Introduction

            The isolation of Lactobacillus helveticus from spoiled milk required the use of enrichment methods to prevent the growth of other natural milk contaminants, including Streptococci.  Enrichment methods include any method used to enhance the growth of a particular bacterium, while selecting against the growth of an unwanted bacterium.  In order to determine the ideal enrichment method, it was necessary to examine the metabolic and physical characteristics of Lactobacillus helveticus and Streptococci.  Bergey’s manual was consulted to determine such characteristics. 

            Lactobacillus helveticus is a gram positive rod.  This non-sporing facultative anaerobe is catalase negative and preferentially grows at 45 degrees Celsius (Bergeys).  L helveticus ferments lactose and glucose, but not sucrose and mannitol (The Prokaryotes).  Steptococci is a gram positive cocci.  Its optimum growth temperature is 37 degrees Celsius, ferments sucrose, grows at a higher pH, and is catalase positive.

            The difference between the two organisms’ shapes (rod and cocci) prompted the use of a Gram Stain as part of the enrichment method. Since Streptococci grow optimally at 37 degrees Celsius, while L helveticus prefers a temperature of 45 degrees Celsius, the media used in this isolation was incubated at 45 degrees Celsius.  Metabolic differences including the fermentation of sucrose and the presence of catalase provoked the use of sucrose fermentation tubes and a catalase test to confirm the presence of L helveticus. Furthermore, acidic Tomato Juice Agar (TJA) was used since L helveticus prefers such an environment, while Steptococci prefers a more basic environment.  It was hypothesized that by growing spoiled milk on TJA, performing a Gram Stain, and running various other tests, Lactobacillus helveticus could be successfully isolated from spoiled milk.