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2. DEFINITION
SENSITIVITY TESTING
It is the degree of activity of the selected
antimicrobial agent against the infecting
bacterial strains.

3.  Usually almost all the bacteria in infectious
disease are drug resistant.
 Hence sensitivity test is performed to select
the correct antimicrobial drug of choice.
 It may also help to identify the pathogen.
INTRODUCTION

4. LIMITATIONS
o It helps us to measure only the antimicrobial
activity against a bacteria under laboratory
conditions and not in the patients.
o The patients clinical condition, type and site
of infection, drug hypersensitivity, ADME,
characters of the patients are not taken in to
consideration in sensitivity testing
techniques.

5.  Sensitivity testing can be performed by
 Diffusion technique and
 Dilution technique

6. 1. Agar diffusion sensitivity test
 A disc of blotting paper is impregnated with
a known volume and appropriate
concentration of antimicrobial placed on a
plate of sensitivity agar inoculated with test
organism.
 The antimicrobial diffuses from the disc in to
the medium. After 24 hours, the culture is
examined for areas of growth around the
disc.

7.  Growth for sensitive strains are inhibited for a
distance while for resistant strains it grows up
to the edge of the disc.
 The zone of inhibition caused by the
antimicrobial is compared with the control.

8.  The volume, moisture content, PH,
constituent of agar medium, concentration,
storage and application of dose influence the
diffusion technique.
 Agar diffusion sensitivity tests are carried out
either by Kirby-Bawer (KB) method, ICS
method or by Stocks method.

9.  Modified KB method is recommended by the
National Committee for clinical Laboratory
Standards (NCCLS) and the WHO.

10. 2. DILUTION SENSITIVITY TESTS.
 Dilution sensitivity tests usually measures
the minimum inhibitory concentration (MIC)
or minimum bactericidal concentration
(MBC) required to kill the bacteria.
 Here dilutions of antimicrobials are added to
the broth or agar.
 A standardised inoculum of test organism is
added.

11.  After overnight the lowest antimicrobial
required to prevent visible growth is taken in
to consideration.
 Dilution technique needs
Careful standardization
Broth and agar medium
Antimicrobial solution
Incubation time and
Dilution time

12. General requirements for sensitivity testing.
1. Sensitivity testing agar.
Suitable media include Mueller Hinton agar,
Iso sensitest agar and Gibco sensitivity
testing agar no.2.

13. Mueller Hinton agar(MHA)
Composition
 Meat infusion 2.0 g/l
 Casein hydrolysate 17.5 g/l
 Starch 1.5 g/l
 Agar-agar 13.0 g/l

14. 2.ANTIMICROBIAL DISC
 This disc should be refrigerated at a
temperature instructed by the manufacturer.
 This should not be used after expiry date.
 The working stock disc should be warmed to
room temperature, avoid keeping in direct
sunlight.

15. ANTIMICROBIAL RESISTANCE
 Antimicrobial resistance can arise in bacteria
in several ways.
 Microbes acquire resistance after a change in
their DNA.
 Such changes may occur by
 genetic mutation ie by alteration in the structure
of their own DNA.
• genetic exchange ie by acquisition of extra-
chromosomal DNA from other bacteria.

16.  Genetic exchange is the most common cause
of serious clinical drug resistance because it
can produce resistance to multiple drugs.
 In genetic exchange , the resistance genes are
transferred from one bacterial species to
another by means of discrete, movable, extra
chromosomal DNA elements called
TRANSPOSONS.

17.  Transfer of transposons between bacteria can
occur by
• Conjugation ie direct physical mating between
bacteria.
• Transduction ie through the agency of
bacteriophages.
• Transposition ie by means of plasmids which are
transferable, extra chromosomal DNA molecule.

18. DRUG RESISTANCE
It refers to unresponsiveness of a micro-
organism to an antimicrobial agent.
They are of 3 types:
i. Natural resistance
ii.Acquired resistance
iii.Cross resistance.

19. NATURAL RESISTANCE
 Some microbes have always been resistant to
certain AMA.
 They lack the metabolic process or the target
site which is affected by the particular drug.
eg. gram negative bacilli are normally
unaffected by Pencillin G
M.tuberculosis is insensitive to tetracyclines

20. ACQUIRED RESISTANCE
 It is the development of resistance by an
organism (which was sensitive before) due to
the use of an AMA over a period of time.

21. CROSS RESISTANCE
 Cross-resistance is the tolerance to a usually
toxic substance as a result of exposure to a
similarly acting substance.
 It is a phenomenon affecting e.g. pesticides
and antibiotics as an example