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

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Presentation on theme: "Antibiotic Sensitivity Testing Methods Technique & Interpretation Factors affecting Tukaram Prabhu K Asst Professor Microbiology PCMS & RC."— Presentation transcript:

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

2 Lecture taken for II yr MBBS ( 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 AntibioticSMSR Amikacin14 mm15-16 mm17 mm Ciprofloxacin15 mm16-20 mm21 mm Ceftriaxone13 mm14-20 mm21 mm Doxycycline12 mm13-15 mm16 mm Gentamicin12 mm13-14 mm15 mm

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 1.pH of the environment 2.Components of the medium 3.Stability of the drugs 4.Size of inoculum 5.Length of incubation 6.Metabolic activity of bacteria 7.Depth of the medium

17 E test

18 Octodiscs


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