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String test to identify Vibrio cholerae

 Vibrio cholerae ( positive) and Aeromonas species (negative) isolated from diarrheal stool, show similar colony characteristics on MacConkey agar and  they are oxidase positive. It also aids to differentiate Vibrio cholerae (positive) from other Vibrio species (negative). Colony morphology, oxidase test positive and  string test positive are very useful for conformation of  Vibrio cholerae.

Introduction

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

Principle of String test

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

Requirements 

• Clean and grease free glass slide
• Test organism
• Bunsen burner and inoculating loop
• 0.5% aqueous solution of bole salts either sodium deoxycholate or sodium taurocholates

Quality Control

To maintain quality of the test, as quality control strains following organisms can be used.

Positive Control: Vibrio cholerae

Negative Control: Aeromonas species

Procedure of String Test

1. Take a clean and grease free slide.
2. Add a drop of 0.5% sodium deoxycholate on the slide.
3. Take colony of test organism using an inoculating loop and mix 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 picture and below video.

Observation

Observe for string formation i.e. thread like structure

 Result and Interpretation 

Positive Test: Appearance of String i.e. thread like structure

Negative Test: No appearance of String

Application of String Test

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

References

• Biochemical tests for Identification of medical Bacteria- The Williams & Wilkins Company Baltimore-Jean F. MacFaddin
• A textbook of Microbiology 9th Edition -Ananthnarayan  & Paniker
• 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