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Antibiotic Sensitivity Testing

Tukaram Prabhu K Asst Professor Microbiology PCMS & RC. Antibiotic Sensitivity Testing. Methods Technique & Interpretation Factors affecting. Lecture taken for II yr MBBS (2008-2009 session).

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Antibiotic Sensitivity Testing

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  1. Tukaram Prabhu K Asst Professor Microbiology PCMS & RC Antibiotic Sensitivity Testing Methods Technique & Interpretation Factors affecting

  2. Lecture taken for II yr MBBS (2008-2009 session) • The following lecture is compiled from different textbooks & websites and is for teaching - learning use only • This lecture is not a substitute for classroom lecture / text book but an adjunct for the same • This lecture may be useful for BSc Nursing, BMLT & DMLT students too

  3. Why is it done? • To determine the ability of an antibiotic agent to inhibit the in-vitro bacterial growth • In the laboratory 2 methods are used • Dilution technique • Diffusion technique

  4. Dilution method • 2 methods • Agar dilution • Broth dilution • Done to determine the MIC & MBC • Minimum concentration of antibiotic required to arrest the multiplication of the bacteria is known as Minimum Inhibitory Concentration • Minimum concentration of antibiotic required to kill the bacteria is known as Minimum Bactericidal Concentration

  5. MIC = 1/8

  6. MBC = 1/4

  7. Dilution method – when to do? • Patient does not respond to treatment thought to be adequate • Relapses while being treated • When there is immunosuppression

  8. Diffusion method • Disc diffusion most commonly used • Disc of blotting paper impregnated with known volume and concentration of antibiotic is plate on a lawn culture of test bacteria • Antibiotic diffuses into the medium • After overnight incubation, the culture is examined for zones of inhibition

  9. Diffusion method • Material required • Trypticase soy broth • Mueller Hinton Agar • Sterile cotton swab • 0.5 McFarland tube • Antibiotic discs

  10. Diffusion method • Kirby Bauer method • Zone of inhibition produced is compared against a previously compared standard chart • Stokes method • Zone of inhibition produced is compared directly with that of control

  11. Mueller Hinton agar with antibiotics showing zones of inhibition

  12. Kirby-Bauer Chart

  13. Interpretation • SENSITIVE • A pathogen reported as sensitive suggests that the infection it has caused is likely to respond to treatment if the antibiotic to which it is susceptible is used in normal recommended doses

  14. Interpretation • INTERMEDIATE / MODERATELY SENSITIVE • A pathogen reported as MS suggests that the infection it has caused is likely to respond to treatment if the antibiotic is used in larger than normal doses or when the antibiotic is concentrated at the site of infection

  15. Interpretation • RESISTANT • A pathogen reported as resistant suggests that the infection it has caused will not respond to treatment with that antibiotic irrespective of dose or site of infection

  16. Factors affecting AST • pH of the environment • Components of the medium • Stability of the drugs • Size of inoculum • Length of incubation • Metabolic activity of bacteria • Depth of the medium

  17. E test

  18. Octodiscs

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