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 37 o C 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 PlateSwabbing

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 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 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.