Bacitracin Susceptibility Test / Taxo A

Bacitracin Susceptibility Test

The test is used for presumptive identification and differentiation of beta-hemolytic group A streptococci (Streptococcus pyogenes– susceptible) from other beta-hemolytic streptococci.
It is also used to distinguish staphylococci species (resistant) from micrococci (susceptible).

Method

1. Using an inoculating loop, streak two or three suspected 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. Image result for susceptibility test

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.

Medium Used:

Mostly, blood agar plate is used (trypticase soy agar + 5% sheep blood).

Expected Results

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

Interpretation

Performance depends on the integrity of the disk. Proper storage and expiration dates should be maintained.
Culture media must be freshly prepared for optimum diffusion of the antibiotic.
Lancefield groups C and G streptococci may occasionally also show susceptibility to bacitracin.
When testing isolates, a light inoculum may result in false zone of inhibition.

References

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