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