Bacitracin and Sulfamethoxazole Trimethoprim Susceptibility Test: Introduction, Principle, Test Procedure and Result Interpretaion
Introduction of Bacitracin and Sulfamethoxazole Trimethoprim Susceptibility Test
Bacitracin and Sulfamethoxazole Trimethoprim Susceptibility Test is employed to identify the different species of the genus, Streptococcus especially Group A (Streptococcus pyogenes) and Group B beta-hemolytic streptococci (Streptococcus agalactiae).
Principle of Bacitracin and Sulfamethoxazole Trimethoprim Susceptibility Test
Group A beta-hemolytic streptococci (Streptococcus pyogenes) is susceptible to 0.04 unit bacitracin but resistant to 1.25µg sulfamethoxazole-trimethoprim (SXT) or also called Co-trimoxazole. Group B beta-hemolytic streptococci (Streptococcus agalactiae) is resistant to both bacitracin and Co-trimoxazole.
Test Requirements for Bacitracin and Sulfamethoxazole Trimethoprim Susceptibility Test
- Test organisms
- McFarland Densitometer
- Glass Test tubes
- Blood agar/Mueller Hinton Agar supplemented with 5%
Sheep blood (preferred) - Antimicrobial disks ( Bacitracin and Sulfamethoxazole Trimethoprim)
- Swab stick
- Bunsen burner
- Inoculating wire
- Incubator
- Disk applicator/ forceps
Test Procedure for Bacitracin and Sulfamethoxazole Trimethoprim Susceptibility Test
- Blood agar/Mueller Hinton Agar supplemented with 5%
Sheep blood is used in these antibiotic susceptibility tests due to being fastidious nature of organisms. - Emulsify heavy colonies in sterile saline matching the turbidity that standard (0.5 McFarland).
- Place a sterile cotton swab in the bacterial suspension and remove the excess fluid by pressing and rotating the cotton against the inside of the tube above the fluid level.
- The swab is streaked in three directions over the surface of the antimicrobial sensitivity testing agar (Blood agar/MHAsupplemented with 5%
Sheep blood to obtain uniform growth. - Allow the plates to dry for 10-15 minutes.
- Using sterile forceps or a suitable disk dispenser, place paper disks impregnated with a fixed concentration of an antibiotic, on the surface agar plates having distance disc to disc 25 mm while plate border to disk 15 mm.
- Incubate the plates at 37°C for 24 hours.
- Following overnight incubation, measure the diameter of the zone of inhibition in millimeters (mm) around each disk.
Result Interpretation of Bacitracin and Sulfamethoxazole Trimethoprim Susceptibility Test
Susceptible: Any zone of inhibition around either antibiotic disk
Resistant: Growth up to the disk (no zone of inhibition)
Organism – Bacitracin – Co-trimoxazole
Group A -Susceptible – Resistant
Group B- Resistant – Resistant
Group C,F,G -Resistant- Susceptible