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yeast on mannitol salt agar? - is it or isn't it? (Jul/03/2006 )


I am collecting feline swabs and plating them out on blood agar and mannitol salt agar.

I had a strange result the other day. The ear MSA plate after 1 night incubation at 37 had just 1 pink colony on it. I reincubated the plate overnight to look for delayed mannitol fermentation which resulted in the appearance of lots of small creamy yellow colonies. The plate had also gone completely yellow. I did a gram stain and the cells stained purple and def look like yeast cells. They are fairly big and seem to be cuboid in shape.

Has anyone come across this before? Any ideas on what my mystery organism is? I didn't realise that yeasts would grown on MSA.


Hi LittleMiss

i think ur plate has got contaminated.

de yellow color colonies on MSA r usually Staphylococcus aureus and they are gram positive cocci and r arranged as grape like clusters(when viewed under de microscope)

dis is how S.aureus appears under a microscope:

red color colonies are usually Staphylococci (other than aureus)

and dis is how yeast appears under a microscope:

pls check dis link which gives de info on MSA and also look at de plate diagram in dis article

Mannitol Salt Agar

all de best smile.gif


im thinking also that your plate got contaminated. did u do it under a hood? just re-plate your sample and see if that organisms grows again. if it grows again, subculture in a TSA or NA, then check the phenotype.


I think it's possible Little Miss is correct...but it's easy enough to test

just take some known yeast and plate it on the MSA, do the 2-day incubation and see what you get; unless your contaminant is some odd mutation that might answer your question

based on your clinical experience (I remember some of your posts tongue.gif ) I bet you can tell the difference microscopically between yeast and staph, and I assume you would have realized if it were staphylococcal contamination


Assuming that it didn't get contaminated in between checking it out and putting it in for reincubation (and I'd doubt that, unless you were checking it with your nose in it wink.gif ), my hunch is, based on timing alone is that it's yeast. Staph contamination (or indeed most of the regular suspects) will grow on a plate after one overnight to great size---yeast is a slow grower sometimes, and the two-day incubation is just what it likes to come up with 1-3 mm colonies. The colour, however, is odd for yeast (but on that agar . . .?) My yeast is always white, tan, or red . . . never really yellow. But that might just be my stuff.

Aimikins is right . . . test 'er out.