Dear colleague,

I strongly recommend, simply to use the microscope. You can hardly mix both types of cells up. Even the size will tell you. This is cheap and fast.

Good luck and best wishes - Johannes.

Hi,

Generally yeast colony shapes are buttery and are different from routine bacteria. such as E. coli but it is not recommended to distinguish based on macroscopic shape. It is better to stain the cells and observe under a simple light microscope.

I agree with Johannes Wostemeyer and Sharareh Harirchi , you can use stain and microscope to distinguish in shape and size.

Dear Zina,

We are working in the field of Medical and Veterinary Mycology since 1973. It is difficult to distinguished between yeast with bacteria just on the basis of colony appearance on the media plate.The morphology is imperative to distinguished between yeast with bacteria. We have developed PHOL (Pal, Hasegawa, Ono, Lee) stain (1990) and Narayan stain (1998) to study the morphology of yeast, molds and algae.You can employ our stain for morphological studies of fungi and algae.

We had tried potassium hydroxide technique (KOH) on the culture plate to differentiate between Gram positive and Gram negative bacteria. When 3% aqueous solution of KOH is mixed with colony by inoculation loop on the clean glass slide, Gram negative bacteria (Escherichia coli) become viscous and string out (positive reaction) when inoculation loop is lifted where as Gram positive bacteria (Bacillus cereus) do not form viscous gel and string out (negative reaction).

As Saccharomyces is a Gram positive yeast, KOH reaction gives negative reaction.If you wish, you may please try this simple and low cost KOH technique with other yeasts, such as Candidia, Cryptococcus, Rhodotorula, Trichosporon, Pichia etc. and inform me the results of your experiment. I still advise you to go for morphological studies to distinguished between yeast with bacteria.

Stay safe and healthy.

With best wishes and good luck in your career.

Prof.Dr.Mahendra Pal

Founder Director of Narayan Consultancy on Veterinary Public Health and Microbiology, Anand ,India

Gram stain is good enough

As I think, its very difficult to distinguish between bacteria and yeast depending on their features and shape of colonies appearance on the plates.

Yeast should appear larger than bacteria (when viewed under a microscope). Also, you should be able to isolate the yeast strain on agar, depending on how contaminated the sample is, although it may take a few tries. If you properly streak the sample, you'll definitely be able to tell the difference between yeast and bacteria

Hi Zina

The best results obtained by microscopic slides, but in case of you " Maybe some reagent that react with yeast cell wall resulting in change in color? ", i think you want to focus on Chromoagar and chromogenic substrates, but choromoagars is more expensive rather than common culture media, if you want work deeply, you must focused on metabolism pathways in yeasts and use different chemical substrates that metabolized by yeast. the below papers may be work as a flashlight for you.

- New developments in chromogenic and fluorogenic culture media.

- Fluorogenic and chromogenic enzyme substrates in culture media and identification tests.

Regard

I am with Mahendra Pal as he has given the answer with detailed descriptions, however, I think, based on their features and shape of colonies appearance on the plates, it is not an easy task to separate bacteria and yeast.

You coud add chloramphenicol 10 mg/ml to your culture plates to avoid bacteria growth. Maybe try replica plating in order to determine which colonies correspond to bacteria and which colonies correspond to yeasts.

Use a microscope and based on size, very quickly you will know if you have bacteria fungi or a mixed culture.

If you want to avoid bacteria growth in the culture plates, you can add Chloramphenicol with a concentration of 100 mg per one liter before autoclaving.