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antibiotic-sensitivity testing

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  • 1. Antibiotic Sensitivity Testing
  • 2. Modern era in Antibiotics begins with Fleming.
  • 3. History of Antibiotic Discovery
  • 4. Uses of Antibiotic Sensitivity Testing
    • Antibiotic sensitivity test: A laboratory test which determines how effective antibiotic therapy is against a bacterial infections.
    • Antibiotic sensitivity testing will control the use of Antibiotics in clinical practice
    • Testing will assist the clinicians in the choice of drugs for the treatment of infections.
  • 5. Components of Antibiotic Sensitivity Testing
    • 1.The identification of relevant pathogens in exudates and body fluids collected from patients
    • 2. Sensitivity tests done to determine the degree of sensitivity or resistance of pathogens isolated from patient to an appropriate range of antimicrobial drugs
    • 3. Assay of the concentration of an administered drug in the blood or body fluid of patient required to control the schedule of dosage.
  • 6. Antibiotic Sensitivity Testing Is Essential of selection of Antibiotics
  • 7. Isolation and Identification of Bacteria precedes the selection of Antibiotic Testing Methods
  • 8. Uses of Antibiotic Sensitivity Testing
    • Helps to guide the Veterinarian in choosing Antibiotics
    • The accumulated results on different pathogens their sensitivity will guide the veterinarian in choosing empirical treatment in serious patients before the individual’s laboratory results are analyzed in the Microbiology laboratory.
    • Reveals the changing trends in the local isolates.
    • Helps the local pattern of antibiotic prescribing.
  • 9. Why Need continues for testing for Antibiotic Sensitivity
    • Bacteria have the ability to develop resistance following repeated or subclinical (insufficient) doses, so more advanced antibiotics and synthetic antimicrobials are continually required to overcome them.
  • 10. Testing for Antibiotic sensitivity
    • The method includes several steps including obtaining a bacterial sample; identifying the type of bacteria in the bacterial sample; selecting a set of antibiotics based on the identity of the bacteria in the bacterial sample; obtaining a control sample from the bacterial sample;
  • 11. Testing Antibiotic Susceptibility
    • Antibiotic sensitivity test: A laboratory test which determines how effective antibiotic therapy is against a bacterial infections.
    • Antibiotic sensitivity test: the in vitro testing of bacterial cultures with antibiotics to determine susceptibility of bacteria to antibiotic therapy. Bauer -Kirby test.
  • 12. How results to be Reported after Sensitivity Testing
    • One should follow guidelines in reporting the results and make matters simple with clear words as
    • Organism A isolated and sensitive ( or resistant ) to Antibiotic B
    • Such a report is relevant to present clinical condition “ that the minimum inhibitory concentration( MIC ) of the antibiotic for it has been measured in some way and that, if the organism is reported as sensitive the MIC is less than a half or quarter of the concentration of antibiotics likely to be found in the infected tissues of a patient given the usual schedule of doses i.e. that the infection is treatable”
  • 13. What is Resistance in Antibiotic Sensitivity Testing
    • Resistance implies that the infection is not treatable with the tested Antibiotic because its MIC exceeds achievable safe tissue or urine levels.
    • Intermediate sensitivity means they show a unimodal distribution, but they can often be reclassified as sensitive or resistant if retested.
  • 14. Kirby-Bauer methods A commonly used method in basic laboratories
    • Kirby-Bauer antibiotic testing ( KB testing or disk diffusion antibiotic sensitivity testing ) is a test which uses antibiotic -impregnated wafers to test whether particular bacteria are susceptible to specific antibiotics
  • 15. How to perform Kirby- Bauer testing
    • The basics are easy:  The bacterium is swabbed on the agar and the antibiotic discs are placed on top.  The antibiotic diffuses from the disc into the agar in decreasing amounts the further it is away from the disc.  If the organism is killed or inhibited by the concentration of the antibiotic, there will be NO growth in the immediate area around the disc: This is called the zone of inhibition . 
