1. Antibiotic Sensitivity Testing
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 Done to determine the MIC & MBC
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 Stokes 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 Antibiotic S MS R Amikacin 14 mm 15-16 mm 17 mm
Ciprofloxacin
15 mm
16-20 mm
21 mm
Ceftriaxone
13 mm
14-20 mm
Doxycycline
12 mm
13-15 mm
16 mm
Gentamicin
13-14 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 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