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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.
Result and interpretation:
Formation of a thread like mucoid string indicates positive test
Reference: https://microbeonline.com/string-test-laboratory-diagnosis-vibrio-cholerae/
Chicken RBC agglutination test
Fresh chicken or sheep red blood cells can be used for this assay. A 2.5% (vol/vol) suspension of washed
(3 times) and packed (by centrifugation) cells is made in normal saline after the final wash. A large
loopful of the red cell suspension is placed on a glass slide. A small portion of the growth from a
nonselective agar slant is added to the red cells with a needle or loop, and is mixed well. In a positive test,
agglutination n of the red cells occurs within 30 to 60 seconds (Figure VI-12). Hemagglutinating ting (El Tor)
and nonhemagglutinating (classical) control strains should be used with each new suspension of red cells.
Strains of classical V. cholerae O1 that have aged in the laboratory or have undergone repeated passage in
broth may cause hemagglutination
and should not be used as controls.

Reference: laboratory identification of vibrio cholera
0/129 susceptibility test
Shewan and Hodgkiss recognized the sensitivity of Vibrio to the vibrio-static agent 0129 (2,4-diamino-6,7-di-
isopropylpteridine
phosphate)(1). 0129 was found to be useful in the differentiation of V ibrio fr om other gram-negative bacteria
especially Aeromonas , which are characteristically resistant to 0129(2). Even among the genus V i brio , different species
show different sensitivities to 0129(3); hence two different concentration discs are to be simultaneously tested to
determine the
degree of sensitivity of the species. 0129 discs of two concentrations are available: 10-μg and 150-μg. Methods for
standardized
disc antimicrobial susceptibility testing are employed, with any zone of inhibition around 0129 disks being regarded as
sensitive.
Medium to be used should be supplemented with 0.5% Sodium Chloride, as sodium ions stimulate the growth of all V i
brio
species and are required by most.
Interpret the results as follows
Sensitive - Zone of inhibition around both 10μg(DD047)and 150μg(DD048)disc.
Resistant - No zone of inhibition around both the disc (DD047) and (DD048) Partially sensitive - Zone of inhibition
around
150μg(DD048)disc and no zone around 10μg(DD047)disc.
Reference: HiMedia Laboratories Pvt. Ltd. A-516,Swastik Disha Business Park,Via Vadhani Ind. Est., LBS Marg, Mumbai-400086, India.
Urease test
Urease test principle
Urea is a diamide of carbonic acid. It is hydrolyzed with the release of ammonia and carbon dioxide.
Many organisms especially those that infect the urinary tract, have an urease enzyme which is able to
split urea in the presence of water to release ammonia and carbon dioxide. The ammonia combines with
carbon dioxide and water to form ammonium carbonate which turns the medium alkaline, turning the
indicator phenol red from its original orange yellow color to bright pink.
Result and Interpretation
Organisms that hydrolyze urea rapidly (e.g. Proteus spp) may produce positive reactions
within 1 or 2 hours; less active species (e.g. Klebsiella spp) may require 3 or more days. In
routine diagnostic laboratories the urease test result is read within 24 hours.
If organism produces urease enzyme, the color of the slant changes from light orange to magenta.
If organism do not produce urease the agar slant and butt remain light orange (medium retains original
color).
Reference: http://microbeonline.com/urease-test-principle-procedure-interpretation-and-
urease-positive-organsims/
Cholera red test
Cholera red reaction: When vibrio cholerae are grown for 24 hours in peptone water medium containing
ng
adequate amount of tryptophan and nitrate, they produce indole and reduce nitrate to nitrite. On adding a few

drops of sulphuric acid, nitroso-indole is formed, which is red in color.
Source Microbiology of Vibrio Cholera | MEDCHROME