Bacitracin Susceptibility

Question 1

What is the Purpose of bacitracin susceptibility ?

Answer 1

This test used for identification and differentiation of beta -hemolytic group A streptococci from other beta-hemolytic streptococci. It also used to distinguish staphylococci species (resistant) from micrococci (susceptible).

Question 2

Principle of bacitracin:

Answer 2

Principle The antibiotic bacitracin inhibits the synthesis of bacterial cell walls. A disk (TaxoA) impreg- nated with a small amount of bacitracin (0.04 units) is placed on an agar plate, allowing the antibiotic to diffuse into the medium and inhibit the growth of susceptible organisms. After incu- bation, the inoculated plates are examined for zones of inhibition surrounding the disks.

Question 3

Method of bacitracin:

Answer 3

1. Using an inoculating loop, streak two or three suspect colonies of a pure culture onto a blood agar plate.
2. Using heated forceps, place a bacitracin disk in the first quadrant (area of heaviest growth). Gently tap the disk to ensure adequate contact with the agar surface.
3. Incubate the plate for 18 to 24 hours at 35°-37°C in ambient air for staphylococci and in 5% to 10% carbon dioxide (CO2) for streptococci differentiation.
4. Look for a zone of inhibition around the disk.

Question 4

Expected results of bacitracin:

Answer 4

Positive: Any zone of inhibition greater than 10 mm; susceptible
Negative: No zone of inhibition: resistant

Question 5

Limitation of bacitracin:

Answer 5

Performance depends on the integrity of the disk. Proper storage and expiration dates should be maintained.

Question 6

Quality control of bacitracin:

Answer 6

Positive: Streptococcus pyogenes (ATCC19615)-susceptible
Micrococcus luteus (ATCC10240)-susceptible

Negative: Streptococcus agalactiae (ATCC27956)-resistant
Staphylococcus aureus (ATCC25923)-resistant