Direct Sensitivity 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 Routine culture must be also 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 If Proteus swarms across its inhibition zone, no problem if the zone is Confirm direct sensitivity by indirect sensitivity

Indirect Sensitivity The inoculum must be a pure Match inoculum by turbidity Growth of test and control strains must not be too heavy or too Radius of control inhibition zone should measure at least 8-15 If growth is not confluent (containing colonies), repeat sensitivity.

Sensitivity Disc diffusion sensitivity Dilution sensitivity Etest sensitivity Rosco sensitivity technique.

Disc diffusion sensitivity techniques: Two techniques are Stokes disc diffusion Kirby-Bauer disc diffusion Stokes technique: Test inhibition zone is compared with control inhibition 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 Not performed Performed when: * Patient is not responding to therapy * Patient is 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 The organism is added to all After overnight incubation, MIC is reported (the last tube where there is no Reading: compare your MIC with known MIC of the drug

Minimal Bactericidal Concentration For some infections, e.g. Done to know concentration of drug that kills the organism not the concentration that inhibits the It is determined by subculturing the MIC tubes and the positive control tube on blood 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 Bactericidal drugs have an MBC equal or similar to the But bacteriostatic drugs have an MBC higher than the MIC.

Interpretation of Test is reported: sensitive, intermediate, or resistant a) Sensitive: Test inhibition zone Wider than control inhibition or Equal to control inhibition or Not 3 mm smaller than control inhibition zone.

b) Intermediate: Test inhibition zone Not less than 2-3 mm than control inhibition zone. c) Resistant: Test inhibition zone 2 mm or less than control inhibition 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 Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please

@ Organisms are considered resistant if: * Growth is heaped-up at inhibition zone edge without gradual fading up * Large colonies are seen growing within inhibition With trimethoprim & sulphonamides small colonies within inhibition zone are due to presence of thymidine inhibitors.