While working with the yeast strain W303-1A, after incubation at 30ºC and storage at 4ºC for around 1 week, white colonies start to appear. Also W303-1A yeast transformed with plasmids and grown in selective media have these white colonies. Can this happen? Or is it for sure a contaminant?
Hi,
Your description is maybe insufficient. This strain has an ade2-1 mutation, therefore, colonies have a tendency to become reddish after some time. This is normal and due to this mutation. What you describe is the opposite. Are these white colonies smaller than the others and appear much later? As an essential test, you should pick one of the "original" colonies and restreak it on fresh medium and do the same with one of the white weird colonies. After a while, will you find again this dual distribution with the original colony or not? Will the white one give only whites? In this latter case, it is very likely that you've got a contamination.
Best.
Hi. I understand that it has ade mutation. I do not know this strain, but color depends on oxidative status of the cell. As I remember well only two of ade mutants are red. Respiratory deficient strains are almost white. Pellets are usually pink in rho- mutants. I remember that many years ago a paper appeared showing that during subcloning progeny of really red spores became pink. This red color is a consequence of polymerization of one of intermediates. Simply check them on minimal medium and if they grow on YPglycerol. Good luck. .
Could you please elaborate more on these "white colonies"? Are they smaller in general than the W303-1A colonies?
First thing I would do if I suspect a contamination is to look at those colonies under the microscope (Dissolve some cells in liquid, then spot some on microscope slides). Bacteria cells are much smaller than yeast cells, and you will be able to distinguish them under the microscope.
W303-1A exhibited a red phenotype on YPD plates, however sometimes small and white colonies appear due to loss of mitochondrial function.
In selective media you must have 20 mg/liter of adenine to obtain red colonies.
Antonio is right. These are most likely petites, partial or complete deletion of mitochondrial DNA leading to loss of mitochondrial function. As a consequence, adenine is no longer limiting for growth, expression of genes in the adenine biosynthesis pathway is reduced, so the pigment is no longer produced.
Can you look at it under the microscope? if different morphology then a contaminant. Sacc cerevisiae is a bit different in morphology from other yeast that are contaminants
Yes, both possibilities should be checked: either they are petites or contaminants. Petites do not grow on glycerol as the sole source of carbon, the contaminants, either bacterial or yeast , could be detected microscopically.
Sometimes happen, if you have too much adenine in the medium,
The colony morphology is very common in many microbes. Especially it is common in lot of yeast strains. If you incubate at 30 degrees centigrade from 48 hrs to 144 hours lot many microbes switch to different phenotypes. It is evident if you take a parameter such as by treating with a physical or chemical factors. For example UV exposure for micro seconds and micrograms of nucleotide analogues (Ex. AZT). Suppose if you plate on any experimental condition around 40 plates (20000 colonies in total), you will definitely observe high frequency switching. Phenotype variability is high frequency that is 1 out of 1000 colonies will give morphology variable phenotype. Your observation is right. You can even try for morphological phenotype colonies.
Hello all!
Thank you very much for all your answers!
I have seen both colonies at the microscope and they have similar morphology (round as W303) but unfortunatly also grow as white colonies in rich media. I have a contaminant and I need to re-start my experiments... Nonetheless i learned new things, so thanks!
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