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. (< 50oC)
• 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
