ANTIBIOTIC SUSCEPTIBILITY TESTING
AND
DRUG RESISTANCE
Rashmi S
ANTIBIOTIC SUSCEPTIBILITY
TESTING(AST)
DEFINITION:
AST is the “study of the susceptibility
of bacteria to a specific antibiotic”
Susceptibility testing is done using following
methods:
 Dilution methods
a) Agar dilution
b) Broth dilution
 Diffusion methods
Disc diffusion- Kirby-Bauer method
KIRBY –BAUER DISC
DIFFUSSION METHOD
 Uses small discs impregnated with antibiotics
 These discs are placed on the surface of the
inoculated Mueller Hinton agar plate, and
incubated at 37oC for 24 hrs.
 After incubation ,plates are observed for the
zone of inhibition.
 The diameter of zone of inhibition is measured
using a scale& compared with Kirby-Bauer chart
Mueller Hinton Agar Plate
Swabbing
Disc Placement
AST plate with zone of inhibition
Measurement of zone of inhibition
MINIMUM INHIBITORY
CONCENTRATION (MIC)
MIC is the least concentration of
antimicrobial that will inhibit the growth of
an organism
Clinical use:
For organisms from patients suffering
serious infection like “infective
endocarditis”
DRUG RESISTANCE
DEFINITION:
Organism develop tolerance for the drug
Genetic mechanism of drug resistance:
 Mutation
 Gene Transfer
MUTATIONAL DRUG RESISTANCE
It is of 2 types
1.Step wise mutation
2.One-Step mutation
CLINICAL SIGNIFICANCE:
Mutational resistance is of great importance in
Tuberculosis
DRUG RESISTANCE DUE TO GENE TRANSFER
It can be due to
 Transformation
 Conjugation
 Transduction
 Transposition
Biochemical mechanism of Drug
Resistance
 Decreased permeability to the drug.
 Development of alternative metabolic
pathway.
 Production of enzymes inactivating the
drugs.
 Modification of target site of the
antimicrobial agent.