L55 Antibiotic sensitivity tests

Question 1

What is MIC?

For a given organism, can MICs of 2 different Abx be compared?

Answer 1

Minimal inhibitory concentration = minimal concentration of an anti-microbial that inhibits microbial growth;

no, cannot be directly compared

Question 2

What is MBC?

Answer 2

Minimal bactericidal concentration (MBC): minimal concentration that kills the organism

Question 3

For the disc diffusion method, it is a qualitative test. Briefly describe. Result?

Answer 3

Blotting paper disc impregnated with a standardized concentration of antibiotic is placed onto an agar plate (with bacteria in place)
>
degree of sensitivity is judged by the size of growth inhibition zone around the disc after overnight incubation
>
If zone of clearance diameter is greater = zone of inhibition is greater = more sensitive

Question 4

Compare zone diameter with a scale relating zone size to MIC, using standard conditions form CLSI (Clinical and Laboratory Standards Institute). > What method in disc diffusion?

Answer 4

Kirby-Bauer method

Question 5

Suggest 2 quantitative tests for Antibiotic sensitivity.

Answer 5

Dilution method

1. Broth dilution method
2. Agar dilution method

Question 6

What is Broth dilution method? (4)

Answer 6

1. Tubes containing a range of doubling dilutions of antibiotic + standardised inoculum of the test organism in Mueller-Hinton broth (MH)
   - MIC - lowest concentration of antibiotic which showed no turbidity = clear (can kill the bacteria
2. Standard amounts of broth are then subcultured onto agar plates containing no antibiotics
   - MBC = lowest concentration which yields no growth

Question 7

What is agar dilution method? Difference wtih Broth dilution method?

Answer 7

MH agar plates containing a range of doubling dilutions of antibiotic + standardized inoculum of the test organism.

• only MIC but not MBC can be tested

Question 8

MIC = lowest concentration that inhibits visible growth of organism
- E.g. organism grows in 4 mcg/L but inhibited at 8 mcg/L > actual MIC could be from 4.1 - 8.0 mcg/L

o Detection of MRSA: cefoxitin zone ≤ ____mm
o Detection of VRSA: vancomycin zone ≤ ____mm

Answer 8

19;

14

Question 9

What is Synergism? Give an example.

Answer 9

1+1 >2

e. g.
1. Septrin: Trimethoprim + Sulfamethoxazole
2. Ampicillin + gentamicin for IE

Question 10

What is Antagonism?

Answer 10

1+1 <2
(positive “D” test on agar)
e.g. if one bactericidal abx (requires organism to be actively dividing) is used together with one bacteriostatic abx > may hinder activity

Question 11

E test is bascially a combination of which 2 methods?

Plastic strip with pre-defined concentration gradient of an antibiotic. MIC is the zone edge.

Answer 11

Disc diffusion method and agar dilution method.

Question 12

Detection of beta lactamase is done by:

1. A piece of β-lactamase test paper disc (impregnated with benzylpenicillin and a pH indicator) is placed on a clean microscope slide and is moistened with distilled water.
2. Several colonies (from the specimen) are picked with a wire loop or wooden stick and streaked on to the dampened paper disc.
3. Presence of β-lactamase is indicated by the streaked portion of the paper disc changing to red colour. Why?

Answer 12

Benzylpenicillin broken down > penicilloic acid > lowers pH > turns paper disc into red color within a minute

Question 13

Give an example of an organism which require exact MIC value.

Answer 13

Streptococcus pneumoniae

• for Pen G & Cefotaxime
• report actual MIC

// Vancomycin resistant enterococcus infection