Introduction of Vancomycin Susceptibility Test
Vancomycin Susceptibility Test is used to differentiate Pediococcus from other alpha-hemolytic streptococci. 5 µg of vancomycin disk is applied for the assaying.
Principle of Vancomycin Susceptibility Test
After incubation with 5 µg of vancomycin disk, Pediococcus is not inhibited by the antibiotic whereas Viridans streptococci are susceptible to vancomycin.
Test Requirements for Vancomycin Susceptibility Test
- Test organisms
- McFarland Densitometer
- Glass Test tubes
- Blood agar/Mueller Hinton Agar supplemented with 5%
Sheep blood (preferred) - Antimicrobial disks ( 5 µg of vancomycin)
- Swab stick
- Bunsen burner
- Inoculating wire
- Incubator
- Disk applicator/ forceps
- Control strains
Test Procedure for Vancomycin 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 Vancomycin Susceptibility Test
Susceptible: Any zone of inhibition around the antibiotic disk
Resistant: Growth up to the disk (no zone of inhibition)
Control strains– Pedicoccus: Resistant
Viridans streptococci-Susceptible
Test Organism – Vancomycin Susceptibility Test
Viridans streptococci: Susceptible
Pediococcus: Resistant