ANTIBIOTIC SUSCEPTIBILITY TESTING ANTIBIOTIC SUSCEPTIBILITY TESTING AND AND

DRUG RESISTANCEDRUG RESISTANCE

Rashmi S

ANTIBIOTIC SUSCEPTIBILITY ANTIBIOTIC SUSCEPTIBILITY TESTING(AST)TESTING(AST)

DEFINITION:DEFINITION:

AST is the “study of the susceptibility AST is the “study of the susceptibility of bacteria to a specific antibiotic”of bacteria to a specific antibiotic”

Susceptibility testing is done using following Susceptibility testing is done using following methods:methods:

Dilution methodsDilution methods

a)a) Agar dilution Agar dilution

b)b) Broth dilutionBroth dilution Diffusion methodsDiffusion methods

Disc diffusion- Kirby-Bauer method

KIRBY –BAUER DISC KIRBY –BAUER DISC DIFFUSSION METHODDIFFUSSION METHOD

Uses small discs impregnated with antibioticsUses small discs impregnated with antibiotics These discs are placed on the surface of the These discs are placed on the surface of the

inoculated Mueller Hinton agar plate, and inoculated Mueller Hinton agar plate, and incubated at 37incubated at 37ooC for 24 hrs.C for 24 hrs.

After incubation ,plates are observed for the After incubation ,plates are observed for the zone of inhibition.zone of inhibition.

The diameter of zone of inhibition is measured The diameter of zone of inhibition is measured using a scale& compared with Kirby-Bauer chartusing 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 MINIMUM INHIBITORY CONCENTRATION (MIC)CONCENTRATION (MIC)

MICMIC is the least concentration of is the least concentration of antimicrobial that will inhibit the growth of antimicrobial that will inhibit the growth of an organisman organism

Clinical use:Clinical use:

For organisms from patients suffering For organisms from patients suffering serious infection like “serious infection like “infective infective endocarditis”endocarditis”

DRUG RESISTANCEDRUG RESISTANCE

DEFINITION:DEFINITION:

Organism develop tolerance for the drugOrganism develop tolerance for the drug

Genetic mechanism of drug resistance:Genetic mechanism of drug resistance: MutationMutation Gene TransferGene Transfer

MUTATIONAL DRUG RESISTANCEMUTATIONAL DRUG RESISTANCE

It is of 2 typesIt is of 2 types

1.Step wise mutation1.Step wise mutation

2.One-Step mutation2.One-Step mutation

CLINICAL SIGNIFICANCE:CLINICAL SIGNIFICANCE:

Mutational resistance is of great importance in Mutational resistance is of great importance in TuberculosisTuberculosis

DRUG RESISTANCE DUE TO GENE TRANSFERDRUG RESISTANCE DUE TO GENE TRANSFER

It can be due toIt can be due to

TransformationTransformation

ConjugationConjugation

TransductionTransduction

TranspositionTransposition

Biochemical mechanism of Drug Biochemical mechanism of Drug ResistanceResistance

Decreased permeability to the drug.Decreased permeability to the drug. Development of alternative metabolic Development of alternative metabolic

pathway.pathway. Production of enzymes inactivating the Production of enzymes inactivating the

drugs.drugs. Modification of target site of the Modification of target site of the

antimicrobial agent.antimicrobial agent.