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ANTIMICROBIAL SENSITIVITY @ Used to select effective drugs.

ANTIMICROBIAL SENSITIVITY @ Used to select effective drugs. @ Not indicated if sensitivity pattern of a pathogen can be predicted. @ Not performed on commensals or contaminants @ Used to identify organism if it has a characteristic sensitivity pattern.

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ANTIMICROBIAL SENSITIVITY @ Used to select effective drugs.

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  1. ANTIMICROBIAL SENSITIVITY @ Used to select effective drugs. @ Not indicated if sensitivity pattern of a pathogen can be predicted. @ Not performed on commensals or contaminants @ Used to identify organism if it has a characteristic sensitivity pattern.

  2. Factors Affecting Sensitivity: @ Media containing inhibiting substances (thymidine) that inhibits the action of tetracyclines and trimethoprim. @ False large zones are formed if medium is acidic. @ False small zones are formed if medium is alkaline. @ In vitro measurement does not mean in vivo drug activity.

  3. Requirements for Sensitivity Sensitivity Test Agar: @ Mueller-Hinton agar: Depth 4mm (25ml) @ Avoid too thin & too thick media. Store at 2-8°C for up to 2 weeks. @ Before use dry plates at 37°C. @ Blood is added to MH agar to test for Neisseria, Haemophilus, Streptococcus.

  4. Antimicrobial Discs: @ Select drug after consulting clinicians. @ Review drug list at regular intervals. @ If resistance developed, one member of each drug group is selected. @ Bring discs to room temp. 1 hr before @ Avoid discs dryness that decrease inhibition zone size.

  5. Control Strains: • @ S. aureus: Oxford strain (NCTC 6571) • Used for all specimens except urine. • @ E.coli: (NCTC 10418). Used for all • specimens of urine. • @ Ps. Aeruginosa: (NCTC 10662). Used • for all isolates of Pseudomonas.

  6. @ Using sterile culture loops, inoculate stock strains on nutrient agar slopes @ Store stock strains in darkness at 20-28°C (room Temp.), and Subculture them every 3–6 months @ Every week, take from stock strains and subculture in a nutrient broth and store at 2–8°C, then use this suspension for daily sensitivity testing.

  7. @ Filter paper discs containing the drugs are placed on MH agar medium with the test and control organisms. @ Control organisms are inoculated on same plate around the discs. @ After an overnight incubation, look for inhibition zones of the test and control organisms around the discs.

  8. Control Organism Test Organism

  9. Control Organism Test Organism Control Organism

  10. Turbidity (opacity) Standard: @ Used for matching organisms under test @ Turbidity of standard is equivalent to an overnight broth culture. @ It contains H2SO4, and barium chloride @ Store in darkness at room temp. for 6 months @ Matching is done before incubating organisms for two hrs.

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