ANTIBIOTIC SUSCEPTIBILITY TESTING AND DRUG RESISTANCE

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ANTIBIOTIC SUSCEPTIBILITY TESTING AND DRUG
RESISTANCE
Rashmi S
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ANTIBIOTIC SUSCEPTIBILITY TESTING(AST)

• DEFINITION
• AST is the study of the susceptibility of
  bacteria to a specific antibiotic

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• Susceptibility testing is done using following
  methods
• Dilution methods
• Agar dilution
• Broth dilution
• Diffusion methods
• Disc diffusion- Kirby-Bauer method

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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

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Mueller Hinton Agar Plate
Swabbing
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Disc Placement
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AST plate with zone of inhibition
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Measurement of zone of inhibition
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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

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DRUG RESISTANCE

• DEFINITION
• Organism develop tolerance for the drug
• Genetic mechanism of drug resistance
• Mutation
• Gene Transfer

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• 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

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• DRUG RESISTANCE DUE TO GENE TRANSFER
• It can be due to
• Transformation
• Conjugation
• Transduction
• Transposition

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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.

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