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: [email protected]
@ 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.