Growth on HYA agar:
    After the first incubation period, there were small colonies present on the agar.  We then restreaked isolated colonies onto HYA  and TSA plates and incubated again at 37 degrees Celsius.  Every time, the colonies appeared to be the same, and appeared to be the only type growing on the media.  The colonies were very small (between .5 and 1 millimeter in diameter), and white, which is typical of S. thermophilus.  However, L. bulgaris is also expected to grow on this media, so the following tests were performed to determine which organism was present.  There was a problem in differentiating the two because they have very similar testable characteristics.
Name of test
Expected Result for S. thermophilus
Expected Result for L. bulgaris
Gram Stain gram positive gram positive  gram positive
Catalase negative negative negative
Glucose Fermentation positive positive positive
Mannitol Fermentation negative negative negative
Simple Stain inconclusive:  appears as if both rod shaped and ovoid shaped bacteria were on slide.  The small size of the bacteria made it hard to determine a shape. spherical or ovoid shaped, in chains rod shaped


Simple stain of S. thermophilus

Gram stain of S. thermophilus- purple color indicates gram positive organism

Above, left is light microscope image of a simple stain of S. thermophilus completed in this experiment.  Above, right is an electron microscope picture of S. thermophilus and L. bulgaricus.  Note the lemon-like appearance of S. thermophilus chains that would make it hard to be distinguished from the L. bulgaricus rods.  (Photo from:

Sugar tubes innoculated with isolated organism:  top is mannitol (not fermented), bottom is glucose (fermented).

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