Principle String Test: When an isolated colony (18-24 hour growth) of a suspected bacterium is emulsified in Sodium deoxycholate or Sodium taurocholate (commonly known as bile salt),it lyses the cell wall of the bacterium releasing the DNA .
The suspension loses turbidity and the mixture becomes viscous. A mucoid “string” is formed when an inoculating loop is drawn slowly away from the suspension
The sensitivity of this test is around 86% and specificity is only 70% so the researchers has recommended that when string test is used for screening of Vibrio spp; both positive and negative results have to confirm by other suitable tests such as decarboxylase activity, production of cytochrome oxidase, 0129 susceptibility, salt tolerance etc.
Purpose of String Test:
- To differentiate Vibrio cholerae ( positive) from Aeromonas spp (negative). Both are isolated from diarrheal stool, show same biochemical properties on culture media and are oxidase positive.
- To differentiate Vibrio cholerae (positive) from other Vibrio spp (negative).
Requirements: freshly prepared 0.5% bile salt, glass slide, inoculating loop
Procedure String Test:
- Take a clean grease free slide and put a drop of 0.5% bile salt.
- Emulsify an isolated colony of the bacterium using an inoculating loop.
- Keep on rubbing the loop vigorously for 2-3 mins until it appears viscous.
- Gently, pull the loop upwards from the slide.
Result and interpretation:
Formation of a thread like mucoid string indicates positive test.
Note: Based on colony morphology, positive oxidase test and positive string test, the isolate can be confirmed as Vibrio cholerae.