Laboratory Investigations in Microbiology
One of nature's most elegant and nutritious food sources is milk. Produced by mammals as the sole source of nutrition for the newborn, milk contains crucial nutrients such as protein, sugar, and fat; vitamins; minerals such as calcium; and even antibodies! Not surprisingly, this nutritious fluid also contains many bacteria, including Lactobacillus and Lactococcus. Bacteria in milk may be beneficial for intestinal health (e.g Bifidobacterium and Lactobacillus), contribute to the eventual spoilage of milk through fermentation, and may cause disease (e.g. Salmonella) if present. To prevent disease and slow down spoilage, milk must be pasteurized before being sold. Pasteurization heats milk to temperatures of ~ 63 or 72C, effectively killing Gram-negative bacteria. However, depending on the initial bacterial count, spoilage organisms may still be present in varying numbers. To test the quality of milk, one can measure the rate at which oxygen is being consumed in a milk sample - the more rapidly oxygen is used, the poorer the quality of milk. It is also possible to determine bacterial numbers in milk samples using the same methods we used for food testing.
Lastly, we will also examine some of the different effects bacteria have on milk by examining their reaction in litmus milk (photo on the right) - a complex differential medium containing milk sugar and proteins. Some bacteria ferment lactose (acid), which may also produce a curd. Others do not ferment and thus produce a basic reaction. Some even hydrolyze casein (proteolysis), producing a brownish clear fluid full of amino acids. This photo shows (L to R) Streptococcus lactis (Acid + Curd), Bacillus subtilis (Proteolysis), Alcaligenes faecalis (Base) and E. coli (Acid).
Data sheet & Review Questions
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