Judging from our results, it can safely be concluded that our environmental isolate is a Gram-positive, spherically-shaped bacterium whose cells adopt a chain-like arrangement when grown in T-soy broth.  Presumably, our isolate is a member of the genus Streptococcus because of grouping patterns seen under microscope .  Additionally, our isolate is a facultatively anaerobic, non-motile, gamma-hemolytic specimen incapable of capsule and endospore formation.  Failure of the specimen to grow upon MacConkey’s agar confirmed its Gram-positive status.  It is both catalase and cytochrome oxidase negative.  Optimal temperature range appears to be at or near 37 ^o C, which suggests the presence of a mesophilic organism.  Our isolate demonstrated that it was incapable of lactose fermentation in the carbohydrate utilization tube assays.
 

        The results of the aforementioned confirmatory assays were all consistent with the literature concerning Streptococcus lactis, with the exception of the lactose utilization assay.  S. lactis is unequivocally a lactose-fermenting specimen, and our isolate proved incapable of lactose fermentation.  As a consequence of this discrepancy, we are forced to conclude that we were not successful in isolating S. lactis from whole milk culture.
 

        Bacterial specimens were obtained initially from fresh milk culture and streaked on T-soy agar and prepared tomato juice agar plates to promote growth.  Confirmatory assays then commenced to verify the identify of the organisms isolated from fresh milk.  All indications suggested that we had successfully isolated S. lactis from fresh milk culture; however, the results of the lactose utilization assay conflicted with all other results obtained from isolated specimens.  The observation of a positive reaction in the E. coli lactose-utilization control tube indicated proper functionality of the components of the carbohydrate tube.  Consequently, we were forced to conclude that our isolate was incapable of lactose fermentation, thereby eliminating the possibility that we were successful in our endeavors.
 

        The presence of a non-lactose fermenting bacterium in milk conceivably could be attributable to airborne contamination of the initial milk culture or the inadvertent introduction of environmental specimens.  Had sufficient time been allotted, we should ideally have streaked plates with milk in progressive stages of spoilage to increase the likelihood of isolation of S. lactis.

Why it is Important to Isolate Streptococcus Lactis :

            S. lactis is a common bacterium which thrives in the lactose-rich environment of milk and other associated dairy products.  It readily ferments lactose, generating lactic acid as a metabolic by-product.  Commercially, S. lactis is integral in the production of cheeses (cottage cheese, especially) and cultured butter from dairy precursors.  Since S. lactis is economically valuable, a clear protocol for its isolation and identification is necessary in order to generate acceptable quantities for the commercial synthesis of dairy products.