Direct Sensitivity Testing:

@ Performed when : * Gram stain shows large number

of one type of reaction * To get quick result for serious

cases * Used only for urine, pus, blood samples

@ Routine culture must be also done

@ Add blood to MH agar to perform direct sensitivity

@ Do not report direct sensitivity if: * Growth is too heavy or too light. * Inhibition zone is smaller than

that of the control.

@ If Proteus swarms across its inhibition

zone, no problem if the zone is clear.

@ Confirm direct sensitivity by indirect sensitivity

Indirect Sensitivity Testing:

@ The inoculum must be a pure isolate

@ Match inoculum by turbidity standard.

@ Growth of test and control strains must not be too heavy or too light.

@ Radius of control inhibition zone should measure at least 8-15 mm.

@ If growth is not confluent (containing colonies), repeat sensitivity.

Sensitivity Techniques:

@ Disc diffusion sensitivity techniques.

@ Dilution sensitivity technique.

@ Etest sensitivity technique.

@ Rosco sensitivity technique.

Disc diffusion sensitivity techniques:

Two techniques are used:

@ Stokes disc diffusion technique:@ Kirby-Bauer disc diffusion technique

@ Stokes technique: Test inhibition zone

is compared with control inhibition zone.

@ Kirby-Bauer technique: Test inhibition

zone is measured & compared against a

scale of standard inhibition zones (WHO).

@ Disc diffusion sensitivity techniques

has the following advantages:

* Both test & control organisms are

inoculated on same plate.

* Inoculum gives a growth that is neither too heavy nor too light.

Dilution sensitivity technique:

@ Not performed routinely.

@ Performed when: * Patient is not responding to

therapy * Patient is immunosuppressed.

@ It measures the MIC (the Minimum

inhibition concentration) of the drug

required to inhibit bacterial growth.

@ Dilutions of drug are added to tubes

containing MH broth.

@ The organism is added to all tubes.

@ After overnight incubation, MIC is

reported (the last tube where there

is no growth).

@ Reading: compare your MIC with known MIC of the drug

Minimal Bactericidal Concentration (MBC):

@ For some infections, e.g. endocarditis,

@ Done to know concentration of drug that kills the organism not the concentration that inhibits the growth.

@ It is determined by subculturing the MIC tubes and the positive control tube on blood agar

@ Count colonies of the subcultured tubes

@ Comparing the control sub culture, the lowest concentration that has reduced the number of colonies by 99.9% is the MBC.

@ Bactericidal drugs have an MBC equal or similar to the MIC

@ But bacteriostatic drugs have an MBC higher than the MIC.

Interpretation of results:

@ Test is reported: sensitive, intermediate, or resistant

a) Sensitive: Test inhibition zone is: @ Wider than control inhibition zone @ or Equal to control inhibition zone @ or Not 3 mm smaller than control inhibition zone.

b) Intermediate: Test inhibition zone is: @ Not less than 2-3 mm than control inhibition zone.

c) Resistant: Test inhibition zone is: @ 2 mm or less than control inhibition zone.

@ Or there is no zone of inhibition

Example of a template for interpreting susceptibility.

Andrews J M J. Antimicrob. Chemother. 2005;56:60-76

© The Author 2005. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org

@ Organisms are considered resistant if:

* Growth is heaped-up at inhibition

zone edge without gradual fading up

* Large colonies are seen growing within inhibition zone.

@ With trimethoprim & sulphonamides

small colonies within inhibition zone are

due to presence of thymidine inhibitors.