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water system testing yeast and mold total aerobic count

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#1 ratanowski


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Posted 14 February 2017 - 09:28 PM



When testing water systems, we test several ports for Aerobic and Y&M Counts. Lately the Y&M plate for a port that comes after a carbon media bed has been turning up with high counts. We use m-Green y&f broth from Neogen. Also, the aerobic count for the same port is very low. The colonies that appear on the yeast and mold plate are crateriform, beige, and matte. It is unclear if they are yeast or bacteria at this time. My questions are as follows:


1) Are there circumstances which would allow for growth of bacteria on my y&m plate?


2) What conditions would arise in which I would have a high yeast or bacterial count but a low total aerobic count?


Thank you,



#2 Ryan M.

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Posted 15 February 2017 - 11:54 PM

Hi Rachael,


1. Yes, depending on the nutrients in the media.  We use potato dextrose agar (EasyGel brand) and have found if we use potato dextrose without antibiotic we tend to get TPC growth infiltration on the plate.


2.  Sugar levels perhaps?  In my understanding yeast does not grow well in a competitive environment with other microorganisms.


If there is a microbiological expert out there perhaps they can shed some light.  If I were in your position I would contact Neogen and get some technical assistance from them.



#3 FurFarmandFork


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Posted 22 February 2017 - 06:38 PM

Smaller defined colonies on your media are normally interpreted as yeast on ym media (YM media should contain an antibiotic), and they may crater if they grew quickly and incubation was long (as ym media usually is). I would consider it a yeast issue but that's without having seen the media. Read Neogen's interpretation guidance provided with your media or filtration kit.


You can find high y/m counts with low bacteria because y/m can survive environments that are inhospitable to bacteria. They are spore formers, do well at low temperatures, can grow with low water activity, have an enormous pH range, and airborne spores or buds easily access places that bacteria find more trouble inoculating.


Carbon filters and DI filters can become harborage points for bacteria or fungi, I would see if you can sample water from directly upstream of the bed to hopefully identify it as the source, alternatively try a different sample port from the same carbon bed to eliminate the port you used as the source.

Austin Bouck
Owner/Consultant at Fur, Farm, and Fork.
Consulting for companies needing effective, lean food safety systems and solutions.

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