I have more than 10 Vibrio species in my lab. When I started purification (streaking) on TCBS plates, they were a yellow color within 24 hour incubation at 35˚C, when I continued their incubation for more than 48 hours, some plates changed yellow to green. Why do their colors change?
The TCBS medium, is an ideal medium to isolate vibrio species, this is because of the high concentrations the Thiosulfate-Citrate, bile and a high pH alcaline. The strains that can't degradate the sacarose are the green color, while the strains that can ferment the sacarose are the yelow color because produce acid as from the sugar. This is because of the color change to the pH indicator the Bromthymol Blue and Thymol Blue, which change to a yellow color in acid medium.
It might be the completion of the reaction (Reduction/oxidation) and indicators might be exhausted. i.e., Sodium Thiosulfate, a sulfur source, and acts with Ferric Citrate as an indicator to detect hydrogen sulfide production. Bromthymol Blue and Thymol Blue are pH indicators. Also if the plates are left for long time at room temperature, the colonies will change their color to green.
See following articles, it might help. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC93376/pdf/am1201005819.pdf
https://catalog.hardydiagnostics.com/cp_prod/content/hugo/TCBSAgar.htm
The TCBS medium, is an ideal medium to isolate vibrio species, this is because of the high concentrations the Thiosulfate-Citrate, bile and a high pH alcaline. The strains that can't degradate the sacarose are the green color, while the strains that can ferment the sacarose are the yelow color because produce acid as from the sugar. This is because of the color change to the pH indicator the Bromthymol Blue and Thymol Blue, which change to a yellow color in acid medium.
Thank you for valuable comments, i have doubt why nature of particular yellow color vibrio species (not at all:only two species) will change green color (like vibrio parahaemolyticus) after 36 hour incubation ?
note : not all the vibrio spp will change yellow to green,
Amino acid deamination to use amino acids as C-source increases the pH of the medium due to the release of ammonium ions. This change in color could be to the initial use of sugars or sucrose from the media and changing to amino acids as carbon source when the carbohydrates become limiting for growing. Vibrio parahaemolyticus can grow on sucrose as sole C-source but it does not produce acid from it. Other green colony-producing bugs may have similar behavior.
Check this for the biochemical profile of V. parahaemolyticus.
http://www.int-res.com/articles/dao/35/d035p077.pdf
hi, there is also the chance to mutations, harveyi, parahaemolyticus and vulnificus are very common to change from green to yellow colonies in TCBS (sucrose negative to positive), reasons??, no idea but i found this in the past after stress (e.g infecting shrimp, chlorine exposure, so on). There is a paper (colonel and collaborators) that describe the seroconversion of cholera non 01 to 01. It gives you a good idea of the plasticity of vibrio.
The high pH of TCBS agar encourages the growth of Vibrio spp. while inhibiting other organisms. The ability or inability of an organism to ferment sucrose is the basis for differetiating among various species of Vibrio. Bromthymol blue and thymol blue are incorporated to indicate the pH.
Source: Mahon book
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