String test for Lab diagnosis of Vibrio cholerae

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.

Figure: A positive string test withV. cholerae. Image source: CDC

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 have recommended that when a 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 susceptibilities, salt tolerance, etc.

Purpose of String Test

1. To differentiate Vibrio cholerae (positive) from Aeromonas spp (negative). Both are isolated from diarrheal stool, show similar biochemical properties on culture media, and are oxidase positive.

2. To differentiate Vibrio cholerae (positive) from other Vibrio spp (negative).

Requirements: freshly prepared 0.5% bile salt, glass slide, inoculating loop

Procedure String Test

1. Take a clean grease free slide and put a drop of 0.5% bile salt.
2. Emulsify an isolated colony of the bacterium using an inoculating loop.
3. Keep on rubbing the loop vigorously for 2-3 mins until it appears viscous.
4. 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.

References

1. Grados, P., & Battilana, C. (1994). Detección de portadores de Vibrio cholerae convalecientes mediante el enterotest [Detection of convalescent Vibrio cholerae carriers using the enterotest]. Boletin de la Oficina Sanitaria Panamericana. Pan American Sanitary Bureau, 116(4), 285–289.
2. Neogy, K. N., & Mukherji, A. C. (1970). A study of the “string” test in vibrio identification. Bulletin of the World Health Organization, 42(4), 638–640.
3. Tamrakar, A. K., Goel, A. K., Kamboj, D. V., & Singh, L. (2006). Surveillance methodology for Vibrio cholerae in environmental samples. International journal of environmental health research, 16(4), 305–312. https://doi.org/10.1080/09603120600734303
4. Lam S. Y. (1983). A rapid test for the identification of Vibrio cholerae in stools. Journal of diarrhoeal diseases research, 1(2), 87–89.