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Introduction of Novobiocin Susceptibility Test

Novobiocin Susceptibility Test is applied to differentiate the different species of coagulase-negative staphylococci.

Principle of Novobiocin Susceptibility Test

After incubation with 5 µg of novobiocin disk, Staphylococcus saprophyticus is not inhibited by the antibiotic whereas Staphylococcus epidermidis  is susceptible to novobiocin.

Test Requirements for Vancomycin Susceptibility Test

• Test organisms
• McFarland Densitometer
• Glass Test tubes
• Mueller Hinton Agar
• Antimicrobial disks ( 5 µg of novobiocin)
• Swab stick
• Bunsen burner
• Inoculating wire
• Incubator
• Disk applicator/ forceps
• Control strains

Test Procedure for Vancomycin Susceptibility Test

1.  Muller Hinton agar( MHA) is used in this antibiotic susceptibility test.
2. Emulsify 2-3 colonies in sterile saline matching the turbidity that standard (0.5 McFarland).
3. 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.
4. The swab is streaked in three directions over the surface of the MHA  to obtain uniform growth.
5. Allow the plates to dry for 10-15 minutes.
6. 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.
7. Incubate the plates at 37°C for 24 hours.
8. Following overnight incubation, measure the diameter of the zone of inhibition in millimeters (mm) around each disc.

Result-Interpretation of Novobiocin Susceptibility Test

Susceptible – Zone of inhibition is greater than 16 mm.

Resistant – zone of inhibition diameter less than or equal to 16 mm

Control strains- S. epidermis is sensitive while S. saprophyticus resistant.

Test organisms-Novobiocin susceptibility test

Staphylococcus epidermidis: Susceptible

 Staphylococcus saprophyticus: Resistant

Keynotes

• List of coagulase-positive staphylococci

1. Staphylococcus aureus
2. Staphylococcus schleriferi
3. Staphylococcus felis
4. Staphylococcus intermedius
5. Staphylococcus lutrae
6. S. hyicus
7. S. lodgunensis

• Note: among them only slide positive organisms-

1. Staphylococcus lodgunensis 
2. Staphylococcus schleriferi

• whereas only tube coagulase test positive-

1. Staphylococcus hyicus