  • 16. Steps in Antibiotic sensitivity testing
  • 17. Streaking the Inoculum
    • Kirby-Bauer (also Bauer-Kirby) disk diffusion antibiotic susceptibility testing applies a defined inoculum (compared to McFarland 0.5 OD standard streaked as a lawn onto a large Mueller-Hinton agar or Blood agar plate (in 3 directions to ensure confluence).
  • 18. Making proper inoculum
    • Swab a Mueller-Hinton plate with each of the bacteria.  Dip a sterile swab into the broth and express any excess moisture by pressing the swab against the side of the tube.
  • 19. Bacteria are inoculated as lawn culture
    • Method of inoculation- Good results are obtained by placing a standard loopful of inoculum suspension on the plate and then spreading it with a dry sterile swab.
  • 20. Disk Diffusion Method
    • After completely swabbing the plate, turn it 90 degrees and repeat the swabbing process. (It is not necessary to re-moisten the swab.)  Run the swab around the circumference of the plate before discarding it in the discard bag.
  • 21. Placing the Antibiotic disks
    • Then, using a dispenser such as the one pictured, antibiotic-impregnated disks are placed onto the agar surface. As the bacteria on the lawn grow, they are inhibited to varying degrees by the antibiotic diffusing from the disk
  • 22. Zone sizes differ on sensitivity pattern
    • It has been determined that zones of inhibition of a certain diameter (varies for antibiotic and to a lesser extent, bacterial species) correlate with sensitivity or resistance to the antibiotic tested
  • 23. Look at the Charts for establishing the zones of Sensitivity
    • The zone sizes are looked up on a standardized chart to give a result of sensitivie, resistant, or intermediate. Many charts have a corresponding column that also gives the MIC (minimal inhibitory concentration) for that drug.
  • 24. Interpretation
    • Place the metric ruler across the zone of inhibition, at the widest diameter, and measure from one edge of the zone to the other edge.  HOLDING THE PLATE UP TO THE LIGHT MIGHT HELP.   
    • The disc diameter will actually be part of that number.   If there is NO zone at all, report it as 0---even though the disc itself is around 7 mm.   
    • Zone diameter is reported in millimetres, looked up on the chart, and result reported as S (sensitive), R (resistant), or I (intermediate).   
  • 25. The area of Inhibition is measured with a Scale
    • Record the results for everyone on your table in the table below.  
  • 26. The disk diffusion methods are commonly used for routine testing
  • 27. Read the plates in traanmitted light
  • 28. The zone of inhibition guides the right choice of Antibiotic
  • 29. How results to be Reported after Sensitivity Testing
    • One should follow guidelines in reporting the results and make matters simple with clear words as
    • Organism A isolated and sensitive ( or resistant ) to Antibiotic B
    • Such a report is relevant to present clinical condition “ that the minimum inhibitory concentration( MIC ) of the antibiotic for it has been measured in some way and that, if the organism is reported as sensitive the MIC is less than a half or quarter of the concentration of antibiotics likely to be found in the infected tissues of a patient given the usual schedule of doses i.e. that the infection is treatable”
  • 30. The strips with multiple Antibiotics can be tested in one go
  • 31. Other methods of Antibiotic susceptibility testing
    • Other methods to test antimicrobial susceptibility include the Stokes method, E-test (also based on antibiotic diffusion). Agar and Broth dilution methods for Minimum Inhibitory Concentration determination.
  • 32. Limitation of Disk Diffusion Methods
    • Disk diffusion methods are not suitable for slow grwoing bacterial pathogens
    • The great limitation being for testing on Mycobacterium tuberculosis which needs varied, technically demanding methods
  • 33. Antibiotic Sensitivity testing can be done with automation
  • 34. There is a growing need for Automation in Antibiotic sensitivity testing
  • 35. Limitation of Antibiotic Sensitivity usage
    • Both Microbiologists and Clinician should however bear in mind that the response therapy in vivo may not always reflect the results of testing the sensitivity of patient's pathogen in vitro.

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