String Test to Identify Vibrio cholerae: Introduction, Principle, Procedure, Result-Interpretation, and Uses

Introduction of String Test

String test is a special test of bacteria to identify Vibrio cholerae (String test positive) from Vibrio species and other bacteria like Aeromonas species and Plesiomonas shigelloides. This assay can perform on a glass slide or plastic petri dish, suspending overnight growth of Vibrio cholerae from a non-inhibitory medium like nutrient agar (NA) or Muller-Hinton agar (MHA) in a drop of 0.5% aqueous solution of bile salt that may be either sodium deoxycholate or Sodium taurocholate.

Principle of String Test

Bile salt lyses the bacterial cells (Vibrio cholerae), the suspension loses turbidity and deoxyribonucleic acid (DNA) releases from those lysed cells causing the mixture to become viscous. After picking the suspension, we will get the mucoid string structure.

Requirements for String Test

• Clean and grease-free glass slide
• Test organism/bacteria
• Bunsen burner and inoculating loop
• 0.5% aqueous solution of bile salts either sodium deoxycholate or sodium taurocholates
• Quality Control Strains-To maintain the quality of the test, quality control strains following organisms can be used. Positive Control (Vibrio cholerae) and Negative Control (Aeromonas species)

Procedure of String Test

1. Take a clean and grease-free slide
2. Put a drop of 0.5% sodium deoxycholate on the slide.
3. Take a colony of the test organism using an inoculating loop and mix it with the drop.
4. Keep on rubbing the loop vigorously for 2-3 minutes until the liquid appears viscous.
5. Then finally pull the inoculating loop upwards from the slide.
6. Observe for a mucoid “string” formation as shown above footage.

Observation of String Test

Observe for string formation i.e. thread-like structure

Result and Interpretation of String Test

Positive Test: Appearance of string i.e. thread-like structure

Negative Test: No appearance of String

Positive control: Positive

Negative control: Negative

Uses of String Test

1. The string test applies to separate Vibrio species from Aeromonas species and P. shigelloides.
2. The string test uses to differentiate Vibrio cholerae (string test positive) from other Vibrio species (string test negative).

Further Reading

1. Biochemical tests for Identification of Medical Bacteria- The Williams & Wilkins Company Baltimore-Jean F. MacFaddin
2. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996
3. A textbook of Microbiology 9th Edition -Ananthnarayan  & Paniker