Judging by the results we observed, we believe that we have successfully isolated Streptococcus thermophilus.  First, only small white colonies grew on the media designed to grow the organism.  This is the expected result for S. thermophilus.  Secondly, the organism grown fermented glucose, but not mannitol, another expected result.  The catalase test came out negative, again agreeing with the expected result.  The gram stain showed gram positive organisms that were either bacilli or streptococci, but upon further investigation, we determined that the bacteria were in fact streptococci.  It was originally difficult to determine the shape of the grown organism because the chains of streptococci resembled the rod shape of bacillus.  (See Results section photos.)  The tests we performed would not differentiate between the two organisms (see Results section table), so microscopic viewing was the only way to determine which organism was present.

    Streptococcus thermophilusdoes not have a known ecological niche.  Its use in fermentation of food products (mainly dairy) was accidentally introduced before medieval times, and has been used continuously since then, with some modification.

    Yogurt is made when S.thermophilus and L. bulgaricus are introduced to milk, and allowed to ferment.  S. thermophilusgrows in the milk first, producing methanoic acid and carbon dioxide.  This creates conditions well suited for the growth of L. bulgaricus, which releases peptides that promote growth of S. thermophilus.  The lactic acid produced by both species makes the yogurt thicken and get its flavor and aroma.  The lactose-digesting organisms are beneficial to humans, as they aid in the digestion of lactose in the gastrointestinal tract.

    S. thermophilus has other uses besides yogurt production.  It is used to make cheddar and mozzerella cheeses, soy milk, and is even used as an anti-diarrheal agent in infants.

Links for More Information

Home Page