Presentation on theme: "Direct Sensitivity Performed when : * Gram stain shows large number of one type of reaction * To get quick result for serious cases * Used only."— Presentation transcript:
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.
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
@ 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.