapy to treat UTI
  • nephrotoxicity, ototoxicity
  • but if pyelonephritis, septicaemia, consider to give it on top of augmentin
  • 3 days to have bactericidal effect
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13
Q

50/M with fracture of femur and intraoperative pus swab are Methicillin resistant Staph.aureus (MRSA).
Drug of choice? Things to note?

Sensitive to:
1. Vancomycin

Resistant to

  1. Cloxacillin
  2. Fusidic act
  3. Erythromicin
A

MRSA = resistant to all penicillins, cephalosporins and carbapenems!
Standard contact precautions are required!

Vancomycin

  • time & concentration dependent, high dose
  • red man syndrome if admission is too quick, have to slowly infuse over 1-2 hours
  • monitor nephrotoxicity and ototoxicity
  • Trough level before 5th dose: aim at 5-10 in mild infections
  • 15-20 in severe infections (e.g. pneumonia)
  • > 20 need to stop drug
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14
Q

48/M with chronic bronchitis presented with SOB and increased sputum production.
A sputum yielded Haemophilus influenzae.

Mucoid sputum.
Gram stain: large number of WBCs seen, few epithelial cell, mixed normal flora

Sensitive to:

  1. Amoxycillin/clavulanate
  2. Cefaclor
  3. Levofloxacin
  4. Clarithromycin

Resistant to:

  • Ampicillin
  • Cotrimoxazole
  • beta-lactamase +
  • Heavy growth of oral commensals. Why?
A

clavulanate: beta-lactamase inhibitor
Augmentin ok!

Cefaclor - 2nd generation cephalosporins ok!

Levofloxacin is for TB, may induce drug resistance. (Quinolone) - have side effects, last resort!

Clarithromycin: for penicillin resistance (macrolide) - bacteriostatic, better for atypical pneumonia

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15
Q

MIC penicillin = 0.5 ug/ml meaning?

A

Sensitive

<2 sensitive
4 intermediate
8 resistant

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16
Q

75/M with 2 month history of fever and SOB.
Cardiac murmurs are noted, echo shows vegetation on aortic valve.
He had been seen in the hospital 4 months previous, requiring cystoscopy for investigation of haematura.

Blood agar: shows alpha haemolytic acitivity
Grows on MacConkey agar (pink = lactose fermenting) and Gram stain shows G+ cocci chains.

Suggestive organism?

A

Enterococci (Group D strep)

alpha haemolytic: either viridans/enterococci
viridian’s does not grow on macconkey

17
Q

Penicillin MIC tested:
1mg/L is clear
0.5mg/L is turbid

Penicillin + Gentamicin MIC tested:

  1. 25 clear
  2. .12 turbid

What effect is shown when gentamicin is added?
How should this infection be treated? What would happen if the patient is treated with penicillin alone?

A

Synergistic effect

IE is treated by ampicillin/penicillin + gentamicin for 6 weeks
- linezolid (broader, bacteriostatic)

Resistance

18
Q

Gentamicin is given 80mg every 8 hourly.
Predose: 2.37mg (<2)
Postdose: 6.2317 (5-10)

Why do we need to assay for gentamicin levels?
Do we need to adjust dosage?

A

Due to toxicity, narrow therapeutic window
Also amino glycoside is concentration dependent killing (peak high is good, trough cannot be high)

Higher than normal predose and normal postpose =
Reduce frequency

19
Q

18/F with congenital heart disease admitted for aortic valve replacement. The procedure is uneventful but 6 weeks later, she develops a fever.

Colonies grow on blood agar,
G+ cocci in cluster
Negative coagulase test.

Possible organism?
Likely diagnosis?

A

Staph epidermidis

Prosthetic valve endocarditis

20
Q

What is the treatment for prosthetic valve endocarditis by S. epidermis? (3)

A

Vancomycin
* nephrotoxicity

+ Rifampacin (prosthetic)
+ Gentamicin (Synergistic - Aminoglycosides) *nephrotoxicity

21
Q

When is Etest and Broth microdulition test more preferred than disc diffusion?

A

E-test and broth dilution is for MIC!
use when:

  1. MIC affects outcome of Abx
    e. g. use of Vancomycin where [drug] corresponds with Tx outcome
  2. Diffusion test is not reliable e.g. too small
  3. Anaerobes with no zone size criteria
22
Q

If patient has prosthetic implants, what should be given for soft tissue infection prophylaxis?

A

Rifampicin

23
Q

If patient is allergic to vancomycin, what should be considered?

A
  1. Linezolid (but suppresses bone marrow), for VRSA

2. Dalfopristin

24
Q

Which of the following should be monitored closely when vancomycin is given?

A. Renal impairment
B. Morbid obesity
C. ICU 
D. Severe burn
E. Spinal injury
A

all of the above

25
Q

If patient has severe SOB, broader spectrum Abx should be used. Such as?

A
  1. Linezolid
  2. Ciprofloxacin
  3. Moxifloxacin
  4. Vancomycin

MICBI - General microbiology (3 decks)

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