Determination of spectrum of activity of antibiotics

Antibiotic spectrum Definition: The spectrum of antimicrobial activity against different pathogenic M.O WHY? Define the antibiotic spectrum against different species of pathogenic M.O to allow proper use of the new antibiotic

Determination of Antibiotic spectrum Principle –A strip with antibiotic is placed in the center of the plate –Various test organisms are streaked at right angles with the antibiotic strip –Incubation for 24 hour –Observation of the growth ability

Principle

Detailed procedure Materials: –A sterile Petri dish –A sterile nutrient agar –Packet containing filter paper strips – 3 test tubes containing 3 different microbial cultures –Inoculation loop –EVERYONE SHOULD BRING FORCEPS

Detailed procedure A B C A B C

Precautions Pour the agar at optimum temp. (< 50 o C) Flame the forceps Avoid overloading the F.P strip with the antibiotic solution FLAME the loop very well between every test organism Cool the loop before transferring each organism

Expected results A B C A B c A is sensitive B is sensitive C is resistant

Expected Results Interpretation Narrow spectrum antibiotic : inhibit only gram positive or Gram negative Broad spectrum antibiotic: inhibit both Gram positive and Gram negative organisms